问题:什么是NullReferenceException,如何解决?

我有一些代码,执行时会抛出NullReferenceException,说:

对象引用未设置为对象的实例。

这是什么意思,我该怎么做才能解决此错误?

标签:c#,.net,vb.net,null,nullreferenceexception

Q: What is a NullReferenceException, and how do I fix it?

I have some code and when it executes, it throws a NullReferenceException, saying:

Object reference not set to an instance of an object.

What does this mean, and what can I do to fix this error?

回答1:

是什么原因?

底线

您正在尝试使用null(或在VB.NET中为Nothing)。这意味着您要么将其设置为null,要么根本不将其设置为任何东西。

像其他任何东西一样,null被传递。如果在方法" A"中为null ,则可能是方法" B"将null 传递给了方法" A"。

null可以具有不同的含义:

  1. 未初始化的对象变量,因此没有指向任何对象。。在这种情况下,如果访问此类对象的属性或方法,则会导致NullReferenceException
  2. 开发人员有意使用null表示没有有意义的值。注意,C#具有变量的可空数据类型的概念(例如数据库表可以具有可空值字段)-您可以将null分配给它们,以指示其中没有存储任何值,例如int?a = null; ,其中问号指示允许在变量a中存储null。您可以使用if(a.HasValue){...}或使用if(a==null){...}进行检查。可空变量(例如本示例中的a)允许通过a.Value显式访问值,或者像通常一样通过a进行访问。
    请注意,如果,则通过a.Value访问它会抛出InvalidOperationException而不是NullReferenceException a null-您应该事先进行检查,即,如果您还有另一个on-nullable变量intb;,则应该执行如果(a.HasValue){b = a.Value; } 或更短的if(a!=null){b=a;}

本文的其余部分将更详细地介绍并显示许多程序员经常犯的错误,这些错误可能导致NullReferenceException

更具体地说

运行时抛出NullReferenceException 总是的意思是相同的:您正在尝试使用引用,并且引用未初始化(或者曾经是 初始化,但不再初始化)。

这意味着引用为null,并且您不能通过null引用访问成员(例如方法)。最简单的情况:

string foo = null;
foo.ToUpper();

这将在第二行引发NullReferenceException,因为您不能在string引用指向上调用实例方法ToUpper()改为null

调试

如何找到NullReferenceException的来源?除了查看异常本身(将其准确地抛出在异常发生的位置)之外,还适用Visual Studio中的常规调试规则:放置战略断点和检查变量,方法是将鼠标悬停在变量名称上,打开(Quick)Watch窗口,或使用各种调试面板(例如Locals和Autos)。

如果要查找引用的设置位置或未设置的位置,请右键单击其名称,然后选择"查找所有引用"。然后,可以在每个找到的位置放置一个断点,并在连接了调试器的情况下运行程序。每次调试器在这样的断点处中断时,您都需要确定引用是否为非空值,检查变量并在需要时验证它是否指向实例。

按照这种程序流程进行操作,您可以找到实例不应为null的位置以及未正确设置实例的原因。

示例

一些可能引发异常的常见情况:

Generic

ref1.ref2.ref3.member

如果ref1或ref2或ref3为空,则您将得到NullReferenceException。如果您想解决问题,则可以通过将表达式重写为更简单的等式来找出哪个为空:

var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member

具体来说,在HttpContext.Current.User.Identity.Name中,HttpContext.Current可以为null,或者User属性可以为空,或者Identity属性可以为空。

间接

public class Person {
    public int Age { get; set; }
}
public class Book {
    public Person Author { get; set; }
}
public class Example {
    public void Foo() {
        Book b1 = new Book();
        int authorAge = b1.Author.Age; // You never initialized the Author property.
                                       // there is no Person to get an Age from.
    }
}

如果要避免子(Person)空引用,可以在父(Book)对象的构造函数中对其进行初始化。

嵌套对象初始化器

嵌套对象初始化程序也是如此:

Book b1 = new Book { Author = { Age = 45 } };

这翻译成

Book b1 = new Book();
b1.Author.Age = 45;

使用new关键字时,它只会创建Book的新实例,而不会创建Person的新实例,因此Author的属性仍然为null

嵌套集合初始化程序

public class Person {
    public ICollection<Book> Books { get; set; }
}
public class Book {
    public string Title { get; set; }
}

嵌套集合初始值设定项的行为相同:

Person p1 = new Person {
    Books = {
        new Book { Title = "Title1" },
        new Book { Title = "Title2" },
    }
};

这翻译成

Person p1 = new Person();
p1.Books.Add(new Book { Title = "Title1" });
p1.Books.Add(new Book { Title = "Title2" });

新人仅创建Person的实例,但是Books集合仍为null。集合初始值设定项语法不会为p1.Books创建集合,而是仅转换为p1.Books.Add(...)语句。

数组

int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.

数组元素

Person[] people = new Person[5];
people[0].Age = 20 // people[0] is null. The array was allocated but not
                   // initialized. There is no Person to set the Age for.

锯齿形阵列

long[][] array = new long[1][];
array[0][0] = 3; // is null because only the first dimension is yet initialized.
                 // Use array[0] = new long[2]; first.

收藏/列表/词典

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
                               // There is no Dictionary to perform the lookup.

范围变量(间接/延迟)

public class Person {
    public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Exception is thrown here, but actually occurs
                                  // on the line above.  "p" is null because the
                                  // first element we added to the list is null.

事件

public class Demo
{
    public event EventHandler StateChanged;

    protected virtual void OnStateChanged(EventArgs e)
    {        
        StateChanged(this, e); // Exception is thrown here 
                               // if no event handlers have been attached
                               // to StateChanged event
    }
}

错误命名约定:

如果您对字段命名与本地名称不同,则可能已经意识到您从未初始化过该字段。

public class Form1 {
    private Customer customer;

    private void Form1_Load(object sender, EventArgs e) {
        Customer customer = new Customer();
        customer.Name = "John";
    }

    private void Button_Click(object sender, EventArgs e) {
        MessageBox.Show(customer.Name);
    }
}

可以通过遵循以下约定为字段加下划线作为前缀来解决:

private Customer _customer;

ASP.NET页面生命周期:

public partial class Issues_Edit : System.Web.UI.Page
{
    protected TestIssue myIssue;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            // Only called on first load, not when button clicked
            myIssue = new TestIssue(); 
        }
    }

    protected void SaveButton_Click(object sender, EventArgs e)
    {
        myIssue.Entry = "NullReferenceException here!";
    }
}

ASP.NET会话值

// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();

ASP.NET MVC空视图模型

如果在ASP.NET MVC视图中引用@Model的属性时发生异常,则需要了解在操作方法中设置了Model,当您返回视图时。当您从控制器返回空模型(或模型属性)时,视图访问它时会发生异常:

// Controller
public class Restaurant:Controller
{
    public ActionResult Search()
    {
         return View();  // Forgot the provide a Model here.
    }
}

// Razor view 
@foreach (var restaurantSearch in Model.RestaurantSearch)  // Throws.
{
}

<p>@Model.somePropertyName</p> <!-- Also throws -->

WPF控件创建顺序和事件

WPF控件是在调用InitializeComponent期间创建的,其顺序是它们在视觉树中的显示顺序。对于带有事件处理程序等的早期创建的控件,将引发NullReferenceException,该事件在引用早期创建的控件的InitializeComponent期间触发。

例如:

<Grid>
    <!-- Combobox declared first -->
    <ComboBox Name="comboBox1" 
              Margin="10"
              SelectedIndex="0" 
              SelectionChanged="comboBox1_SelectionChanged">
        <ComboBoxItem Content="Item 1" />
        <ComboBoxItem Content="Item 2" />
        <ComboBoxItem Content="Item 3" />
    </ComboBox>

    <!-- Label declared later -->
    <Label Name="label1" 
           Content="Label"
           Margin="10" />
</Grid>

label1之前创建comboBox1。如果comboBox1_SelectionChanged尝试引用`label1,则尚未创建。

private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    label1.Content = comboBox1.SelectedIndex.ToString(); // NullReference here!!
}

更改XAML中的声明顺序(即在comboBox1之前列出label1),而忽略了设计哲学问题,至少会解决NullReferenceException 此处。

使用as

进行投射
var myThing = someObject as Thing;

这不会引发InvalidCastException,而是在强制转换失败时(并且someObject本身为null时)返回null。因此请注意。

LINQ FirstOrDefault()和SingleOrDefault()

普通版本First()Single()在没有内容时会引发异常。在这种情况下," OrDefault"版本返回null。因此请注意。

foreach

尝试迭代null集合时,会抛出

foreach。通常是由返回集合的方法的意外null结果引起的。

 List<int> list = null;    
 foreach(var v in list) { } // exception

更实际的示例-从XML文档中选择节点。如果未找到节点但初始调试显示所有属性均有效,则会抛出该错误:

 foreach (var node in myData.MyXml.DocumentNode.SelectNodes("//Data"))

要避免的方式

明确检查null并忽略空值。

如果您希望引用有时为空,则可以在访问实例成员之前检查其是否为null

void PrintName(Person p) {
    if (p != null) {
        Console.WriteLine(p.Name);
    }
}

明确检查null并提供默认值。

您希望返回实例的

方法调用可以返回null,例如,在找不到要查找的对象时。在这种情况下,您可以选择返回默认值:

string GetCategory(Book b) {
    if (b == null)
        return "Unknown";
    return b.Category;
}

从方法调用中明确检查null并引发自定义异常。

您还可以引发自定义异常,仅在调用代码中捕获该异常:

string GetCategory(string bookTitle) {
    var book = library.FindBook(bookTitle);  // This may return null
    if (book == null)
        throw new BookNotFoundException(bookTitle);  // Your custom exception
    return book.Category;
}

如果值永远不应为null,请使用Debug.Assert,以便在异常发生之前更早地发现问题。

当您在开发过程中知道某个方法可以但不能返回null时,可以使用Debug.Assert()使其尽快中断。确实发生了:

string GetTitle(int knownBookID) {
    // You know this should never return null.
    var book = library.GetBook(knownBookID);  

    // Exception will occur on the next line instead of at the end of this method.
    Debug.Assert(book != null, "Library didn't return a book for known book ID.");

    // Some other code

    return book.Title; // Will never throw NullReferenceException in Debug mode.
}

尽管此检查不会最终出现在您的发行版中,导致在发布模式下运行时book==null时,它将再次引发NullReferenceException

GetValueOrDefault()用于可空值类型,以在它们为null时提供默认值。

DateTime? appointment = null;
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the default value provided (DateTime.Now), because appointment is null.

appointment = new DateTime(2022, 10, 20);
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the appointment date, not the default

使用空合并运算符:?? [C#]或If() [VB]。

遇到null时提供默认值的简写:

IService CreateService(ILogger log, Int32? frobPowerLevel)
{
    var serviceImpl = new MyService(log ?? NullLog.Instance);

    // Note that the above "GetValueOrDefault()" can also be rewritten to use
    // the coalesce operator:
    serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;
}

对数组使用空条件运算符:?.?[x](在C#6和VB.NET 14中可用):

有时也称为安全导航或Elvis(在其形状之后)运算符。如果运算符左侧的表达式为null,则不会对右侧进行求值,而是返回null。这意味着像这样的情况:

var title = person.Title.ToUpper();

如果此人没有标题,这将引发异常,因为它试图在具有空值的属性上调用ToUpper

在C#5及以下版本中,可以使用以下方法加以保护:

var title = person.Title == null ? null : person.Title.ToUpper();

现在title变量将为null而不是引发异常。 C#6为此引入了一个较短的语法:

var title = person.Title?.ToUpper();

这将导致标题变量为null,如果person.Title,则不会调用ToUpper

当然,您仍然必须检查title是否为空,或者将空条件运算符与空合并运算符(?? )以提供默认值:

// regular null check
int titleLength = 0;
if (title != null)
    titleLength = title.Length; // If title is null, this would throw NullReferenceException

// combining the `?` and the `??` operator
int titleLength = title?.Length ?? 0;

同样,对于数组,您可以如下使用?[i]

int[] myIntArray=null;
var i=5;
int? elem = myIntArray?[i];
if (!elem.HasValue) Console.WriteLine("No value");

这将执行以下操作:如果myIntArray为null,则表达式返回null,您可以安全地对其进行检查。如果它包含数组,则将执行以下操作:elem=myIntArray[i];并返回第i th 元素。

使用空上下文(在C#8中可用):

在C#8中引入了null上下文和nullable引用类型,它们对变量执行静态分析,并在值可能为null或已设置为null时提供编译器警告。可空引用类型允许将类型明确允许为空。

可以使用csproj文件中的Nullable元素为项目设置可为空的注释上下文和可为空的警告上下文。该元素配置编译器如何解释类​​型的可空性以及生成什么警告。有效设置为:

  • enable:可空注释上下文已启用。可空警告上下文已启用。引用类型的变量(例如字符串)是不可为空的。启用所有可空性警告。
  • disable:可空注释上下文已禁用。可空警告上下文已禁用。引用类型的变量是忽略的,就像C#的早期版本一样。所有可空性警告均已禁用。
  • safeonly:启用了可为空的注释上下文。可为空的警告上下文仅是安全的。引用类型的变量不可为空。启用所有安全性为空的警告。
  • 警告:可空注释上下文已禁用。可空警告上下文已启用。引用类型的变量是忽略的。启用所有可空性警告。
  • safeonlywarnings:可空注释上下文已禁用。可为空的警告上下文仅是安全的。引用类型的变量是忽略的。启用所有安全性为空的警告。

使用与可为空的值类型相同的语法记录可为空的引用类型:将?附加到变量的类型。

用于调试和修复迭代器中的空deref的特殊技术

C#支持"迭代器块"(在某些其他流行语言中称为"生成器")。由于推迟执行,空的解除引用异常在迭代器块中调试特别棘手:

public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
    for (int i = 0; i < count; ++i)
      yield return f.MakeFrob();
}
...
FrobFactory factory = whatever;
IEnumerable<Frobs> frobs = GetFrobs();
...
foreach(Frob frob in frobs) { ... }

如果任何结果为null,则MakeFrob将抛出。现在,您可能会认为正确的做法是:

// DON'T DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
    if (f == null) 
      throw new ArgumentNullException("f", "factory must not be null");
    for (int i = 0; i < count; ++i)
      yield return f.MakeFrob();
}

为什么这是错误的?因为直到foreach为止,迭代器块才真正运行 !对GetFrobs的调用仅返回一个对象,该对象在进行迭代时将运行迭代器块。

通过这样编写null检查,可以防止null取消引用,但是可以将null参数异常移动到 iteration 的位置,而不是 call 的位置>,这调试起来非常混乱

正确的解决方法是:

// DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
    // No yields in a public method that throws!
    if (f == null) 
      throw new ArgumentNullException("f", "factory must not be null");
    return GetFrobsForReal(f, count);
}
private IEnumerable<Frob> GetFrobsForReal(FrobFactory f, int count)
{
    // Yields in a private method
    Debug.Assert(f != null);
    for (int i = 0; i < count; ++i)
      yield return f.MakeFrob();
}

也就是,制作一个具有迭代器块逻辑的私有帮助器方法,以及一个进行空检查并返回迭代器的公共表面方法。现在,当调用GetFrobs时,立即执行空检查,然后在迭代序列时执行GetFrobsForReal

如果检查LINQ to Objects的参考源,您会发现该技术始终被使用。编写起来有点笨拙,但是它使调试null错误更加容易。 优化代码以方便调用者,而不是作者方便

关于不安全代码中空引用的注意事项

顾名思义,

C#具有"不安全"模式,因为不强制执行提供内存安全和类型安全的常规安全机制,因此它极为危险。 除非您对内存的工作原理有透彻和深入的了解,否则不应编写不安全的代码

在不安全模式下,您应该了解两个重要事实:

  • 取消引用空的指针会产生与取消引用空的引用
  • 相同的异常。
  • 取消引用无效的非null指针可以在某些情况下产生该异常

要了解为什么会这样,它首先有助于了解.NET如何产生null取消引用异常。 (这些详细信息适用于Windows上运行的.NET;其他操作系统使用类似的机制。)

内存在Windows中已虚拟化;每个进程都会获得由操作系统跟踪的许多"页面"内存的虚拟内存空间。内存的每一页上都设置了标志,这些标志确定如何使用它:读取,写入,执行等。 最低页面被标记为"如果以任何方式使用都会产生错误"。

C#中的空指针和空引用在内部都表示为数字零,因此任何尝试将其取消引用到其相应的内存中的操作都会导致操作系统产生错误。然后,.NET运行时将检测到此错误,并将其转变为null取消引用异常。

这就是为什么同时引用空指针和空引用会产生相同的异常的原因。

第二点呢?取消引用位于虚拟内存最低页中的 any 无效指针会导致相同的操作系统错误,从而导致相同的异常。

这为什么有意义?好吧,假设我们有一个包含两个int的结构,以及一个等于null的非托管指针。如果我们尝试取消引用结构中的第二个int,CLR将不会尝试访问零位置的存储;它将访问第四位置的存储。但是从逻辑上讲,这是一个空取消引用,因为我们通过空通过到达该地址。

如果您使用的是不安全的代码,并且会收到null解除引用异常,则请注意,有问题的指针不必为null。它可以位于最低页面的任何位置,并且会产生此异常。

A1:

What is the cause?

Bottom Line

You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null, or you never set it to anything at all.

Like anything else, null gets passed around. If it is null in method "A", it could be that method "B" passed a null to method "A".

null can have different meanings:

  1. Object variables which are uninitialized and hence point to nothing. In this case, if you access properties or methods of such objects, it causes a NullReferenceException.
  2. The developer is using null intentionally to indicate there is no meaningful value available. Note that C# has the concept of nullable datatypes for variables (like database tables can have nullable fields) - you can assign null to them to indicate there is no value stored in it, for example int? a = null; where the question mark indicates it is allowed to store null in variable a. You can check that either with if (a.HasValue) {...} or with if (a==null) {...}. Nullable variables, like a this example, allow to access the value via a.Value explicitly, or just as normal via a.
    Note that accessing it via a.Value throws an InvalidOperationException instead of a NullReferenceException if a is null - you should do the check beforehand, i.e. if you have another on-nullable variable int b; then you should do assignments like if (a.HasValue) { b = a.Value; } or shorter if (a != null) { b = a; }.

The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.

More Specifically

The runtime throwing a NullReferenceException always means the same thing: you are trying to use a reference, and the reference is not initialized (or it was once initialized, but is no longer initialized).

This means the reference is null, and you cannot access members (such as methods) through a null reference. The simplest case:

string foo = null;
foo.ToUpper();

This will throw a NullReferenceException at the second line because you can't call the instance method ToUpper() on a string reference pointing to null.

Debugging

How do you find the source of a NullReferenceException? Apart from looking at the exception itself, which will be thrown exactly at the location where it occurs, the general rules of debugging in Visual Studio apply: place strategic breakpoints and inspect your variables, either by hovering the mouse over their names, opening a (Quick)Watch window or using the various debugging panels like Locals and Autos.

If you want to find out where the reference is or isn't set, right-click its name and select "Find All References". You can then place a breakpoint at every found location and run your program with the debugger attached. Every time the debugger breaks on such a breakpoint, you need to determine whether you expect the reference to be non-null, inspect the variable and and verify that it points to an instance when you expect it to.

By following the program flow this way, you can find the location where the instance should not be null, and why it isn't properly set.

Examples

Some common scenarios where the exception can be thrown:

Generic

ref1.ref2.ref3.member

If ref1 or ref2 or ref3 is null, then you'll get a NullReferenceException. If you want to solve the problem, then find out which one is null by rewriting the expression to its simpler equivalent:

var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member

Specifically, in HttpContext.Current.User.Identity.Name, the HttpContext.Current could be null, or the User property could be null, or the Identity property could be null.

Indirect

public class Person {
    public int Age { get; set; }
}
public class Book {
    public Person Author { get; set; }
}
public class Example {
    public void Foo() {
        Book b1 = new Book();
        int authorAge = b1.Author.Age; // You never initialized the Author property.
                                       // there is no Person to get an Age from.
    }
}

If you want to avoid the child (Person) null reference, you could initialize it in the parent (Book) object's constructor.

Nested Object Initializers

The same applies to nested object initializers:

Book b1 = new Book { Author = { Age = 45 } };

This translates to

Book b1 = new Book();
b1.Author.Age = 45;

While the new keyword is used, it only creates a new instance of Book, but not a new instance of Person, so the Author the property is still null.

Nested Collection Initializers

public class Person {
    public ICollection<Book> Books { get; set; }
}
public class Book {
    public string Title { get; set; }
}

The nested collection initializers behave the same:

Person p1 = new Person {
    Books = {
        new Book { Title = "Title1" },
        new Book { Title = "Title2" },
    }
};

This translates to

Person p1 = new Person();
p1.Books.Add(new Book { Title = "Title1" });
p1.Books.Add(new Book { Title = "Title2" });

The new Person only creates an instance of Person, but the Books collection is still null. The collection initializer syntax does not create a collection for p1.Books, it only translates to the p1.Books.Add(...) statements.

Array

int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.

Array Elements

Person[] people = new Person[5];
people[0].Age = 20 // people[0] is null. The array was allocated but not
                   // initialized. There is no Person to set the Age for.

Jagged Arrays

long[][] array = new long[1][];
array[0][0] = 3; // is null because only the first dimension is yet initialized.
                 // Use array[0] = new long[2]; first.

Collection/List/Dictionary

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
                               // There is no Dictionary to perform the lookup.

Range Variable (Indirect/Deferred)

public class Person {
    public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Exception is thrown here, but actually occurs
                                  // on the line above.  "p" is null because the
                                  // first element we added to the list is null.

Events

public class Demo
{
    public event EventHandler StateChanged;

    protected virtual void OnStateChanged(EventArgs e)
    {        
        StateChanged(this, e); // Exception is thrown here 
                               // if no event handlers have been attached
                               // to StateChanged event
    }
}

Bad Naming Conventions:

If you named fields differently from locals, you might have realized that you never initialized the field.

public class Form1 {
    private Customer customer;

    private void Form1_Load(object sender, EventArgs e) {
        Customer customer = new Customer();
        customer.Name = "John";
    }

    private void Button_Click(object sender, EventArgs e) {
        MessageBox.Show(customer.Name);
    }
}

This can be solved by following the convention to prefix fields with an underscore:

private Customer _customer;

ASP.NET Page Life cycle:

public partial class Issues_Edit : System.Web.UI.Page
{
    protected TestIssue myIssue;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            // Only called on first load, not when button clicked
            myIssue = new TestIssue(); 
        }
    }

    protected void SaveButton_Click(object sender, EventArgs e)
    {
        myIssue.Entry = "NullReferenceException here!";
    }
}

ASP.NET Session Values

// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();

ASP.NET MVC empty view models

If the exception occurs when referencing a property of @Model in an ASP.NET MVC view, you need to understand that the Model gets set in your action method, when you return a view. When you return an empty model (or model property) from your controller, the exception occurs when the views access it:

// Controller
public class Restaurant:Controller
{
    public ActionResult Search()
    {
         return View();  // Forgot the provide a Model here.
    }
}

// Razor view 
@foreach (var restaurantSearch in Model.RestaurantSearch)  // Throws.
{
}

<p>@Model.somePropertyName</p> <!-- Also throws -->

WPF Control Creation Order and Events

WPF controls are created during the call to InitializeComponent in the order they appear in the visual tree. A NullReferenceException will be raised in the case of early-created controls with event handlers, etc. , that fire during InitializeComponent which reference late-created controls.

For example :

<Grid>
    <!-- Combobox declared first -->
    <ComboBox Name="comboBox1" 
              Margin="10"
              SelectedIndex="0" 
              SelectionChanged="comboBox1_SelectionChanged">
        <ComboBoxItem Content="Item 1" />
        <ComboBoxItem Content="Item 2" />
        <ComboBoxItem Content="Item 3" />
    </ComboBox>

    <!-- Label declared later -->
    <Label Name="label1" 
           Content="Label"
           Margin="10" />
</Grid>

Here comboBox1 is created before label1. If comboBox1_SelectionChanged attempts to reference `label1, it will not yet have been created.

private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    label1.Content = comboBox1.SelectedIndex.ToString(); // NullReference here!!
}

Changing the order of the declarations in the XAML (i.e., listing label1 before comboBox1, ignoring issues of design philosophy, would at least resolve the NullReferenceException here.

Cast with as

var myThing = someObject as Thing;

This doesn't throw an InvalidCastException but returns a null when the cast fails (and when someObject is itself null). So be aware of that.

LINQ FirstOrDefault() and SingleOrDefault()

The plain versions First() and Single() throw exceptions when there is nothing. The "OrDefault" versions return null in that case. So be aware of that.

foreach

foreach throws when you try to iterate null collection. Usually caused by unexpected null result from methods that return collections.

 List<int> list = null;    
 foreach(var v in list) { } // exception

More realistic example - select nodes from XML document. Will throw if nodes are not found but initial debugging shows that all properties valid:

 foreach (var node in myData.MyXml.DocumentNode.SelectNodes("//Data"))

Ways to Avoid

Explicitly check for null and ignore null values.

If you expect the reference sometimes to be null, you can check for it being null before accessing instance members:

void PrintName(Person p) {
    if (p != null) {
        Console.WriteLine(p.Name);
    }
}

Explicitly check for null and provide a default value.

Methods call you expect to return an instance can return null, for example when the object being sought cannot be found. You can choose to return a default value when this is the case:

string GetCategory(Book b) {
    if (b == null)
        return "Unknown";
    return b.Category;
}

Explicitly check for null from method calls and throw a custom exception.

You can also throw a custom exception, only to catch it in the calling code:

string GetCategory(string bookTitle) {
    var book = library.FindBook(bookTitle);  // This may return null
    if (book == null)
        throw new BookNotFoundException(bookTitle);  // Your custom exception
    return book.Category;
}

Use Debug.Assert if a value should never be null, to catch the problem earlier than the exception occurs.

When you know during development that a method maybe can, but never should return null, you can use Debug.Assert() to break as soon as possible when it does occur:

string GetTitle(int knownBookID) {
    // You know this should never return null.
    var book = library.GetBook(knownBookID);  

    // Exception will occur on the next line instead of at the end of this method.
    Debug.Assert(book != null, "Library didn't return a book for known book ID.");

    // Some other code

    return book.Title; // Will never throw NullReferenceException in Debug mode.
}

Though this check will not end up in your release build, causing it to throw the NullReferenceException again when book == null at runtime in release mode.

Use GetValueOrDefault() for nullable value types to provide a default value when they are null.

DateTime? appointment = null;
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the default value provided (DateTime.Now), because appointment is null.

appointment = new DateTime(2022, 10, 20);
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the appointment date, not the default

Use the null coalescing operator: ?? [C#] or If() [VB].

The shorthand to providing a default value when a null is encountered:

IService CreateService(ILogger log, Int32? frobPowerLevel)
{
    var serviceImpl = new MyService(log ?? NullLog.Instance);

    // Note that the above "GetValueOrDefault()" can also be rewritten to use
    // the coalesce operator:
    serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;
}

Use the null condition operator: ?. or ?[x] for arrays (available in C# 6 and VB.NET 14):

This is also sometimes called the safe navigation or Elvis (after its shape) operator. If the expression on the left side of the operator is null, then the right side will not be evaluated, and null is returned instead. That means cases like this:

var title = person.Title.ToUpper();

If the person does not have a title, this will throw an exception because it is trying to call ToUpper on a property with a null value.

In C# 5 and below, this can be guarded with:

var title = person.Title == null ? null : person.Title.ToUpper();

Now the title variable will be null instead of throwing an exception. C# 6 introduces a shorter syntax for this:

var title = person.Title?.ToUpper();

This will result in the title variable being null, and the call to ToUpper is not made if person.Title is null.

Of course, you still have to check title for null or use the null condition operator together with the null coalescing operator (??) to supply a default value:

// regular null check
int titleLength = 0;
if (title != null)
    titleLength = title.Length; // If title is null, this would throw NullReferenceException

// combining the `?` and the `??` operator
int titleLength = title?.Length ?? 0;

Likewise, for arrays you can use ?[i] as follows:

int[] myIntArray=null;
var i=5;
int? elem = myIntArray?[i];
if (!elem.HasValue) Console.WriteLine("No value");

This will do the following: If myIntArray is null, the expression returns null and you can safely check it. If it contains an array, it will do the same as: elem = myIntArray[i]; and returns the ith element.

Use null context (available in C# 8):

Introduced in C# 8 there null context's and nullable reference types perform static analysis on variables and provides a compiler warning if a value can be potentially null or have been set to null. The nullable reference types allows types to be explicitly allowed to be null.

The nullable annotation context and nullable warning context can be set for a project using the Nullable element in your csproj file. This element configures how the compiler interprets the nullability of types and what warnings are generated. Valid settings are:

  • enable: The nullable annotation context is enabled. The nullable warning context is enabled. Variables of a reference type, string for example, are non-nullable. All nullability warnings are enabled.
  • disable: The nullable annotation context is disabled. The nullable warning context is disabled. Variables of a reference type are oblivious, just like earlier versions of C#. All nullability warnings are disabled.
  • safeonly: The nullable annotation context is enabled. The nullable warning context is safeonly. Variables of a reference type are nonnullable. All safety nullability warnings are enabled.
  • warnings: The nullable annotation context is disabled. The nullable warning context is enabled. Variables of a reference type are oblivious. All nullability warnings are enabled.
  • safeonlywarnings: The nullable annotation context is disabled. The nullable warning context is safeonly. Variables of a reference type are oblivious. All safety nullability warnings are enabled.

A nullable reference type is noted using the same syntax as nullable value types: a ? is appended to the type of the variable.

Special techniques for debugging and fixing null derefs in iterators

C# supports "iterator blocks" (called "generators" in some other popular languages). Null dereference exceptions can be particularly tricky to debug in iterator blocks because of deferred execution:

public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
    for (int i = 0; i < count; ++i)
      yield return f.MakeFrob();
}
...
FrobFactory factory = whatever;
IEnumerable<Frobs> frobs = GetFrobs();
...
foreach(Frob frob in frobs) { ... }

If whatever results in null then MakeFrob will throw. Now, you might think that the right thing to do is this:

// DON'T DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
    if (f == null) 
      throw new ArgumentNullException("f", "factory must not be null");
    for (int i = 0; i < count; ++i)
      yield return f.MakeFrob();
}

Why is this wrong? Because the iterator block does not actually run until the foreach! The call to GetFrobs simply returns an object which when iterated will run the iterator block.

By writing a null check like this you prevent the null dereference, but you move the null argument exception to the point of the iteration, not to the point of the call, and that is very confusing to debug.

The correct fix is:

// DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
    // No yields in a public method that throws!
    if (f == null) 
      throw new ArgumentNullException("f", "factory must not be null");
    return GetFrobsForReal(f, count);
}
private IEnumerable<Frob> GetFrobsForReal(FrobFactory f, int count)
{
    // Yields in a private method
    Debug.Assert(f != null);
    for (int i = 0; i < count; ++i)
      yield return f.MakeFrob();
}

That is, make a private helper method that has the iterator block logic, and a public surface method that does the null check and returns the iterator. Now when GetFrobs is called, the null check happens immediately, and then GetFrobsForReal executes when the sequence is iterated.

If you examine the reference source for LINQ to Objects you will see that this technique is used throughout. It is slightly more clunky to write, but it makes debugging nullity errors much easier. Optimize your code for the convenience of the caller, not the convenience of the author.

A note on null dereferences in unsafe code

C# has an "unsafe" mode which is, as the name implies, extremely dangerous because the normal safety mechanisms which provide memory safety and type safety are not enforced. You should not be writing unsafe code unless you have a thorough and deep understanding of how memory works.

In unsafe mode, you should be aware of two important facts:

  • dereferencing a null pointer produces the same exception as dereferencing a null reference
  • dereferencing an invalid non-null pointer can produce that exception in some circumstances

To understand why that is, it helps to understand how .NET produces null dereference exceptions in the first place. (These details apply to .NET running on Windows; other operating systems use similar mechanisms.)

Memory is virtualized in Windows; each process gets a virtual memory space of many "pages" of memory that are tracked by the operating system. Each page of memory has flags set on it which determine how it may be used: read from, written to, executed, and so on. The lowest page is marked as "produce an error if ever used in any way".

Both a null pointer and a null reference in C# are internally represented as the number zero, and so any attempt to dereference it into its corresponding memory storage causes the operating system to produce an error. The .NET runtime then detects this error and turns it into the null dereference exception.

That's why dereferencing both a null pointer and a null reference produces the same exception.

What about the second point? Dereferencing any invalid pointer that falls in the lowest page of virtual memory causes the same operating system error, and thereby the same exception.

Why does this make sense? Well, suppose we have a struct containing two ints, and an unmanaged pointer equal to null. If we attempt to dereference the second int in the struct, the CLR will not attempt to access the storage at location zero; it will access the storage at location four. But logically this is a null dereference because we are getting to that address via the null.

If you are working with unsafe code and you get a null dereference exception, just be aware that the offending pointer need not be null. It can be any location in the lowest page, and this exception will be produced.

回答2:

NullReference异常— Visual Basic

Visual Basic NullReference异常 C#中的相同。毕竟,它们都报告了它们都使用的.NET Framework中定义的相同异常。 Visual Basic特有的原因很少(也许只有一个)。

此答案将使用Visual Basic术语,语法和上下文。使用的示例来自大量过去的Stack Overflow问题。这是通过使用帖子中经常出现的情况的<种类> 来最大化相关性。还为可能需要的人提供了更多解释。 非常可能会在此处列出与您类似的示例。

注意:

  1. 这是基于概念的:没有代码可粘贴到您的项目中。它旨在帮助您了解导致NullReferenceException(NRE)的原因,如何找到它,如何修复它以及如何避免它。 NRE可能由多种原因引起,因此这不可能是您唯一遇到的问题。
  2. 示例(来自Stack Overflow的帖子)并不总是显示出最佳的做某事的最佳方法。
  3. 通常,使用最简单的补救方法。

基本含义

消息"对象未设置为对象的实例" 表示您正在尝试使用尚未初始化的对象。归结为以下之一:

  • 您的代码声明了一个对象变量,但没有对其进行初始化(创建实例或" 实例化")
  • 您的代码假设会初始化对象的东西
  • 可能,其他代码过早地使仍在使用的对象无效

找到原因

由于问题是一个对象引用,该对象引用是Nothing,所以答案是检查它们以找出哪个。然后确定为什么不初始化。将鼠标悬停在各个变量上,Visual Studio(VS)将显示其值-罪魁祸首是Nothing

 IDE调试显示

您还应该从相关代码中删除任何Try / Catch块,尤其是在Catch块中没有任何内容的地方。尝试使用Nothing的对象时,这将导致您的代码崩溃。 这就是您想要的,因为它将识别出问题的确切位置,并允许您识别导致问题的对象。

在Catch中显示Errorwhile...MsgBox几乎没有帮助。这种方法还会导致非常糟糕的堆栈溢出问题,因为您无法描述实际的异常,所涉及的对象甚至发生异常的代码行。

您还可以使用LocalsWindow( Debug-> Windows-> Locals )检查对象。

一旦您知道问题所在和出处,通常比发布新问题要容易得多,而且解决起来也快。

另请参阅:

示例和补救措施

类对象/创建实例

Dim reg As CashRegister
...
TextBox1.Text = reg.Amount         ' NRE

问题是Dim不会创建CashRegister object ;它仅声明该类型的名为reg的变量。 声明一个对象变量和创建一个实例是两件事。

补救措施

New运算符通常可以在声明实例时用于创建实例:

Dim reg As New CashRegister        ' [New] creates instance, invokes the constructor

' Longer, more explicit form:
Dim reg As CashRegister = New CashRegister

仅在以后创建实例时:

Private reg As CashRegister         ' Declare
  ...
reg = New CashRegister()            ' Create instance

注意:请勿在过程中再次使用Dim,包括构造函数(SubNew):

Private reg As CashRegister
'...

Public Sub New()
   '...
   Dim reg As New CashRegister
End Sub

这将创建一个 local 变量reg,该变量仅在该上下文(子)中存在。模块级别为Scopereg变量将在其他任何地方使用,仍然为Nothing

缺少New运算符是在所检查的堆栈溢出问题中看到的NullReference异常 的第一原因。

Visual Basic尝试使用 New 重复使过程更清晰:使用 New 运算符可创建< strong> new 对象,并调用 SubNew -构造函数-您的对象可以在其中执行任何其他初始化。

为清楚起见,Dim(或Private)仅声明变量及其Type。变量的 Scope (是否存在于整个模块/类中还是过程的局部变量)由声明的 where 确定。 私有|朋友| Public 定义访问级别,而不是 Scope

有关更多信息,请参见:


数组

还必须实例化数组:

Private arr as String()

仅声明了此数组,未创建。有几种初始化数组的方法:

Private arr as String() = New String(10){}
' or
Private arr() As String = New String(10){}

' For a local array (in a procedure) and using 'Option Infer':
Dim arr = New String(10) {}

注意:从VS 2010开始,使用文字和OptionInferAs New初始化本地数组时,元素是可选的:

Dim myDbl As Double() = {1.5, 2, 9.9, 18, 3.14}
Dim myDbl = New Double() {1.5, 2, 9.9, 18, 3.14}
Dim myDbl() = {1.5, 2, 9.9, 18, 3.14}

从分配的数据中推断出数据类型和数组大小。类/模块级别的声明仍然需要As OptionStrict

Private myDoubles As Double() = {1.5, 2, 9.9, 18, 3.14}

示例:类对象的数组

Dim arrFoo(5) As Foo

For i As Integer = 0 To arrFoo.Count - 1
   arrFoo(i).Bar = i * 10       ' Exception
Next

已创建数组,但其中的Foo对象尚未创建。

补救措施

For i As Integer = 0 To arrFoo.Count - 1
    arrFoo(i) = New Foo()         ' Create Foo instance
    arrFoo(i).Bar = i * 10
Next

使用List(OfT)将使得很难拥有没有有效对象的元素:

Dim FooList As New List(Of Foo)     ' List created, but it is empty
Dim f As Foo                        ' Temporary variable for the loop

For i As Integer = 0 To 5
    f = New Foo()                    ' Foo instance created
    f.Bar =  i * 10
    FooList.Add(f)                   ' Foo object added to list
Next

有关更多信息,请参见:


列表和集合

.NET集合(种类繁多-列表,字典等)也必须实例化或创建。

Private myList As List(Of String)
..
myList.Add("ziggy")           ' NullReference

由于相同的原因,您将获得相同的异常-仅声明了myList,但未创建任何实例。补救措施是相同的:

myList = New List(Of String)

' Or create an instance when declared:
Private myList As New List(Of String)

常见的疏忽是使用集合Type

的类。
Public Class Foo
    Private barList As List(Of Bar)

    Friend Function BarCount As Integer
        Return barList.Count
    End Function

    Friend Sub AddItem(newBar As Bar)
        If barList.Contains(newBar) = False Then
            barList.Add(newBar)
        End If
    End Function

这两个过程都将导致NRE,因为barList仅声明而不是实例化。创建Foo的实例也不会创建内部barList的实例。可能是打算在构造函数中执行此操作:

Public Sub New         ' Constructor
    ' Stuff to do when a new Foo is created...
    barList = New List(Of Bar)
End Sub

和以前一样,这是不正确的:

Public Sub New()
    ' Creates another barList local to this procedure
     Dim barList As New List(Of Bar)
End Sub

有关更多信息,请参见 List(属于T)


数据提供者对象

使用数据库为NullReference提供了很多机会,因为可以有很多对象(CommandConnectionTransactionDatasetDataTableDataRows ....)一次使用。 注意:不管使用哪个数据提供程序-MySQL,SQL Server,OleDB等-概念都是相同的。

示例1

Dim da As OleDbDataAdapter
Dim ds As DataSet
Dim MaxRows As Integer

con.Open()
Dim sql = "SELECT * FROM tblfoobar_List"
da = New OleDbDataAdapter(sql, con)
da.Fill(ds, "foobar")
con.Close()

MaxRows = ds.Tables("foobar").Rows.Count      ' Error

和以前一样,声明了ds数据集对象,但从未创建实例。 DataAdapter将填充现有的DataSet,而不创建一个。在这种情况下,由于ds是局部变量,因此 IDE警告您这可能会发生:

 img

当声明为模块/类级别的变量时(如con所示),编译器无法知道对象是否由上游过程创建。不要忽略警告。

补救措施

Dim ds As New DataSet

示例2

ds = New DataSet
da = New OleDBDataAdapter(sql, con)
da.Fill(ds, "Employees")

txtID.Text = ds.Tables("Employee").Rows(0).Item(1)
txtID.Name = ds.Tables("Employee").Rows(0).Item(2)

这里有一个拼写错误:EmployeesEmployee。没有创建名为" Employee"的DataTable,因此,NullReferenceException导致尝试访问它。另一个潜在的问题是假设将存在Items,而当SQL包含WHERE子句时,情况可能并非如此。

补救措施

因为它使用一个表,所以使用Tables(0)可以避免拼写错误。检查Rows.Count也可以:

If ds.Tables(0).Rows.Count > 0 Then
    txtID.Text = ds.Tables(0).Rows(0).Item(1)
    txtID.Name = ds.Tables(0).Rows(0).Item(2)
End If

填充是一个函数,该函数返回受影响的的数量,也可以对其进行测试:

If da.Fill(ds, "Employees") > 0 Then...

示例3

Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO,
        TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN
        FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con)
Dim ds As New DataSet
da.Fill(ds)

If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then

如上例所示,DataAdapter将提供TableNames,但不会解析SQL或数据库表中的名称。结果,ds.Tables("TICKET_RESERVATION")引用了一个不存在的表。

补救措施是相同的,请按索引引用该表:

If ds.Tables(0).Rows.Count > 0 Then

另请参见 DataTable类< / a>。


对象路径/嵌套

If myFoo.Bar.Items IsNot Nothing Then
   ...

该代码仅测试Items,而myFooBar可能也没有。 补救方法是一次测试一个对象的整个链或路径:

If (myFoo IsNot Nothing) AndAlso
    (myFoo.Bar IsNot Nothing) AndAlso
    (myFoo.Bar.Items IsNot Nothing) Then
    ....

Andso很重要。一旦遇到第一个False条件,将不执行后续测试。这允许代码一次安全地"钻"入对象一个"级别",仅在(如果确定)myFoo之后评估myFoo.Bar是有效的。编码复杂对象时,对象链或路径可能会变得很长:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

不可能引用null对象的任何"下游"对象。这也适用于控件:

myWebBrowser.Document.GetElementById("formfld1").InnerText = "some value"

在这里,myWebBrowserDocument可能为空,或者formfld1元素可能不存在。


UI控件

Dim cmd5 As New SqlCommand("select Cartons, Pieces, Foobar " _
     & "FROM Invoice where invoice_no = '" & _
     Me.ComboBox5.SelectedItem.ToString.Trim & "' And category = '" & _
     Me.ListBox1.SelectedItem.ToString.Trim & "' And item_name = '" & _
     Me.ComboBox2.SelectedValue.ToString.Trim & "' And expiry_date = '" & _
     Me.expiry.Text & "'", con)

除其他事项外,此代码无法预期用户可能未在一个或多个UI控件中选择某些内容。 ListBox1.SelectedItem可能是Nothing,所以ListBox1.SelectedItem.ToString将产生NRE。

补救措施

在使用之前验证数据(也使用OptionStrict和SQL参数):

Dim expiry As DateTime         ' for text date validation
If (ComboBox5.SelectedItems.Count > 0) AndAlso
    (ListBox1.SelectedItems.Count > 0) AndAlso
    (ComboBox2.SelectedItems.Count > 0) AndAlso
    (DateTime.TryParse(expiry.Text, expiry) Then

    '... do stuff
Else
    MessageBox.Show(...error message...)
End If

或者,您可以使用(ComboBox5.SelectedItemIsNothing)And......


Visual Basic表单

Public Class Form1

    Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _
                   Controls("TextBox2"), Controls("TextBox3"), _
                   Controls("TextBox4"), Controls("TextBox5"), _
                   Controls("TextBox6")}

    ' same thing in a different format:
    Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...}

    ' Immediate NRE:
    Private somevar As String = Me.Controls("TextBox1").Text

这是获得NRE的相当普遍的方法。在C#中,根据其编码方式,IDE将报告在当前上下文中不存在Controls或"无法引用非静态成员"。因此,在某种程度上,这是仅VB的情况。这也很复杂,因为它可能导致级联失败。

无法以这种方式初始化数组和集合。此初始化代码将在构造器创建Form之前的之前运行.控件。结果:

  • 列表和集合将完全为空
  • 数组将包含Nothing的五个元素
  • somevar分配将导致立即产生NRE,因为什么都没有.Text属性

稍后引用数组元素将导致NRE。如果在Form_Load中执行此操作,则由于一个奇怪的错误,IDE 可能不会在发生异常时报告该异常。当您的代码尝试使用数组时,该异常将弹出稍后。此"静默例外"是在这篇文章中详细介绍的。出于我们的目的,关键是当创建表单时发生灾难性事件(SubNewFormLoad事件)时,异常可能不会报告,代码将退出过程,并且只是显示表单。

由于NRE之后,SubNewFormLoad事件中将不会再运行其他代码,因此还有很多其他事情未初始化。

Sub Form_Load(..._
   '...
   Dim name As String = NameBoxes(2).Text        ' NRE
   ' ...
   ' More code (which will likely not be executed)
   ' ...
End Sub

注意,这适用于所有和所有控件和组件引用,这些引用使它们在以下位置非法:

Public Class Form1

    Private myFiles() As String = Me.OpenFileDialog1.FileName & ...
    Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..."
    Private studentName As String = TextBox13.Text

部分补救措施

很好奇VB没有提供警告,但补救措施是在表单级别声明容器,但在表单加载事件处理程序中初始化它们控件执行存在。只要您的代码在InitializeComponent调用之后,即可在SubNew中完成:

' Module level declaration
Private NameBoxes as TextBox()
Private studentName As String

' Form Load, Form Shown or Sub New:
'
' Using the OP's approach (illegal using OPTION STRICT)
NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...)
studentName = TextBox32.Text           ' For simple control references

阵列代码可能尚未走出困境。容器控件中的任何控件(例如GroupBoxPanel)都不会在Me.Controls中找到;它们将位于该Panel或GroupBox的Controls集合中。控件名称拼写错误("TeStBox2")时,也不返回控件。在这种情况下,Nothing将不再存储在那些数组元素中,并且在您尝试引用它时将产生NRE。

现在,您已经知道要查找的内容,这些应该很容易找到: VS会向您显示错误方式

" Button2"位于Panel

补救措施

使用控件引用代替使用表单的Controls集合按名称进行间接引用:

' Declaration
Private NameBoxes As TextBox()

' Initialization -  simple and easy to read, hard to botch:
NameBoxes = New TextBox() {TextBox1, TextBox2, ...)

' Initialize a List
NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...})
' or
NamesList = New List(Of TextBox)
NamesList.AddRange({TextBox1, TextBox2, TextBox3...})

函数什么也不返回

Private bars As New List(Of Bars)        ' Declared and created

Public Function BarList() As List(Of Bars)
    bars.Clear
    If someCondition Then
        For n As Integer = 0 to someValue
            bars.Add(GetBar(n))
        Next n
    Else
        Exit Function
    End If

    Return bars
End Function

在这种情况下,IDE会警告您" 并非所有路径都返回值,并且可能会导致NullReferenceException "。您可以通过将ExitFunction替换为ReturnNothing来抑制警告,但这不能解决问题。当someCondition=False时,任何尝试使用返回值的东西都会导致NRE:

bList = myFoo.BarList()
For Each b As Bar in bList      ' EXCEPTION
      ...

补救措施

在函数中用ReturnbList替换ExitFunction。返回 empty List不同于返回Nothing。如果返回的对象有可能是Nothing,请在使用前进行测试:

 bList = myFoo.BarList()
 If bList IsNot Nothing Then...

尝试/捕获实施不佳

实施不当的"尝试/捕获"功能可能会隐藏问题所在并导致出现新问题:

Dim dr As SqlDataReader
Try
    Dim lnk As LinkButton = TryCast(sender, LinkButton)
    Dim gr As GridViewRow = DirectCast(lnk.NamingContainer, GridViewRow)
    Dim eid As String = GridView1.DataKeys(gr.RowIndex).Value.ToString()
    ViewState("username") = eid
    sqlQry = "select FirstName, Surname, DepartmentName, ExtensionName, jobTitle,
             Pager, mailaddress, from employees1 where username='" & eid & "'"
    If connection.State <> ConnectionState.Open Then
        connection.Open()
    End If
    command = New SqlCommand(sqlQry, connection)

    'More code fooing and barring

    dr = command.ExecuteReader()
    If dr.Read() Then
        lblFirstName.Text = Convert.ToString(dr("FirstName"))
        ...
    End If
    mpe.Show()
Catch

Finally
    command.Dispose()
    dr.Close()             ' <-- NRE
    connection.Close()
End Try

这是一个对象未​​按预期创建的情况,但还演示了空Catch的反作用。

SQL中(在"邮件地址"之后)有一个多余的逗号,导致在.ExecuteReader处出现异常。在Catch不执行任何操作之后,最后尝试执行清理,但是由于您无法Close关闭空的DataReader对象,一个全新的NullReferenceException结果。

一个空的Catch块是魔鬼的游乐场。这个OP困惑于为什么他在Finally块中获得了NRE。在其他情况下,空的Catch可能会导致其他事情进一步向下游蔓延,并导致您花费时间在错误的位置寻找问题所在。 (上述"静默例外"提供了相同的娱乐价值。)

补救措施

不要使用空的Try / Catch块-让代码崩溃,以便您可以a)查明原因b)查明位置并c)采取适当的补救措施。 "尝试/捕获"块无意于向唯一有资格对其进行修复的人员-开发人员隐藏异常。


DBNull与Nothing不同

For Each row As DataGridViewRow In dgvPlanning.Rows
    If Not IsDBNull(row.Cells(0).Value) Then
        ...

IsDBNull函数用于测试是否等于System.DBNull从MSDN:

System.DBNull值指示该Object表示缺少或不存在的数据。 DBNull与Nothing不同,表示变量尚未初始化。

补救措施

If row.Cells(0) IsNot Nothing Then ...

像以前一样,您可以先测试Nothing,然后再测试特定值:

If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then

示例2

Dim getFoo = (From f In dbContext.FooBars
               Where f.something = something
               Select f).FirstOrDefault

If Not IsDBNull(getFoo) Then
    If IsDBNull(getFoo.user_id) Then
        txtFirst.Text = getFoo.first_name
    Else
       ...

FirstOrDefault返回第一项或默认值,对于引用类型,为Nothing,从不为DBNull

If getFoo IsNot Nothing Then...

控件

Dim chk As CheckBox

chk = CType(Me.Controls(chkName), CheckBox)
If chk.Checked Then
    Return chk
End If

如果找不到带有chkNameCheckBox(或存在于GroupBox中),则chk将为Nothing,并且尝试引用任何属性都将导致异常。

补救措施

If (chk IsNot Nothing) AndAlso (chk.Checked) Then ...

DataGridView

DGV有一些定期出现的怪癖:

dgvBooks.DataSource = loan.Books
dgvBooks.Columns("ISBN").Visible = True       ' NullReferenceException
dgvBooks.Columns("Title").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Author").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Price").DefaultCellStyle.Format = "C"

如果dgvBooks具有AutoGenerateColumns=True,它将创建列,但没有命名它们,因此当上面的代码按名称引用它们时,上述代码将失败。 / p>

补救措施

手动命名列,或按索引引用:

dgvBooks.Columns(0).Visible = True

示例2-提防NewRow

xlWorkSheet = xlWorkBook.Sheets("sheet1")

For i = 0 To myDGV.RowCount - 1
    For j = 0 To myDGV.ColumnCount - 1
        For k As Integer = 1 To myDGV.Columns.Count
            xlWorkSheet.Cells(1, k) = myDGV.Columns(k - 1).HeaderText
            xlWorkSheet.Cells(i + 2, j + 1) = myDGV(j, i).Value.ToString()
        Next
    Next
Next

当您的DataGridViewAllowUserToAddRows作为True(默认值)时,Cells为空白/新底部的所有行都将包含Nothing。大多数尝试使用内容的尝试(例如,ToString)将导致NRE。

补救措施

使用For/Each循环并测试IsNewRow属性以确定它是否是最后一行。无论AllowUserToAddRows是否为真,此方法都有效:

For Each r As DataGridViewRow in myDGV.Rows
    If r.IsNewRow = False Then
         ' ok to use this row

如果确实使用Forn循环,则当IsNewRow为true时,修改行数或使用ExitFor


我的设置(StringCollection)

在某些情况下,尝试使用来自My.Settings的项目(其为StringCollection)可能会在您首次使用NullReference时出现。解决方案是相同的,但不是很明显。考虑:

My.Settings.FooBars.Add("ziggy")         ' foobars is a string collection

由于VB正在为您管理"设置",因此可以预期它会初始化集合。它将,但前提是您先前已在集合中添加了初始条目(在"设置"编辑器中)。由于添加集合时(显然)初始化了集合,因此在"设置"编辑器中没有要添加的项目时,集合保持Nothing

补救措施

如果需要的话,在表单的Load事件处理程序中初始化设置集合:

If My.Settings.FooBars Is Nothing Then
    My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

通常,Settings集合仅在应用程序第一次运行时才需要初始化。另一种解决方法是在 Project-> Settings | FooBars ,保存项目,然后删除假值。


要点

您可能忘记了New运算符。

您认为可以完美执行的操作可以将初始化的对象返回到您的代码中,

永远不要忽略编译器警告,并始终使用OptionStrictOn(可选)。


MSDN NullReference异常

A2:

NullReference Exception — Visual Basic

The NullReference Exception for Visual Basic is no different from the one in C#. After all, they are both reporting the same exception defined in the .NET Framework which they both use. Causes unique to Visual Basic are rare (perhaps only one).

This answer will use Visual Basic terms, syntax, and context. The examples used come from a large number of past Stack Overflow questions. This is to maximize relevance by using the kinds of situations often seen in posts. A bit more explanation is also provided for those who might need it. An example similar to yours is very likely listed here.

Note:

  1. This is concept-based: there is no code for you to paste into your project. It is intended to help you understand what causes a NullReferenceException (NRE), how to find it, how to fix it, and how to avoid it. An NRE can be caused many ways so this is unlikely to be your sole encounter.
  2. The examples (from Stack Overflow posts) do not always show the best way to do something in the first place.
  3. Typically, the simplest remedy is used.

Basic Meaning

The message "Object not set to an instance of Object" means you are trying to use an object which has not been initialized. This boils down to one of these:

  • Your code declared an object variable, but it did not initialize it (create an instance or 'instantiate' it)
  • Something which your code assumed would initialize an object, did not
  • Possibly, other code prematurely invalidated an object still in use

Finding The Cause

Since the problem is an object reference which is Nothing, the answer is to examine them to find out which one. Then determine why it is not initialized. Hold the mouse over the various variables and Visual Studio (VS) will show their values - the culprit will be Nothing.

You should also remove any Try/Catch blocks from the relevant code, especially ones where there is nothing in the Catch block. This will cause your code to crash when it tries to use an object which is Nothing. This is what you want because it will identify the exact location of the problem, and allow you to identify the object causing it.

A MsgBox in the Catch which displays Error while... will be of little help. This method also leads to very bad Stack Overflow questions, because you can't describe the actual exception, the object involved or even the line of code where it happens.

You can also use the Locals Window (Debug -> Windows -> Locals) to examine your objects.

Once you know what and where the problem is, it is usually fairly easy to fix and faster than posting a new question.

See also:

Examples and Remedies

Class Objects / Creating an Instance

Dim reg As CashRegister
...
TextBox1.Text = reg.Amount         ' NRE

The problem is that Dim does not create a CashRegister object; it only declares a variable named reg of that Type. Declaring an object variable and creating an instance are two different things.

Remedy

The New operator can often be used to create the instance when you declare it:

Dim reg As New CashRegister        ' [New] creates instance, invokes the constructor

' Longer, more explicit form:
Dim reg As CashRegister = New CashRegister

When it is only appropriate to create the instance later:

Private reg As CashRegister         ' Declare
  ...
reg = New CashRegister()            ' Create instance

Note: Do not use Dim again in a procedure, including the constructor (Sub New):

Private reg As CashRegister
'...

Public Sub New()
   '...
   Dim reg As New CashRegister
End Sub

This will create a local variable, reg, which exists only in that context (sub). The reg variable with module level Scope which you will use everywhere else remains Nothing.

Missing the New operator is the #1 cause of NullReference Exceptions seen in the Stack Overflow questions reviewed.

Visual Basic tries to make the process clear repeatedly using New: Using the New Operator creates a new object and calls Sub New -- the constructor -- where your object can perform any other initialization.

To be clear, Dim (or Private) only declares a variable and its Type. The Scope of the variable - whether it exists for the entire module/class or is local to a procedure - is determined by where it is declared. Private | Friend | Public defines the access level, not Scope.

For more information, see:


Arrays

Arrays must also be instantiated:

Private arr as String()

This array has only been declared, not created. There are several ways to initialize an array:

Private arr as String() = New String(10){}
' or
Private arr() As String = New String(10){}

' For a local array (in a procedure) and using 'Option Infer':
Dim arr = New String(10) {}

Note: Beginning with VS 2010, when initializing a local array using a literal and Option Infer, the As <Type> and New elements are optional:

Dim myDbl As Double() = {1.5, 2, 9.9, 18, 3.14}
Dim myDbl = New Double() {1.5, 2, 9.9, 18, 3.14}
Dim myDbl() = {1.5, 2, 9.9, 18, 3.14}

The data Type and array size are inferred from the data being assigned. Class/Module level declarations still require As <Type> with Option Strict:

Private myDoubles As Double() = {1.5, 2, 9.9, 18, 3.14}

Example: Array of class objects

Dim arrFoo(5) As Foo

For i As Integer = 0 To arrFoo.Count - 1
   arrFoo(i).Bar = i * 10       ' Exception
Next

The array has been created, but the Foo objects in it have not.

Remedy

For i As Integer = 0 To arrFoo.Count - 1
    arrFoo(i) = New Foo()         ' Create Foo instance
    arrFoo(i).Bar = i * 10
Next

Using a List(Of T) will make it quite difficult to have an element without a valid object:

Dim FooList As New List(Of Foo)     ' List created, but it is empty
Dim f As Foo                        ' Temporary variable for the loop

For i As Integer = 0 To 5
    f = New Foo()                    ' Foo instance created
    f.Bar =  i * 10
    FooList.Add(f)                   ' Foo object added to list
Next

For more information, see:


Lists and Collections

.NET collections (of which there are many varieties - Lists, Dictionary, etc.) must also be instantiated or created.

Private myList As List(Of String)
..
myList.Add("ziggy")           ' NullReference

You get the same exception for the same reason - myList was only declared, but no instance created. The remedy is the same:

myList = New List(Of String)

' Or create an instance when declared:
Private myList As New List(Of String)

A common oversight is a class which uses a collection Type:

Public Class Foo
    Private barList As List(Of Bar)

    Friend Function BarCount As Integer
        Return barList.Count
    End Function

    Friend Sub AddItem(newBar As Bar)
        If barList.Contains(newBar) = False Then
            barList.Add(newBar)
        End If
    End Function

Either procedure will result in an NRE, because barList is only declared, not instantiated. Creating an instance of Foo will not also create an instance of the internal barList. It may have been the intent to do this in the constructor:

Public Sub New         ' Constructor
    ' Stuff to do when a new Foo is created...
    barList = New List(Of Bar)
End Sub

As before, this is incorrect:

Public Sub New()
    ' Creates another barList local to this procedure
     Dim barList As New List(Of Bar)
End Sub

For more information, see List(Of T) Class.


Data Provider Objects

Working with databases presents many opportunities for a NullReference because there can be many objects (Command, Connection, Transaction, Dataset, DataTable, DataRows....) in use at once. Note: It does not matter which data provider you are using -- MySQL, SQL Server, OleDB, etc. -- the concepts are the same.

Example 1

Dim da As OleDbDataAdapter
Dim ds As DataSet
Dim MaxRows As Integer

con.Open()
Dim sql = "SELECT * FROM tblfoobar_List"
da = New OleDbDataAdapter(sql, con)
da.Fill(ds, "foobar")
con.Close()

MaxRows = ds.Tables("foobar").Rows.Count      ' Error

As before, the ds Dataset object was declared, but an instance was never created. The DataAdapter will fill an existing DataSet, not create one. In this case, since ds is a local variable, the IDE warns you that this might happen:

When declared as a module/class level variable, as appears to be the case with con, the compiler can't know if the object was created by an upstream procedure. Do not ignore warnings.

Remedy

Dim ds As New DataSet

Example 2

ds = New DataSet
da = New OleDBDataAdapter(sql, con)
da.Fill(ds, "Employees")

txtID.Text = ds.Tables("Employee").Rows(0).Item(1)
txtID.Name = ds.Tables("Employee").Rows(0).Item(2)

A typo is a problem here: Employees vs Employee. There was no DataTable named "Employee" created, so a NullReferenceException results trying to access it. Another potential problem is assuming there will be Items which may not be so when the SQL includes a WHERE clause.

Remedy

Since this uses one table, using Tables(0) will avoid spelling errors. Examining Rows.Count can also help:

If ds.Tables(0).Rows.Count > 0 Then
    txtID.Text = ds.Tables(0).Rows(0).Item(1)
    txtID.Name = ds.Tables(0).Rows(0).Item(2)
End If

Fill is a function returning the number of Rows affected which can also be tested:

If da.Fill(ds, "Employees") > 0 Then...

Example 3

Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO,
        TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN
        FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con)
Dim ds As New DataSet
da.Fill(ds)

If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then

The DataAdapter will provide TableNames as shown in the previous example, but it does not parse names from the SQL or database table. As a result, ds.Tables("TICKET_RESERVATION") references a non-existent table.

The Remedy is the same, reference the table by index:

If ds.Tables(0).Rows.Count > 0 Then

See also DataTable Class.


Object Paths / Nested

If myFoo.Bar.Items IsNot Nothing Then
   ...

The code is only testing Items while both myFoo and Bar may also be Nothing. The remedy is to test the entire chain or path of objects one at a time:

If (myFoo IsNot Nothing) AndAlso
    (myFoo.Bar IsNot Nothing) AndAlso
    (myFoo.Bar.Items IsNot Nothing) Then
    ....

AndAlso is important. Subsequent tests will not be performed once the first False condition is encountered. This allows the code to safely 'drill' into the object(s) one 'level' at a time, evaluating myFoo.Bar only after (and if) myFoo is determined to be valid. Object chains or paths can get quite long when coding complex objects:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

It is not possible to reference anything 'downstream' of a null object. This also applies to controls:

myWebBrowser.Document.GetElementById("formfld1").InnerText = "some value"

Here, myWebBrowser or Document could be Nothing or the formfld1 element may not exist.


UI Controls

Dim cmd5 As New SqlCommand("select Cartons, Pieces, Foobar " _
     & "FROM Invoice where invoice_no = '" & _
     Me.ComboBox5.SelectedItem.ToString.Trim & "' And category = '" & _
     Me.ListBox1.SelectedItem.ToString.Trim & "' And item_name = '" & _
     Me.ComboBox2.SelectedValue.ToString.Trim & "' And expiry_date = '" & _
     Me.expiry.Text & "'", con)

Among other things, this code does not anticipate that the user may not have selected something in one or more UI controls. ListBox1.SelectedItem may well be Nothing, so ListBox1.SelectedItem.ToString will result in an NRE.

Remedy

Validate data before using it (also use Option Strict and SQL parameters):

Dim expiry As DateTime         ' for text date validation
If (ComboBox5.SelectedItems.Count > 0) AndAlso
    (ListBox1.SelectedItems.Count > 0) AndAlso
    (ComboBox2.SelectedItems.Count > 0) AndAlso
    (DateTime.TryParse(expiry.Text, expiry) Then

    '... do stuff
Else
    MessageBox.Show(...error message...)
End If

Alternatively, you can use (ComboBox5.SelectedItem IsNot Nothing) AndAlso...


Visual Basic Forms

Public Class Form1

    Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _
                   Controls("TextBox2"), Controls("TextBox3"), _
                   Controls("TextBox4"), Controls("TextBox5"), _
                   Controls("TextBox6")}

    ' same thing in a different format:
    Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...}

    ' Immediate NRE:
    Private somevar As String = Me.Controls("TextBox1").Text

This is a fairly common way to get an NRE. In C#, depending on how it is coded, the IDE will report that Controls does not exist in the current context, or "cannot reference non-static member". So, to some extent, this is a VB-only situation. It is also complex because it can result in a failure cascade.

The arrays and collections cannot be initialized this way. This initialization code will run before the constructor creates the Form or the Controls. As a result:

  • Lists and Collection will simply be empty
  • The Array will contain five elements of Nothing
  • The somevar assignment will result in an immediate NRE because Nothing doesn't have a .Text property

Referencing array elements later will result in an NRE. If you do this in Form_Load, due to an odd bug, the IDE may not report the exception when it happens. The exception will pop up later when your code tries to use the array. This "silent exception" is detailed in this post. For our purposes, the key is that when something catastrophic happens while creating a form (Sub New or Form Load event), exceptions may go unreported, the code exits the procedure and just displays the form.

Since no other code in your Sub New or Form Load event will run after the NRE, a great many other things can be left uninitialized.

Sub Form_Load(..._
   '...
   Dim name As String = NameBoxes(2).Text        ' NRE
   ' ...
   ' More code (which will likely not be executed)
   ' ...
End Sub

Note this applies to any and all control and component references making these illegal where they are:

Public Class Form1

    Private myFiles() As String = Me.OpenFileDialog1.FileName & ...
    Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..."
    Private studentName As String = TextBox13.Text

Partial Remedy

It is curious that VB does not provide a warning, but the remedy is to declare the containers at the form level, but initialize them in form load event handler when the controls do exist. This can be done in Sub New as long as your code is after the InitializeComponent call:

' Module level declaration
Private NameBoxes as TextBox()
Private studentName As String

' Form Load, Form Shown or Sub New:
'
' Using the OP's approach (illegal using OPTION STRICT)
NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...)
studentName = TextBox32.Text           ' For simple control references

The array code may not be out of the woods yet. Any controls which are in a container control (like a GroupBox or Panel) will not be found in Me.Controls; they will be in the Controls collection of that Panel or GroupBox. Nor will a control be returned when the control name is misspelled ("TeStBox2"). In such cases, Nothing will again be stored in those array elements and an NRE will result when you attempt to reference it.

These should be easy to find now that you know what you are looking for:

"Button2" resides on a Panel

Remedy

Rather than indirect references by name using the form's Controls collection, use the control reference:

' Declaration
Private NameBoxes As TextBox()

' Initialization -  simple and easy to read, hard to botch:
NameBoxes = New TextBox() {TextBox1, TextBox2, ...)

' Initialize a List
NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...})
' or
NamesList = New List(Of TextBox)
NamesList.AddRange({TextBox1, TextBox2, TextBox3...})

Function Returning Nothing

Private bars As New List(Of Bars)        ' Declared and created

Public Function BarList() As List(Of Bars)
    bars.Clear
    If someCondition Then
        For n As Integer = 0 to someValue
            bars.Add(GetBar(n))
        Next n
    Else
        Exit Function
    End If

    Return bars
End Function

This is a case where the IDE will warn you that 'not all paths return a value and a NullReferenceException may result'. You can suppress the warning, by replacing Exit Function with Return Nothing, but that does not solve the problem. Anything which tries to use the return when someCondition = False will result in an NRE:

bList = myFoo.BarList()
For Each b As Bar in bList      ' EXCEPTION
      ...

Remedy

Replace Exit Function in the function with Return bList. Returning an empty List is not the same as returning Nothing. If there is a chance that a returned object can be Nothing, test before using it:

 bList = myFoo.BarList()
 If bList IsNot Nothing Then...

Poorly Implemented Try/Catch

A badly implemented Try/Catch can hide where the problem is and result in new ones:

Dim dr As SqlDataReader
Try
    Dim lnk As LinkButton = TryCast(sender, LinkButton)
    Dim gr As GridViewRow = DirectCast(lnk.NamingContainer, GridViewRow)
    Dim eid As String = GridView1.DataKeys(gr.RowIndex).Value.ToString()
    ViewState("username") = eid
    sqlQry = "select FirstName, Surname, DepartmentName, ExtensionName, jobTitle,
             Pager, mailaddress, from employees1 where username='" & eid & "'"
    If connection.State <> ConnectionState.Open Then
        connection.Open()
    End If
    command = New SqlCommand(sqlQry, connection)

    'More code fooing and barring

    dr = command.ExecuteReader()
    If dr.Read() Then
        lblFirstName.Text = Convert.ToString(dr("FirstName"))
        ...
    End If
    mpe.Show()
Catch

Finally
    command.Dispose()
    dr.Close()             ' <-- NRE
    connection.Close()
End Try

This is a case of an object not being created as expected, but also demonstrates the counter usefulness of an empty Catch.

There is an extra comma in the SQL (after 'mailaddress') which results in an exception at .ExecuteReader. After the Catch does nothing, Finally tries to perform clean up, but since you cannot Close a null DataReader object, a brand new NullReferenceException results.

An empty Catch block is the devil's playground. This OP was baffled why he was getting an NRE in the Finally block. In other situations, an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem. (The "silent exception" described above provides the same entertainment value.)

Remedy

Don't use empty Try/Catch blocks - let the code crash so you can a) identify the cause b) identify the location and c) apply a proper remedy. Try/Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them - the developer.


DBNull is not the same as Nothing

For Each row As DataGridViewRow In dgvPlanning.Rows
    If Not IsDBNull(row.Cells(0).Value) Then
        ...

The IsDBNull function is used to test if a value equals System.DBNull: From MSDN:

The System.DBNull value indicates that the Object represents missing or non-existent data. DBNull is not the same as Nothing, which indicates that a variable has not yet been initialized.

Remedy

If row.Cells(0) IsNot Nothing Then ...

As before, you can test for Nothing, then for a specific value:

If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then

Example 2

Dim getFoo = (From f In dbContext.FooBars
               Where f.something = something
               Select f).FirstOrDefault

If Not IsDBNull(getFoo) Then
    If IsDBNull(getFoo.user_id) Then
        txtFirst.Text = getFoo.first_name
    Else
       ...

FirstOrDefault returns the first item or the default value, which is Nothing for reference types and never DBNull:

If getFoo IsNot Nothing Then...

Controls

Dim chk As CheckBox

chk = CType(Me.Controls(chkName), CheckBox)
If chk.Checked Then
    Return chk
End If

If a CheckBox with chkName can't be found (or exists in a GroupBox), then chk will be Nothing and be attempting to reference any property will result in an exception.

Remedy

If (chk IsNot Nothing) AndAlso (chk.Checked) Then ...

The DataGridView

The DGV has a few quirks seen periodically:

dgvBooks.DataSource = loan.Books
dgvBooks.Columns("ISBN").Visible = True       ' NullReferenceException
dgvBooks.Columns("Title").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Author").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Price").DefaultCellStyle.Format = "C"

If dgvBooks has AutoGenerateColumns = True, it will create the columns, but it does not name them, so the above code fails when it references them by name.

Remedy

Name the columns manually, or reference by index:

dgvBooks.Columns(0).Visible = True

Example 2 — Beware of the NewRow

xlWorkSheet = xlWorkBook.Sheets("sheet1")

For i = 0 To myDGV.RowCount - 1
    For j = 0 To myDGV.ColumnCount - 1
        For k As Integer = 1 To myDGV.Columns.Count
            xlWorkSheet.Cells(1, k) = myDGV.Columns(k - 1).HeaderText
            xlWorkSheet.Cells(i + 2, j + 1) = myDGV(j, i).Value.ToString()
        Next
    Next
Next

When your DataGridView has AllowUserToAddRows as True (the default), the Cells in the blank/new row at the bottom will all contain Nothing. Most attempts to use the contents (for example, ToString) will result in an NRE.

Remedy

Use a For/Each loop and test the IsNewRow property to determine if it is that last row. This works whether AllowUserToAddRows is true or not:

For Each r As DataGridViewRow in myDGV.Rows
    If r.IsNewRow = False Then
         ' ok to use this row

If you do use a For n loop, modify the row count or use Exit For when IsNewRow is true.


My.Settings (StringCollection)

Under certain circumstances, trying to use an item from My.Settings which is a StringCollection can result in a NullReference the first time you use it. The solution is the same, but not as obvious. Consider:

My.Settings.FooBars.Add("ziggy")         ' foobars is a string collection

Since VB is managing Settings for you, it is reasonable to expect it to initialize the collection. It will, but only if you have previously added an initial entry to the collection (in the Settings editor). Since the collection is (apparently) initialized when an item is added, it remains Nothing when there are no items in the Settings editor to add.

Remedy

Initialize the settings collection in the form's Load event handler, if/when needed:

If My.Settings.FooBars Is Nothing Then
    My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

Typically, the Settings collection will only need to be initialized the first time the application runs. An alternate remedy is to add an initial value to your collection in Project -> Settings | FooBars, save the project, then remove the fake value.


Key Points

You probably forgot the New operator.

or

Something you assumed would perform flawlessly to return an initialized object to your code, did not.

Don't ignore compiler warnings (ever) and use Option Strict On (always).


MSDN NullReference Exception

回答3:

另一种情况是将空对象转换为值类型。例如,下面的代码:

object o = null;
DateTime d = (DateTime)o;

它将在演员表上抛出NullReferenceException。在上面的示例中,这似乎很明显,但是这可能发生在更复杂的"后期绑定"场景中,其中空对象是从您不拥有的某些代码中返回的,而强制转换例如是由某些自动系统生成的。

一个示例是带有Calendar控件的简单ASP.NET绑定片段:

<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />

在这里,SelectedDate实际上是Calendar Web控件类型的DateTime类型的属性,并且绑定可以完美地返回空的东西。隐式ASP.NET生成器将创建一段与上面的强制转换代码等效的代码。这将引发一个很难发现的NullReferenceException,因为它位于ASP.NET生成的代码中,可以很好地编译...

A3:

Another scenario is when you cast a null object into a value type. For example, the code below:

object o = null;
DateTime d = (DateTime)o;

It will throw a NullReferenceException on the cast. It seems quite obvious in the above sample, but this can happen in more "late-binding" intricate scenarios where the null object has been returned from some code you don't own, and the cast is for example generated by some automatic system.

One example of this is this simple ASP.NET binding fragment with the Calendar control:

<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />

Here, SelectedDate is in fact a property - of DateTime type - of the Calendar Web Control type, and the binding could perfectly return something null. The implicit ASP.NET Generator will create a piece of code that will be equivalent to the cast code above. And this will raise a NullReferenceException that is quite difficult to spot, because it lies in ASP.NET generated code which compiles fine...

回答4:

这意味着所讨论的变量没有指向任何对象。我可以这样生成:

SqlConnection connection = null;
connection.Open();

这将引发错误,因为当我声明变量" connection"时,它没有指向任何东西。当我尝试将成员称为" Open"时,没有可解决的引用,它将引发错误。

为避免此错误:

  1. 在尝试对它们执行任何操作之前,请始终对其进行初始化。
  2. 如果不确定对象是否为空,请使用object==null进行检查。

JetBrains的Resharper工具将识别代码中可能存在空引用错误的每个位置,从而使您可以进行空检查。该错误是错误的第一来源,恕我直言。

A4:

It means that the variable in question is pointed at nothing. I could generate this like so:

SqlConnection connection = null;
connection.Open();

That will throw the error because while I've declared the variable "connection", it's not pointed to anything. When I try to call the member "Open", there's no reference for it to resolve, and it will throw the error.

To avoid this error:

  1. Always initialize your objects before you try to do anything with them.
  2. If you're not sure whether the object is null, check it with object == null.

JetBrains' Resharper tool will identify every place in your code that has the possibility of a null reference error, allowing you to put in a null check. This error is the number one source of bugs, IMHO.

回答5:

这意味着您的代码使用了设置为null的对象引用变量(即,它没有引用实际的对象实例)。

为防止错误,可能为null的对象在使用前应进行null测试。

if (myvar != null)
{
    // Go ahead and use myvar
    myvar.property = ...
}
else
{
    // Whoops! myvar is null and cannot be used without first
    // assigning it to an instance reference
    // Attempting to use myvar here will result in NullReferenceException
}

A5:

It means your code used an object reference variable that was set to null (i.e. it did not reference an actual object instance).

To prevent the error, objects that could be null should be tested for null before being used.

if (myvar != null)
{
    // Go ahead and use myvar
    myvar.property = ...
}
else
{
    // Whoops! myvar is null and cannot be used without first
    // assigning it to an instance reference
    // Attempting to use myvar here will result in NullReferenceException
}

回答6:

请注意,无论哪种情况,.NET中的原因始终相同:

您正在尝试使用值为Nothing / null的引用变量。当引用变量的值为Nothing / null时,这意味着它实际上并不持有对堆上存在的任何对象的实例的引用。

您要么从未为变量分配任何东西,从未创建分配给该变量的值的实例,要么手动将变量设置为等于Nothing / null,或者您调用了一个将变量设置为Nothing / null的函数。

A6:

Be aware that regardless of the scenario, the cause is always the same in .NET:

You are trying to use a reference variable whose value is Nothing/null. When the value is Nothing/null for the reference variable, that means it is not actually holding a reference to an instance of any object that exists on the heap.

You either never assigned something to the variable, never created an instance of the value assigned to the variable, or you set the variable equal to Nothing/null manually, or you called a function that set the variable to Nothing/null for you.

回答7:

抛出此异常的一个示例是:当您尝试检查某些内容时,该值为null。

例如:

string testString = null; //Because it doesn't have a value (i.e. it's null; "Length" cannot do what it needs to do)

if (testString.Length == 0) // Throws a nullreferenceexception
{
    //Do something
} 

当您尝试对尚未实例化的内容(即上面的代码)执行操作时,.NET运行时将抛出NullReferenceException。

与ArgumentNullException相比,如果方法期望传递给它的内容不为null,则通常将其作为防御措施抛出。

更多信息在 C#NullReferenceException和Null参数 中。

A7:

An example of this exception being thrown is: When you are trying to check something, that is null.

For example:

string testString = null; //Because it doesn't have a value (i.e. it's null; "Length" cannot do what it needs to do)

if (testString.Length == 0) // Throws a nullreferenceexception
{
    //Do something
} 

The .NET runtime will throw a NullReferenceException when you attempt to perform an action on something which hasn't been instantiated i.e. the code above.

In comparison to an ArgumentNullException which is typically thrown as a defensive measure if a method expects that what is being passed to it is not null.

More information is in C# NullReferenceException and Null Parameter.

回答8:

更新C#8.0,2019:可为空的引用类型

C#8.0引入了可空引用类型不可空引用类型。因此,仅必须检查可为空的引用类型,以避免 NullReferenceException


如果尚未初始化引用类型,并且要设置或读取其属性之一,则它将引发 NullReferenceException

示例:

Person p = null;
p.Name = "Harry"; // NullReferenceException occurs here.

您可以通过检查变量是否不为空来简单地避免这种情况:

Person p = null;
if (p!=null)
{
    p.Name = "Harry"; // Not going to run to this point
}

要完全理解为什么会引发NullReferenceException,了解值类型和[引用类型] [3]。

因此,如果您要处理值类型,则可能不会发生NullReferenceException。尽管在处理引用类型时需要保持警惕!

顾名思义,只有引用类型可以保存引用或从字面上指向任何内容(或" null")。值类型始终包含一个值。

引用类型(必须检查这些类型):

  • 动态
  • 对象
  • 字符串

值类型(您可以忽略这些值类型):

  • 数值类型
  • 整数类型
  • 浮点类型
  • 十进制
  • 布尔
  • 用户定义的结构

A8:

Update C#8.0, 2019: Nullable reference types

C#8.0 introduces nullable reference types and non-nullable reference types. So only nullable reference types must be checked to avoid a NullReferenceException.


If you have not initialized a reference type, and you want to set or read one of its properties, it will throw a NullReferenceException.

Example:

Person p = null;
p.Name = "Harry"; // NullReferenceException occurs here.

You can simply avoid this by checking if the variable is not null:

Person p = null;
if (p!=null)
{
    p.Name = "Harry"; // Not going to run to this point
}

To fully understand why a NullReferenceException is thrown, it is important to know the difference between value types and [reference types][3].

So, if you're dealing with value types, NullReferenceExceptions can not occur. Though you need to keep alert when dealing with reference types!

Only reference types, as the name is suggesting, can hold references or point literally to nothing (or 'null'). Whereas value types always contain a value.

Reference types (these ones must be checked):

  • dynamic
  • object
  • string

Value types (you can simply ignore these ones):

  • Numeric types
  • Integral types
  • Floating-point types
  • decimal
  • bool
  • User defined structs

回答9:

NullReferenceExceptions可能发生的另一种情况是 as运算符

class Book {
    public string Name { get; set; }
}
class Car { }

Car mycar = new Car();
Book mybook = mycar as Book;   // Incompatible conversion --> mybook = null

Console.WriteLine(mybook.Name);   // NullReferenceException

此处,BookCar是不兼容的类型; Car不能转换/广播为Book。如果此强制转换失败,则as返回null。此后使用mybook会导致NullReferenceException

通常,应按以下方式使用强制转换或as

如果您希望类型转换始终成功(即,您知道对象应该提前执行什么操作),则应使用强制转换:

ComicBook cb = (ComicBook)specificBook;

如果不确定类型,但是想尝试将其用作特定类型,请使用as

ComicBook cb = specificBook as ComicBook;
if (cb != null) {
   // ...
}

A9:

Another case where NullReferenceExceptions can happen is the (incorrect) use of the as operator:

class Book {
    public string Name { get; set; }
}
class Car { }

Car mycar = new Car();
Book mybook = mycar as Book;   // Incompatible conversion --> mybook = null

Console.WriteLine(mybook.Name);   // NullReferenceException

Here, Book and Car are incompatible types; a Car cannot be converted/cast to a Book. When this cast fails, as returns null. Using mybook after this causes a NullReferenceException.

In general, you should use a cast or as, as follows:

If you are expecting the type conversion to always succeed (ie. you know what the object should be ahead of time), then you should use a cast:

ComicBook cb = (ComicBook)specificBook;

If you are unsure of the type, but you want to try to use it as a specific type, then use as:

ComicBook cb = specificBook as ComicBook;
if (cb != null) {
   // ...
}

回答10:

您正在使用包含空值引用的对象。因此,它给出了一个空异常。在该示例中,字符串值为null,并且在检查其长度时,发生了异常。

示例:

string value = null;
if (value.Length == 0) // <-- Causes exception
{
    Console.WriteLine(value); // <-- Never reached
}

异常错误是:

未处理的异常:

System.NullReferenceException:对象引用未设置为对象的实例。在Program.Main()

A10:

You are using the object that contains the null value reference. So it's giving a null exception. In the example the string value is null and when checking its length, the exception occurred.

Example:

string value = null;
if (value.Length == 0) // <-- Causes exception
{
    Console.WriteLine(value); // <-- Never reached
}

The exception error is:

Unhandled Exception:

System.NullReferenceException: Object reference not set to an instance of an object. at Program.Main()

回答11:

什么导致 NullReferenceExceptions avoid / fix 这样的异常的方法已在其他答案中得到了解决,许多程序员尚未了解的是如何独立地 debug

在Visual Studio中,这通常很容易,这要感谢 Visual Studio调试器


首先,请确保将捕获正确的错误-请参见如何在VS2010中如何允许中断" System.NullReferenceException" ? Note 1

然后"> em>从调试开始(F5) 将[VS调试器]附加到正在运行的进程 。有时使用 Debugger.Break ,它将提示启动调试器。

现在,当引发(或未处理)NullReferenceException时,调试器将在发生异常的那行停止(是否记住上面设置的规则?)。有时错误很容易发现。

例如,在以下行中,可以引起异常的唯一代码是myString评估为null时。可以通过查看监视窗口或在即时窗口中运行表达式。

var x = myString.Trim();

在更高级的情况下,例如以下情况,您需要使用上面的一种技术("监视"或"即时Windows")来检查表达式,以确定str1是否为空或< code> str2 为空。

var x = str1.Trim() + str2.Trim();

一旦找到了引发异常的位置 ,通常通常不容易进行倒推以找出在[错误地]引入了null值的位置-

花点时间了解异常原因。检查空表达式。检查可能导致这种空表达式的先前表达式。添加断点,并根据需要逐步执行该程序。 使用调试器。


1 如果"抛出时中断"过于激进,并且调试器在.NET或3rd-party库中的NPE上停止,则用户未处理的破坏可用于限制捕获的异常。此外,VS2012引入了我的代码,我建议也启用该功能。 / p>

如果在启用"仅我的代码"的情况下进行调试,则行为会稍有不同。启用"仅我的代码"后,调试器将忽略在"我的代码"之外抛出且不会通过"我的代码"传递的第一时间公共语言运行时(CLR)异常

A11:

While what causes a NullReferenceExceptions and approaches to avoid/fix such an exception have been addressed in other answers, what many programmers haven't learned yet is how to independently debug such exceptions during development.

In Visual Studio this is usually easy thanks to the Visual Studio Debugger.


First, make sure that the correct error is going to be caught - see How do I allow breaking on 'System.NullReferenceException' in VS2010? Note1

Then either Start with Debugging (F5) or Attach [the VS Debugger] to Running Process. On occasion it may be useful to use Debugger.Break, which will prompt to launch the debugger.

Now, when the NullReferenceException is thrown (or unhandled) the debugger will stop (remember the rule set above?) on the line on which the exception occurred. Sometimes the error will be easy to spot.

For instance, in the following line the only code that can cause the exception is if myString evaluates to null. This can be verified by looking at the Watch Window or running expressions in the Immediate Window.

var x = myString.Trim();

In more advanced cases, such as the following, you'll need to use one of the techniques above (Watch or Immediate Windows) to inspect the expressions to determine if str1 was null or if str2 was null.

var x = str1.Trim() + str2.Trim();

Once where the exception is throw has been located, it's usually trivial to reason backwards to find out where the null value was [incorrectly] introduced --

Take the time required to understand the cause of the exception. Inspect for null expressions. Inspect the previous expressions which could have resulted in such null expressions. Add breakpoints and step through the program as appropriate. Use the debugger.


1 If Break on Throws is too aggressive and the debugger stops on an NPE in the .NET or 3rd-party library, Break on User-Unhandled can be used to limit the exceptions caught. Additionally, VS2012 introduces Just My Code which I recommend enabling as well.

If you are debugging with Just My Code enabled, the behavior is slightly different. With Just My Code enabled, the debugger ignores first-chance common language runtime (CLR) exceptions that are thrown outside of My Code and do not pass through My Code

回答12:

Simon Mourier给出了此示例

object o = null;
DateTime d = (DateTime)o;  // NullReferenceException

其中 取消装箱 转换(投射)来自 对象(或来自其中一个类别System.ValueType System.Enum,或从接口类型)值类型(Nullable<>除外)本身给出了NullReferenceException

从另一个方向来说,是 装箱 转换,是来自具有HasValue的Nullable<> 等于false 的引用类型,可以提供null引用,该引用随后可以导致NullReferenceException 。经典示例是:

DateTime? d = null;
var s = d.ToString();  // OK, no exception (no boxing), returns ""
var t = d.GetType();   // Bang! d is boxed, NullReferenceException

有时候拳击会以另一种方式发生。例如,使用这种非通用扩展方法:

public static void MyExtension(this object x)
{
  x.ToString();
}

以下代码会出现问题:

DateTime? d = null;
d.MyExtension();  // Leads to boxing, NullReferenceException occurs inside the body of the called method, not here.

出现这些情况是由于运行时对Nullable<>实例进行装箱时使用了特殊规则。

A12:

Simon Mourier gave this example:

object o = null;
DateTime d = (DateTime)o;  // NullReferenceException

where an unboxing conversion (cast) from object (or from one of the classes System.ValueType or System.Enum, or from an interface type) to a value type (other than Nullable<>) in itself gives the NullReferenceException.

In the other direction, a boxing conversion from a Nullable<> which has HasValue equal to false to a reference type, can give a null reference which can then later lead to a NullReferenceException. The classic example is:

DateTime? d = null;
var s = d.ToString();  // OK, no exception (no boxing), returns ""
var t = d.GetType();   // Bang! d is boxed, NullReferenceException

Sometimes the boxing happens in another way. For example with this non-generic extension method:

public static void MyExtension(this object x)
{
  x.ToString();
}

the following code will be problematic:

DateTime? d = null;
d.MyExtension();  // Leads to boxing, NullReferenceException occurs inside the body of the called method, not here.

These cases arise because of the special rules the runtime uses when boxing Nullable<> instances.

回答13:

添加一种情况,即实体框架中使用的实体的类名与Web表单代码隐藏文件的类名相同。

假设您有一个Web表单Contact.aspx,其代码背后的类是Contact,并且您有一个实体名称Contact。

然后,当您调用context.SaveChanges()

时,以下代码将引发NullReferenceException。
Contact contact = new Contact { Name = "Abhinav"};
var context = new DataContext();
context.Contacts.Add(contact);
context.SaveChanges(); // NullReferenceException at this line

为了完整性DataContext类

public class DataContext : DbContext 
{
    public DbSet<Contact> Contacts {get; set;}
}

和联系实体类。有时,实体类是局部类,因此您也可以将它们扩展到其他文件中。

public partial class Contact 
{
    public string Name {get; set;}
}

当实体和代码隐藏类都在同一名称空间中时,将发生错误。要解决此问题,请为Contact.aspx重命名实体类或代码隐藏类。

原因我仍然不确定原因。但是只要任何实体类将扩展System.Web.UI.Page,就会发生此错误。

有关讨论,请查看DbContext.saveChanges()中的 NullReferenceException

A13:

Adding a case when the class name for entity used in entity framework is same as class name for a web form code-behind file.

Suppose you have a web form Contact.aspx whose codebehind class is Contact and you have an entity name Contact.

Then following code will throw a NullReferenceException when you call context.SaveChanges()

Contact contact = new Contact { Name = "Abhinav"};
var context = new DataContext();
context.Contacts.Add(contact);
context.SaveChanges(); // NullReferenceException at this line

For the sake of completeness DataContext class

public class DataContext : DbContext 
{
    public DbSet<Contact> Contacts {get; set;}
}

and Contact entity class. Sometimes entity classes are partial classes so that you can extend them in other files too.

public partial class Contact 
{
    public string Name {get; set;}
}

The error occurs when both the entity and codebehind class are in same namespace. To fix this, rename the entity class or the codebehind class for Contact.aspx.

Reason I am still not sure about the reason. But whenever any of the entity class will extend System.Web.UI.Page this error occurs.

For discussion have a look at NullReferenceException in DbContext.saveChanges()

回答14:

另一种可能会收到此异常的一般情况涉及在单元测试期间模拟类。无论使用哪种模拟框架,都必须确保正确模拟了类层次结构的所有适当级别。特别是,必须模拟被测试代码引用的HttpContext的所有属性。

请参阅" 测试自定义AuthorizationAttribute时引发的NullReferenceException "例子。

A14:

Another general case where one might receive this exception involves mocking classes during unit testing. Regardless of the mocking framework being used, you must ensure that all appropriate levels of the class hierarchy are properly mocked. In particular, all properties of HttpContext which are referenced by the code under test must be mocked.

See "NullReferenceException thrown when testing custom AuthorizationAttribute" for a somewhat verbose example.

回答15:

我对此有不同的看法。这类回答"我还能采取什么措施避免它?"

例如在MVC应用程序中跨不同层工作时,控制器需要服务来调用业务操作。在这种情况下,依赖注入容器可用于初始化服务以避免 NullReferenceException 。因此,这意味着您无需担心检查null的情况,只需从控制器调用服务,就好像它们将始终作为单例或原型可用(并初始化)一样。

public class MyController
{
    private ServiceA serviceA;
    private ServiceB serviceB;

    public MyController(ServiceA serviceA, ServiceB serviceB)
    {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
    }

    public void MyMethod()
    {
        // We don't need to check null because the dependency injection container 
        // injects it, provided you took care of bootstrapping it.
        var someObject = serviceA.DoThis();
    }
}

A15:

I have a different perspective to answering this. This sort of answers "what else can I do to avoid it?"

When working across different layers, for example in an MVC application, a controller needs services to call business operations. In such scenarios Dependency Injection Container can be used to initialize the services to avoid the NullReferenceException. So that means you don't need to worry about checking for null and just call the services from the controller as though they will always to available (and initialized) as either a singleton or a prototype.

public class MyController
{
    private ServiceA serviceA;
    private ServiceB serviceB;

    public MyController(ServiceA serviceA, ServiceB serviceB)
    {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
    }

    public void MyMethod()
    {
        // We don't need to check null because the dependency injection container 
        // injects it, provided you took care of bootstrapping it.
        var someObject = serviceA.DoThis();
    }
}

回答16:

关于"我该怎么办" ,可以有很多答案。

在开发过程中防止此类错误情况的一种"正式"方法正在应用 按合同设计 。这意味着在开发时,您需要在系统上设置类不变量,甚至函数/方法前提条件后置条件

简而言之,类不变式确保您的类中存在一些在正常使用中不会违反的约束(因此,该类将不会进入状态不一致)。 前提条件表示作为函数/方法的输入提供的数据必须遵循某些约束集,并且从不违反它们,而后置条件表示函数/方法方法输出必须再次遵循设置的约束,而不能违反它们。在执行无错误程序期间,不得违反合同条件,因此,在调试模式下会在实践中检查按合同进行的设计,而在发布中将其禁用,以最大限度地提高开发出的系统性能。

这样,您可以避免由于违反约束集而导致的NullReferenceException情况。例如,如果您在类中使用对象属性X,然后尝试调用其方法之一,并且X具有空值,则这将导致 NullReferenceException

public X { get; set; }

public void InvokeX()
{
    X.DoSomething(); // if X value is null, you will get a NullReferenceException
}

但是,如果将"属性X绝不能具有空值"设置为方法前提,则可以防止上述情况:

//Using code contracts:
[ContractInvariantMethod]
protected void ObjectInvariant () 
{
    Contract.Invariant ( X != null );
    //...
}

为此,请 代码存在用于.NET应用程序的合同 项目。

或者,可以使用 断言应用按合同进行设计 < / em>。

更新:值得一提的是,该术语是由Bertrand Meyer创造的与他设计的Eiffel编程语言有关

A16:

On the matter of "what should I do about it", there can be many answers.

A more "formal" way of preventing such error conditions while developing is applying design by contract in your code. This means you need to set class invariants, and/or even function/method preconditions and postconditions on your system, while developing.

In short, class invariants ensure that there will be some constraints in your class that will not get violated in normal use (and therefore, the class will not get in an inconsistent state). Preconditions mean that data given as input to a function/method must follow some constraints set and never violate them, and postconditions mean that a function/method output must follow the set constraints again without ever violating them. Contract conditions should never be violated during execution of a bug-free program, therefore design by contract is checked in practice in debug mode, while being disabled in releases, to maximize the developed system performance.

This way, you can avoid NullReferenceException cases that are results of violation of the constraints set. For example, if you use an object property X in a class and later try to invoke one of its methods and X has a null value, then this will lead to NullReferenceException:

public X { get; set; }

public void InvokeX()
{
    X.DoSomething(); // if X value is null, you will get a NullReferenceException
}

But if you set "property X must never have a null value" as method precondition, then you can prevent the scenario described before:

//Using code contracts:
[ContractInvariantMethod]
protected void ObjectInvariant () 
{
    Contract.Invariant ( X != null );
    //...
}

For this cause, Code Contracts project exists for .NET applications.

Alternatively, design by contract can be applied using assertions.

UPDATE: It is worth mentioning that the term was coined by Bertrand Meyer in connection with his design of the Eiffel programming language.

回答17:

当我们尝试访问空对象的属性或当字符串值变为空并且试图访问字符串方法时,会抛出NullReferenceException

例如:

  1. 访问空字符串的字符串方法时:

    string str = string.Empty;
    str.ToLower(); // throw null reference exception
    
  2. 访问空对象的属性时:

    Public Class Person {
        public string Name { get; set; }
    }
    Person objPerson;
    objPerson.Name  /// throw Null refernce Exception 
    

A17:

A NullReferenceException is thrown when we are trying to access Properties of a null object or when a string value becomes empty and we are trying to access string methods.

For example:

  1. When a string method of an empty string accessed:

    string str = string.Empty;
    str.ToLower(); // throw null reference exception
    
  2. When a property of a null object accessed:

    Public Class Person {
        public string Name { get; set; }
    }
    Person objPerson;
    objPerson.Name  /// throw Null refernce Exception 
    

回答18:

TL; DR:尝试使用Html.Partial而不是Renderpage


当我尝试通过向其发送模型来在视图中呈现视图时,得到的对象引用未设置为对象的实例

@{
    MyEntity M = new MyEntity();
}
@RenderPage("_MyOtherView.cshtml", M); // error in _MyOtherView, the Model was Null

调试显示MyOtherView中的模型为Null。直到我将其更改为:

@{
    MyEntity M = new MyEntity();
}
@Html.Partial("_MyOtherView.cshtml", M);

它奏效了。

此外,我之初没有Html.Partial的原因是因为Visual Studio 有时会在Html.Partial下抛出看似扭曲的行.(即使它不是真正的错误),如果它位于不同构造的foreach循环中:

@inherits System.Web.Mvc.WebViewPage
@{
    ViewBag.Title = "Entity Index";
    List<MyEntity> MyEntities = new List<MyEntity>();
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
}
<div>
    @{
        foreach(var M in MyEntities)
        {
            // Squiggly lines below. Hovering says: cannot convert method group 'partial' to non-delegate type Object, did you intend to envoke the Method?
            @Html.Partial("MyOtherView.cshtml");
        }
    }
</div>

但是我能够在没有任何"错误"问题的情况下运行该应用程序。通过更改foreach循环的结构使其看起来像这样,我可以消除错误:

@foreach(var M in MyEntities){
    ...
}

尽管我有一种感觉,这是因为Visual Studio误读了与号和括号。

A18:

TL;DR: Try using Html.Partial instead of Renderpage


I was getting Object reference not set to an instance of an object when I tried to render a View within a View by sending it a Model, like this:

@{
    MyEntity M = new MyEntity();
}
@RenderPage("_MyOtherView.cshtml", M); // error in _MyOtherView, the Model was Null

Debugging showed the model was Null inside MyOtherView. Until I changed it to:

@{
    MyEntity M = new MyEntity();
}
@Html.Partial("_MyOtherView.cshtml", M);

And it worked.

Furthermore, the reason I didn't have Html.Partial to begin with was because Visual Studio sometimes throws error-looking squiggly lines under Html.Partial if it's inside a differently constructed foreach loop, even though it's not really an error:

@inherits System.Web.Mvc.WebViewPage
@{
    ViewBag.Title = "Entity Index";
    List<MyEntity> MyEntities = new List<MyEntity>();
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
}
<div>
    @{
        foreach(var M in MyEntities)
        {
            // Squiggly lines below. Hovering says: cannot convert method group 'partial' to non-delegate type Object, did you intend to envoke the Method?
            @Html.Partial("MyOtherView.cshtml");
        }
    }
</div>

But I was able to run the application with no problems with this "error". I was able to get rid of the error by changing the structure of the foreach loop to look like this:

@foreach(var M in MyEntities){
    ...
}

Although I have a feeling it was because Visual Studio was misreading the ampersands and brackets.

回答19:

您能做什么?

这里有很多很好的答案,解释什么是空引用以及如何调试它。但是,如何预防该问题或至少使它更容易发现,却很少。

检查参数

例如,方法可以检查不同的参数以查看它们是否为空,并抛出ArgumentNullException,这显然是为此目的而创建的异常。

ArgumentNullException的构造函数甚至将参数名称和一条消息作为参数,以便您可以准确地告诉开发者问题出在哪里。

public void DoSomething(MyObject obj) {
    if(obj == null) 
    {
        throw new ArgumentNullException("obj", "Need a reference to obj.");
    }
}

使用工具

也有一些库可以提供帮助。例如," Resharper"可以在编写代码时向您提供警告,尤其是在使用它们的属性时: NotNullAttribute

在" Microsoft Code Contracts"中,您可以使用像Contract.Requires(obj!=null)这样的语法,它可以为您提供运行时和编译检查:代码合同简介

还有一个" PostSharp",可让您只使用如下属性:

public void DoSometing([NotNull] obj)

这样做,将使PostSharp成为构建过程obj的一部分,将在运行时检查是否为null。请参阅: PostSharp空检查

普通代码解决方案

或者您始终可以使用简单的旧代码编写自己的方法。例如,这里是一个可用于捕获空引用的结构。它以与Nullable

相同的概念建模
[System.Diagnostics.DebuggerNonUserCode]
public struct NotNull<T> where T: class
{
    private T _value;

    public T Value
    {
        get
        {
            if (_value == null)
            {
                throw new Exception("null value not allowed");
            }

            return _value;
        }
        set
        {
            if (value == null)
            {
                throw new Exception("null value not allowed.");
            }

            _value = value;
        }
    }

    public static implicit operator T(NotNull<T> notNullValue)
    {
        return notNullValue.Value;
    }

    public static implicit operator NotNull<T>(T value)
    {
        return new NotNull<T> { Value = value };
    }
}

您将使用与使用Nullable 相同的方法,但其目的完全相反-不允许使用null。以下是一些示例:

NotNull<Person> person = null; // throws exception
NotNull<Person> person = new Person(); // OK
NotNull<Person> person = GetPerson(); // throws exception if GetPerson() returns null

NotNull 隐式地与T进行相互转换,因此您可以在任何需要的地方使用它。例如,您可以将Person对象传递给采用NotNull

的方法。
Person person = new Person { Name = "John" };
WriteName(person);

public static void WriteName(NotNull<Person> person)
{
    Console.WriteLine(person.Value.Name);
}

如上所示,您可以通过Value属性访问基础值。另外,您可以使用显式或隐式强制转换,可以在下面看到带有返回值的示例:

Person person = GetPerson();

public static NotNull<Person> GetPerson()
{
    return new Person { Name = "John" };
}

或者当该方法仅通过执行强制转换返回T(在本例中为Person)时,甚至可以使用它。例如,以下代码与上面的代码一样:

Person person = (NotNull<Person>)GetPerson();

public static Person GetPerson()
{
    return new Person { Name = "John" };
}

与扩展程序合并

使用扩展方法组合NotNull ,您甚至可以涵盖更多情况。这是扩展方法的示例:

[System.Diagnostics.DebuggerNonUserCode]
public static class NotNullExtension
{
    public static T NotNull<T>(this T @this) where T: class
    {
        if (@this == null)
        {
            throw new Exception("null value not allowed");
        }

        return @this;
    }
}

这是一个如何使用它的示例:

var person = GetPerson().NotNull();

GitHub

作为参考,我在GitHub上提供了上面的代码,您可以在以下位置找到它:

https://github.com/luisperezphd/NotNull

相关语言功能

C#6.0引入了"空条件运算符",它对此有所帮助。使用此功能,您可以引用嵌套对象,如果其中任何一个为null,则整个表达式将返回null

这减少了在某些情况下必须执行的空检查次数。语法是在每个点之前加一个问号。以下面的代码为例:

var address = country?.State?.County?.City;

想象一下,countryCountry类型的对象,它具有名为State的属性,依此类推。如果countryStateCountyCitynull,则地址将为.因此,您只需要检查地址是否为为空。

这是一个很棒的功能,但它给您的信息较少。并不清楚这4个中的哪个为空。

像Nullable一样内置吗?

C#是Nullable 的一个很好的简写,您可以通过在诸如int?这样的类型后加一个问号来使某些东西为可空的。

如果C#具有上面的NotNull 结构之类的东西并且具有类似的简写形式(也许是感叹号(!)),那么这很好,这样您就可以编写如下内容:public void WriteName(Person!person)

A19:

What can you do about it?

There is a lot of good answers here explaining what a null reference is and how to debug it. But there is very little on how to prevent the issue or at least make it easier to catch.

Check arguments

For example, methods can check the different arguments to see if they are null and throw an ArgumentNullException, an exception obviously created for this exact purpose.

The constructor for the ArgumentNullException even takes the name of the parameter and a message as arguments so you can tell the developer exactly what the problem is.

public void DoSomething(MyObject obj) {
    if(obj == null) 
    {
        throw new ArgumentNullException("obj", "Need a reference to obj.");
    }
}

Use Tools

There are also several libraries that can help. "Resharper" for example can provide you with warnings while you are writing code, especially if you use their attribute: NotNullAttribute

There's "Microsoft Code Contracts" where you use syntax like Contract.Requires(obj != null) which gives you runtime and compile checking: Introducing Code Contracts.

There's also "PostSharp" which will allow you to just use attributes like this:

public void DoSometing([NotNull] obj)

By doing that and making PostSharp part of your build process obj will be checked for null at runtime. See: PostSharp null check

Plain Code Solution

Or you can always code your own approach using plain old code. For example here is a struct that you can use to catch null references. It's modeled after the same concept as Nullable<T>:

[System.Diagnostics.DebuggerNonUserCode]
public struct NotNull<T> where T: class
{
    private T _value;

    public T Value
    {
        get
        {
            if (_value == null)
            {
                throw new Exception("null value not allowed");
            }

            return _value;
        }
        set
        {
            if (value == null)
            {
                throw new Exception("null value not allowed.");
            }

            _value = value;
        }
    }

    public static implicit operator T(NotNull<T> notNullValue)
    {
        return notNullValue.Value;
    }

    public static implicit operator NotNull<T>(T value)
    {
        return new NotNull<T> { Value = value };
    }
}

You would use very similar to the same way you would use Nullable<T>, except with the goal of accomplishing exactly the opposite - to not allow null. Here are some examples:

NotNull<Person> person = null; // throws exception
NotNull<Person> person = new Person(); // OK
NotNull<Person> person = GetPerson(); // throws exception if GetPerson() returns null

NotNull<T> is implicitly cast to and from T so you can use it just about anywhere you need it. For example, you can pass a Person object to a method that takes a NotNull<Person>:

Person person = new Person { Name = "John" };
WriteName(person);

public static void WriteName(NotNull<Person> person)
{
    Console.WriteLine(person.Value.Name);
}

As you can see above as with nullable you would access the underlying value through the Value property. Alternatively, you can use an explicit or implicit cast, you can see an example with the return value below:

Person person = GetPerson();

public static NotNull<Person> GetPerson()
{
    return new Person { Name = "John" };
}

Or you can even use it when the method just returns T (in this case Person) by doing a cast. For example, the following code would just like the code above:

Person person = (NotNull<Person>)GetPerson();

public static Person GetPerson()
{
    return new Person { Name = "John" };
}

Combine with Extension

Combine NotNull<T> with an extension method and you can cover even more situations. Here is an example of what the extension method can look like:

[System.Diagnostics.DebuggerNonUserCode]
public static class NotNullExtension
{
    public static T NotNull<T>(this T @this) where T: class
    {
        if (@this == null)
        {
            throw new Exception("null value not allowed");
        }

        return @this;
    }
}

And here is an example of how it could be used:

var person = GetPerson().NotNull();

GitHub

For your reference I made the code above available on GitHub, you can find it at:

https://github.com/luisperezphd/NotNull

Related Language Feature

C# 6.0 introduced the "null-conditional operator" that helps with this a little. With this feature, you can reference nested objects and if any one of them is null the whole expression returns null.

This reduces the number of null checks you have to do in some cases. The syntax is to put a question mark before each dot. Take the following code for example:

var address = country?.State?.County?.City;

Imagine that country is an object of type Country that has a property called State and so on. If country, State, County, or City is null then address will benull. Therefore you only have to check whetheraddressisnull`.

It's a great feature, but it gives you less information. It doesn't make it obvious which of the 4 is null.

Built-in like Nullable?

C# has a nice shorthand for Nullable<T>, you can make something nullable by putting a question mark after the type like so int?.

It would be nice if C# had something like the NotNull<T> struct above and had a similar shorthand, maybe the exclamation point (!) so that you could write something like: public void WriteName(Person! person).

回答20:

有趣的是,此页面上的答案均未提及这两种情况,希望我添加它们时不会有人介意:

边缘情况#1:同时访问字典

.NET中的通用词典不是线程安全的,有时 可能会抛出NullReference或什至(更常见)抛出KeyNotFoundException您尝试从两个并发线程访问密钥。在这种情况下,例外情况非常容易引起误解。

边缘案例2:不安全的代码

如果unsafe代码引发了NullReferenceException,则可能会查看指针变量,并检查它们是否为IntPtr.Zero或其他内容。 。这是同一回事("空指针异常"),但是在不安全的代码中,变量通常被转换为值类型/数组等,然后您将头撞墙,想知道值类型如何抛出该值例外。

(顺便说一句,除非您需要,否则不使用不安全代码的另一个原因)

A20:

Interestingly, none of the answers on this page mention the two edge cases, hope no one minds if I add them:

Edge case #1: concurrent access to a Dictionary

Generic dictionaries in .NET are not thread-safe and they sometimes might throw a NullReference or even (more frequent) a KeyNotFoundException when you try to access a key from two concurrent threads. The exception is quite misleading in this case.

Edge case #2: unsafe code

If a NullReferenceException is thrown by unsafe code, you might look at your pointer variables, and check them for IntPtr.Zero or something. Which is the same thing ("null pointer exception"), but in unsafe code, variables are often cast to value-types/arrays, etc., and you bang your head against the wall, wondering how a value-type can throw this exception.

(Another reason for non-using unsafe code unless you need it, by the way)

回答21:

您可以使用c#6中的空条件运算符以干净的方式修复NullReferenceException,并编写更少的代码来处理空检查。

它用于在执行成员访问(?。)或索引(?[)操作之前测试null。

示例

  var name = p?.Spouse?.FirstName;

等效于:

    if (p != null)
    {
        if (p.Spouse != null)
        {
            name = p.Spouse.FirstName;
        }
    }

结果是,当p为null或p.Spouse为null时,名称将为null。

否则,将为变量名分配p.Spouse.FirstName的值。

有关更多详细信息:空-条件运算符

A21:

You can fix NullReferenceException in a clean way using Null-conditional Operators in c#6 and write less code to handle null checks.

It's used to test for null before performing a member access (?.) or index (?[) operation.

Example

  var name = p?.Spouse?.FirstName;

is equivalent to:

    if (p != null)
    {
        if (p.Spouse != null)
        {
            name = p.Spouse.FirstName;
        }
    }

The result is that the name will be null when p is null or when p.Spouse is null.

Otherwise, the variable name will be assigned the value of the p.Spouse.FirstName.

For More details : Null-conditional Operators

回答22:

错误行"对象引用未设置为对象的实例。"表明您尚未将实例对象分配给对象引用,而您仍在访问该对象的属性/方法。

例如:假设您有一个名为myClass的类,它包含一个属性prop1。

public Class myClass
{
   public int prop1 {get;set;}
}

现在,您可以像下面这样在其他一些类中访问此prop1:

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref.prop1 = 1;  //This line throws error
     }
}

上面的行引发错误,因为声明了类myClass的引用但未实例化,或者未将对象实例分配给该类的Referecne。

要解决此问题,您必须实例化(将对象分配给该类的引用)。

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref = new myClass();
        ref.prop1 = 1;  
     }
}

A22:

The error line "Object reference not set to an instance of an object. " states that you have not assigned instance object to a object reference and still you are accessing properies/methods of that object.

for example: let say you have a class called myClass and it contains one property prop1.

public Class myClass
{
   public int prop1 {get;set;}
}

Now you are accessing this prop1 in some other class just like below:

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref.prop1 = 1;  //This line throws error
     }
}

above line throws error because reference of class myClass is declared but not instantiated or an instance of object is not assigned to referecne of that class.

To fix this you have to instantiate (assign object to reference of that class).

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref = new myClass();
        ref.prop1 = 1;  
     }
}

回答23:

当未实例化要使用的类的对象时,会发生

NullReferenceException或未设置为对象实例的对象引用。例如:

假设您有一个名为Student的班级。

public class Student
{
    private string FirstName;
    private string LastName;
    public string GetFullName()
    {
        return FirstName + LastName;
    }
}

现在,考虑尝试在另一个班级检索学生的全名。

public class StudentInfo
{      
    public string GetStudentName()
    {
        Student s;
        string fullname = s.GetFullName();
        return fullname;
    }        
}

如上面的代码所示,语句 Student s -仅声明类型为Student的变量,请注意,此时尚未实例化Student类。因此,当语句 s.GetFullName()被执行时,它将抛出NullReferenceException。

A23:

NullReferenceException or Object reference not set to an instance of an object occurs when an object of the class you are trying to use is not instantiated. For example:

Assume that you have a class named Student.

public class Student
{
    private string FirstName;
    private string LastName;
    public string GetFullName()
    {
        return FirstName + LastName;
    }
}

Now, consider another class where you are trying to retrieve the student's full name.

public class StudentInfo
{      
    public string GetStudentName()
    {
        Student s;
        string fullname = s.GetFullName();
        return fullname;
    }        
}

As seen in the above code, the statement Student s - only declares the variable of type Student, note that the Student class is not instantiated at this point. Hence, when the statement s.GetFullName() gets executed, it will throw the NullReferenceException.

回答24:

简单地说:

您正在尝试访问未创建或当前不在内存中的对象。

那么如何解决这个问题:

  1. 调试并让调试器中断...它将直接带您到已损坏的变量...现在,您的任务就是简单地解决此问题。.使用 new 关键字在适当的位置。

  2. 如果是由于某些数据库命令导致的,因为该对象不存在,那么您需要做的就是进行空检查并处理它:

    if (i == null) {
        // Handle this
    }
    
  3. 最困难的一个..如果 GC 已经收集了对象...如果您尝试使用字符串查找对象,通常会发生这种情况。也就是说,按对象名称查找它,则可能发生了GC可能已经清理了它的情况。 ..这很难找到,并且将成为一个大问题...解决此问题的更好方法是在开发过程中的必要位置进行空检查。这样可以节省很多时间。

通过名称查找,我的意思是某个框架允许您使用字符串来查找对象,并且代码可能如下所示: FindObject(" ObjectName");

A24:

Well, in simple terms:

You are trying to access an object that isn't created or currently not in memory.

So how to tackle this:

  1. Debug and let the debugger break... It will directly take you to the variable that is broken... Now your task is to simply fix this.. Using the new keyword in the appropriate place.

  2. If it is caused on some database commands because the object isn't present then all you need to do is do a null check and handle it:

    if (i == null) {
        // Handle this
    }
    
  3. The hardest one .. if the GC collected the object already... This generally occurs if you are trying to find an object using strings... That is, finding it by name of the object then it may happen that the GC might already cleaned it up... This is hard to find and will become quite a problem... A better way to tackle this is do null checks wherever necessary during the development process. This will save you a lot of time.

By finding by name I mean some framework allow you to FIndObjects using strings and the code might look like this: FindObject("ObjectName");

回答25:

从字面上看,修复NullReferenceExeption的最简单方法有两种。例如,如果您有一个GameObject(带有附加的脚本)和一个名为rb(rigidbody)的变量,则在您启动游戏时该变量将以null开头。这就是为什么会得到NullReferenceExeption的原因,因为计算机中没有存储数据变量。

我将使用RigidBody变量作为示例。
实际上,我们可以通过以下几种方式非常轻松地添加数据:

  1. 使用AddComponent> Physics> Rigidbody
    将RigidBody添加到对象中,然后进入脚本并键入rb=GetComponent ();
    这行代码最有效在Start()Awake()函数下。
  2. 您可以通过编程添加组件,并使用一行代码同时分配变量:rb=AddComponent ();

更多说明:如果您希望统一为您的对象添加一个组件,而您可能忘记了添加一个组件,则可以在类声明上方键入[RequireComponent(typeof(RigidBody))](您使用的所有空间下面的空间。)
享受游戏乐趣!

A25:

Literally the easiest way to fix a NullReferenceExeption has two ways. If you have a GameObject for example with a script attached and a variable named rb (rigidbody) this variable will start null when you start your game.
This is why you get a NullReferenceExeption because the computer does not have data stored in that variable.

I'll be using a RigidBody variable as an example.
We can add data really easily actually in a few ways:

  1. Add a RigidBody to your object with AddComponent > Physics > Rigidbody
    Then go into your script and type rb = GetComponent<Rigidbody>();
    This line of code works best under your Start() or Awake() functions.
  2. You can add a component programmatically and assign the variable at the same time with one line of code: rb = AddComponent<RigidBody>();

Further Notes: If you want unity to add a component to your object and you might have forgotten to add one, you can type [RequireComponent(typeof(RigidBody))] above your class declaration (the space below all of your usings).
Enjoy and have fun making games!

回答26:

如果我们考虑可能引发此异常的常见情况,请在顶部使用对象访问属性。

例如:

string postalcode=Customer.Address.PostalCode; 
//if customer or address is null , this will through exeption

在这里,如果address为null,那么您将获得NullReferenceException。

因此,作为一种做法,在访问此类对象(特别是通用对象)中的属性之前,我们应始终使用null检查

string postalcode=Customer?.Address?.PostalCode;
//if customer or address is null , this will return null, without through a exception

A26:

If we consider common scenarios where this exception can be thrown, accessing properties withing object at the top.

Ex:

string postalcode=Customer.Address.PostalCode; 
//if customer or address is null , this will through exeption

in here , if address is null , then you will get NullReferenceException.

So, as a practice we should always use null check, before accessing properties in such objects (specially in generic)

string postalcode=Customer?.Address?.PostalCode;
//if customer or address is null , this will return null, without through a exception

回答27:

如果在保存或编译内部版本时收到此消息,请关闭所有文件,然后打开任何文件进行编译和保存。

对我来说,原因是我已重命名该文件,而旧文件仍处于打开状态。

A27:

If one is getting this message during saving or compiling the build, just close all the files and then open any file to compile and save.

For me the reason was that I had rename the file and old file was still open.

回答28:

这基本上是一个Null引用异常。如 Microsoft 所述-< / p>

当您尝试访问其值为null的类型的成员时,将引发NullReferenceException异常。

那是什么意思?

这意味着如果某个成员没有任何价值,而我们正在使该成员执行某些任务,那么系统无疑会抛弃一条消息并说出-

"等等,该成员没有值,因此它无法执行您要移交的任务。"

异常本身表明正在引用某些内容,但未设置其值。因此,这表明它仅在使用引用类型时发生,因为值类型不可为空。

如果我们使用值类型成员,则不会发生NullReferenceException。

class Program
{
    static void Main(string[] args)
    {
        string str = null;
        Console.WriteLine(str.Length);
        Console.ReadLine();
    }
}

上面的代码显示了一个简单的字符串,该字符串分配了一个 null 值。

现在,当我尝试打印字符串 str 的长度时,我确实得到了发生'System.NullReferenceException'类型的未处理异常消息,因为成员 str 指向null,并且不能有任何长度的null。

当我们忘记实例化引用类型时,也会出现

" NullReferenceException "。

假设我有一个类和成员方法。我没有实例化我的班级,而只是命名我的班级。现在,如果我尝试使用该方法,编译器将抛出错误或发出警告(取决于编译器)。

class Program
{
    static void Main(string[] args)
    {
        MyClass1 obj;
        obj.foo();  //Use of unassigned local variable 'obj'
    }
}

public class MyClass1
{
    internal void foo()
    {
        Console.WriteLine("hello from foo");

    }
}
上面代码的

编译器会引发一个错误,指出未分配变量 obj ,这表明我们的变量具有空值或什么都没有。上述代码的编译器会产生一个错误,指出未分配变量 obj ,这表明我们的变量具有空值或什么都没有。

为什么会发生?

  • NullReferenceException是由于我们没有检查对象的值而导致的错误。我们经常在代码开发中不检查对象值。

  • 当我们忘记实例化对象时,它也会出现。使用可以返回或设置空值的方法,属性,集合等也可能是导致此异常的原因。

如何避免?

有多种方法和方法可以避免这种著名的异常:

  1. 显式检查:我们应遵循检查对象,属性,方法,数组和集合是否为空的传统。可以使用if-else if-else等条件语句简单地实现。

  2. 异常处理:管理此异常的重要方法之一。使用简单的try-catch-finally块,我们可以控制此异常并对其进行维护。当您的应用程序处于生产阶段时,这将非常有用。

  3. 空运算符:在将值设置为对象,变量,属性和字段时,也可以方便地使用空合并运算符和空条件运算符。

  4. 调试器:对于开发人员而言,我们拥有与我们一起进行调试的强大武器。如果在开发过程中遇到NullReferenceException,则可以使用调试器来获取异常源。

  5. 内置方法:诸如GetValueOrDefault(),IsNullOrWhiteSpace()和IsNullorEmpty()之类的系统方法将检查null并在存在null值时分配默认值。

这里已经有很多好的答案。您也可以在我的博客

希望这也有帮助!

A28:

This is basically is a Null reference exception. As Microsoft states-

A NullReferenceException exception is thrown when you try to access a member of a type whose value is null.

What does that mean?

That means if any member which doesn’t hold any value and we are making that member to perform certain task then the system will undoubtedly toss a message and say-

“Hey wait, that member has no values so it can’t perform the task which you are handing it over.”

The exception itself says that something is being referred but whose value is not being set. So this denotes that it only occurs while using reference types as Value types are non-nullable.

NullReferenceException won't occur if we are using Value type members.

class Program
{
    static void Main(string[] args)
    {
        string str = null;
        Console.WriteLine(str.Length);
        Console.ReadLine();
    }
}

The above code shows simple string which is assigned with a null value.

Now, when I try to print the length of the string str, I do get An unhandled exception of type ‘System.NullReferenceException’ occurred message because member str is pointing to null and there can’t be any length of null.

NullReferenceException’ also occurs when we forget to instantiate a reference type.

Suppose I have a class and member method in it. I have not instantiated my class but only named my class. Now if I try to use the method, the compiler will throw an error or issue a warning (depending on the compiler).

class Program
{
    static void Main(string[] args)
    {
        MyClass1 obj;
        obj.foo();  //Use of unassigned local variable 'obj'
    }
}

public class MyClass1
{
    internal void foo()
    {
        Console.WriteLine("hello from foo");

    }
}

Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing. Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing.

Why it occurs?

  • NullReferenceException arises due to our fault for not checking the object’s value. We often leave the object values unchecked in the code development.

  • It also arises when we forget to instantiate our objects. Using methods, properties, collections etc. which can return or set null values can also be the cause of this exception.

How can it be avoided?

There are various ways and methods to avoid this renowned exception:

  1. Explicit Checking: We should adhere to the tradition of checking the objects, properties, methods, arrays, and collections whether they are null. This can be simply implemented using conditional statements like if-else if-else etc.

  2. Exception handling: One of the important ways of managing this exception. Using simple try-catch-finally blocks we can control this exception and also maintain a log of it. This can be very useful when your application is on production stage.

  3. Null operators: Null Coalescing operator and null conditional operators can also be used in handy while setting values to objects, variables, properties and fields.

  4. Debugger: For developers, we have the big weapon of Debugging with us. If have we face NullReferenceException during the development face we can use the debugger to get to the source of the exception.

  5. In-built method: System methods such as GetValueOrDefault(),IsNullOrWhiteSpace() and IsNullorEmpty() checks for nulls and assign the default value if there is a null value.

There are many good answers already here. You can also check more detailed description with examples on my blog.

Hope this helps too!

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