更新:2007 年 11 月

null 关键字是表示不引用任何对象的 null 引用的文字值。null 是引用类型变量的默认值。普通值类型不能为 null。但是,C# 2.0 引入了可以为 null 值的类型。请参见可空类型(C# 编程指南)

下面的示例演示 null 关键字的一些行为:

C# 复制代码
class Program
{
    class MyClass
    {
        public void MyMethod() { }
    }

    static void Main(string[] args)
    {
        // Set a breakpoint here to see that mc = null.
        // However, the compiler considers it "unassigned."
        // and generates a compiler error if you try to
        // use the variable.
        MyClass mc;

        // Now the variable can be used, but...
        mc = null;

        // ... a method call on a null object raises 
        // a run-time NullReferenceException.
        // Uncomment the following line to see for yourself.
        // mc.MyMethod();

        // Now mc has a value.
        mc = new MyClass();

        // You can call its method.
        mc.MyMethod();

        // Set mc to null again. The object it referenced
        // is no longer accsessible and can now be garbage-collected.
        mc = null;

        // A null string is not the same as an empty string.
        string s = null;
        string t = String.Empty; // Logically the same as ""

        // Equals applied to any null object returns false.
        bool b = (t.Equals(s));
        Console.WriteLine(b);

        // Equality operator also returns false when one
        // operand is null.
        Console.WriteLine("Empty string {0} null string", s == t ? "equals": "does not equal");

        // Returns true.
        Console.WriteLine("null == null is {0}", null == null);


        // A value type cannot be null
        // int i = null; // Compiler error!

        // Use a nullable value type instead:
        int? i = null;

        // Keep the console window open in debug mode.
        System.Console.WriteLine("Press any key to exit.");
        System.Console.ReadKey();

    }
}

C# 语言规范

有关更多信息,请参见 C# 语言规范中的以下各章节:

  • 2.4.4.6 null 文本

请参见

修订记录

日期

修订

原因

2008 年 7 月

添加了代码示例。

内容 Bug 修复