null（C# 参考）
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 可以为 null 的类型（C# 编程指南）。
下面的示例演示 null 关键字的一些行为：
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。该语言规范是 C# 语法和用法的权威资料。