null（C# 参考）

项目
09/22/2022

null 关键字是表示不引用任何对象的空引用的文字值。 null 是引用类型变量的默认值。普通值类型不能为 NULL，可为空的值类型除外。

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 accessible 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();
    }
}

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