Nullable<T> 内部实现了显示和隐式转换

显示转换:

public static explicit operator T(T? value)    {      return value.Value;    }

Nullable<int> nullable = new Nullable<int>(10);

int x = (int)nullable;

 

隐式转换:

public static implicit operator T?(T value)    {      return new T?(value);    }

Nullable<int> i = 100;

装箱拆箱:

 

Nullable
     
       nullable = 100;            object boxed = nullable;//有值的可空类型装箱            Console.WriteLine(boxed.GetType());//System.Int32            int normal = (int)boxed;//有值的可空类型拆箱成非可空类型            Console.WriteLine(normal);            nullable = (Nullable
      
       )boxed;//有值的可空类型拆箱成可空类型            Console.WriteLine(nullable);            nullable = new Nullable
       
        ();            boxed = nullable;//没有值的可空类型装箱            Console.WriteLine(boxed == null);//True            nullable = (Nullable
        
         )boxed;//没有值的可空类型拆箱成可空类型            Console.WriteLine(nullable.HasValue);//False