可空类型 Nullable<T>

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<int> 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<int>)boxed;//有值的可空类型拆箱成可空类型
            Console.WriteLine(nullable);

            nullable = new Nullable<int>();
            boxed = nullable;//没有值的可空类型装箱
            Console.WriteLine(boxed == null);//True

            nullable = (Nullable<int>)boxed;//没有值的可空类型拆箱成可空类型
            Console.WriteLine(nullable.HasValue);//False