可空T实作 [英] Nullable<T> implementation

问题描述

我正在尝试实现Nullable类型。但是下面提到的代码不支持valuetype数据类型的空值。

I am trying to implement Nullable type. But below mentioned code doesn't support null value for valuetype datatypes.

using System;
using System.Runtime;
using System.Runtime.InteropServices;

namespace Nullable
{

    [Serializable, StructLayout(LayoutKind.Sequential)]    
    public struct Nullable<T> where T : struct
    {
        private bool hasValue;

        public bool HasValue
        {
            get { return hasValue; }
        }

        internal T value;

        public Nullable(T value)
        {
            this.value = value;
            this.hasValue = true;
        }

        public T Value
        {
            get
            {
                if (!this.hasValue)
                {
                    new InvalidOperationException("No value assigned");
                }
                return this.value;
            }
        }

        public T GetValueOrDefault()
        {
            return this.value;
        }

        public T GetValueOrDefault(T defaultValue)
        {
            if (!this.HasValue)
            {
                return defaultValue;
            }
            return this.value;

        }

        public override bool Equals(object obj)
        {
            if (!this.HasValue)
            {
                return obj == null;
            }
            if (obj == null)
            {
                return false;
            }
            return this.value.Equals(obj);
        }

        public override int GetHashCode()
        {
            if (!this.hasValue)
            {
                return 0;
            }
            return this.value.GetHashCode();
        }

        public override string ToString()
        {
            if (!this.hasValue)
            {
                return string.Empty;
            }
            return this.value.ToString();
        }

        public static implicit operator Nullable<T>(T value)
        {
            return new Nullable<T>(value);
        }

        public static explicit operator T(Nullable<T> value)
        {
            return value.Value;
        }
    }
}

当我尝试分配值作为null，它会引发错误无法将null转换为'Nullable.Nullable'，因为它是非空值类型

When I trying to assign the value as null, it's throwing an error "Cannot convert null to 'Nullable.Nullable' because it is a non-nullable value type"

我必须解决的问题

推荐答案

将 null 分配给 Nullable< T> 只是分配 new Nullable< T>（）的语法糖，它是C＃语言的一部分，您不能

Assigning null to Nullable<T> is just a syntactic sugar for assigning new Nullable<T>(), it's part of C# language, and you can't add that feature to you custom type.

C＃规范，

> 4.1.10可空类型

可空类型可以表示其基础类型的所有值加上
的附加空值。可空类型写为T ?，其中T是基础类型
。此语法是System.Nullable，
的简写，并且两种形式可以互换使用。

A nullable type can represent all values of its underlying type plus an additional null value. A nullable type is written T?, where T is the underlying type. This syntax is shorthand for System.Nullable, and the two forms can be used interchangeably.

6.1.5空文字转换

从空文字到任何可空$的隐式转换存在b $ b类型。此转换将产生给定
可为空类型的空值（第4.1.1节）。

An implicit conversion exists from the null literal to any nullable type. This conversion produces the null value (§4.1.10) of the given nullable type.

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