如何为空< T>从类似的自定义C＃结构有什么不同？ [英] How is Nullable<T> different from a similar custom C# struct?

问题描述

在可空微的优化，第一部分，埃里克提到，可空< T> 拥有的奇怪的拳击行为的不能由一个类似的用户定义类型来实现。

有什么特殊功能，在C＃语言的赠款，以预定义的可空< T> 键入？尤其是无法作出一个 MyNullable 键入

当然，工作的人，可空< T> 具有特殊的语法糖 T ，但我的问题更多的是语义

<？ DIV CLASS =h2_lin>解决方案

我是越来越善于为：有没有这样的东西作为一个盒装可为空。当你框中 INT ，你得到的是盒装的 INT 的参考。当你框中 INT？，你无论是空引用或引用的盒装 INT 。你永远没有盒装 INT

您可以轻松地制作自己的可选<？。 T> 结构，但你无法实现具有拳击的行为结构。 可空< T> 的特殊行为被烤成运行

这实际上导致了一些奇怪的现象。 。例如：

• C＃4：动态和可空<>




• C＃反思：如何得到一个可空<的类型; INT> ;?




• 无法更改类型泛型方法可空

和FYI还有其他的方式，其中可空< T> 类型是神奇。例如，尽管它是一个结构式，它不满足该结构的约束。有没有办法为你做你自己的结构具有这个属性。

In Nullable micro-optimizations, part one, Eric mentions that Nullable<T> has a strange boxing behaviour that could not be achieved by a similar user-defined type.

What are the special features that the C# language grants to the predefined Nullable<T> type? Especially the ones that could not be made to work on a MyNullable type.

Of course, Nullable<T> has special syntactic sugar T?, but my question is more about semantics.

解决方案

What I was getting at is: there is no such thing as a boxed nullable. When you box an int, you get a reference to a boxed int. When you box an int?, you get either a null reference or a reference to a boxed int. You never get a boxed int?.

You can easily make your own Optional<T> struct, but you can't implement a struct that has that boxing behaviour. Nullable<T>'s special behaviour is baked into the runtime.

This fact leads to a number of oddities. For example:

• C# 4: Dynamic and Nullable<>

• C# Reflection: How to get the type of a Nullable<int>?

• Cannot change type to nullable in generic method

And FYI there are other ways in which the Nullable<T> type is "magical". For instance, though it is a struct type, it does not satisfy the struct constraint. There's no way for you to make your own struct that has that property.

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