为什么会出现在私人变量定义一个问号？ [英] Why is there a questionmark on the private variable definition?

问题描述

我读有关MVVP模式的文章，如何用WPF实现它。在源代码中有多个行，我想不通其中的问号代表什么。

I am reading an article about the MVVP Pattern and how to implement it with WPF. In the source code there are multiple lines where I cannot figure out what the question marks in it stand for.

private DateTime? _value;

什么的？指的是在定义？我试图找到它从VS的帮助，但失败了。

What does the ? mean in the definition? I tried to find it in the help from VS but failed.

推荐答案

这是一个空值。结构，默认情况下，不能为空的，他们必须有一个值，所以在C＃2.0中，可空< T> 类型被引入到.NET Framework。

It's a nullable value. Structs, by default, cannot be nullable, they must have a value, so in C# 2.0, the Nullable<T> type was introduced to the .NET Framework.

C＃实现可空< T> 用一块语法糖，其中后放置一个问号型键入名称，从而使以前的非可空类型，可为空。

C# implements the Nullable<T> type with a piece of syntactic sugar, which places a question mark after the type name, thus making the previously non-nullable type, nullable.

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