C# 为什么私有变量定义上有一个问号？
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Why is there a questionmark on the private variable definition?
提问by Booser
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.
我正在阅读一篇关于 MVVP 模式以及如何使用 WPF 实现它的文章。在源代码中有多行，我无法弄清楚其中的问号代表什么。
private DateTime? _value;
What does the ?mean in the definition? I tried to find it in the help from VS but failed.
?定义中的意思是什么？我试图在 VS 的帮助中找到它，但失败了。
采纳答案by David Morton
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.
这是一个可以为空的值。默认情况下，结构不能为空，它们必须有一个值，因此在 C# 2.0 中，该Nullable<T>类型被引入到 .NET Framework 中。
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.
C#Nullable<T>用一段语法糖实现了类型，在类型名称后面放了一个问号，从而使之前不可为空的类型可以为空。
回答by Thomas Levesque
It means that the field is a Nullable<DateTime>, i.e. a DateTimethat can be null
这意味着该字段是 a Nullable<DateTime>，即 aDateTime可以为空
回答by Svetlozar Angelov
This is a nullable type, you can assign null to it
这是一个可为空的类型，您可以为其分配空值
回答by Raja
Private DateTime? _value - means that the _value is nullable. check out this link for a better explanation.
私人日期时间？_value - 表示 _value 可以为空。查看此链接以获得更好的解释。
http://davidhayden.com/blog/dave/archive/2005/05/23/1047.aspx
http://davidhayden.com/blog/dave/archive/2005/05/23/1047.aspx
Hope this helps.
希望这可以帮助。
Thanks, Raja
谢谢，拉贾
回答by Asad
cannot be null
不能为空
DateTime
DateTime dt = null; // Error: Cannot convert null to 'System.DateTime'
because it is a non-nullable value type
can be null
可以为空
DateTime? / Nullable<DateTime>
DateTime? dt = null; // no problems
