带有问号好奇的C＃语法 [英] A curious C# syntax with a question mark

问题描述

 私人枚举E_Week
{
   周一= 0，
   星期二，
   。 。 。
}
 

什么是以下code是什么意思？

  E_Week？周= NULL;
 

是不是等于下列code？是什么的函数'？'在这里签字？

  E_Week周= NULL;
 

解决方案

您code是使用什么叫做的可空类型。枚举，很像一个int或一个DateTime，就是被称为价值型，这是需要始终有一定的价值。可空类型允许你对待值类型，如果他们这样做允许空值。

例如，这code是无效的，不能编译，因为枚举不能为空：

  E_Week周= NULL;
 

但是，这code是有效的：

  E_Week？周= NULL;
 

和它是完全一样的：

 可空＆LT; E_Week＆GT;周= NULL;
 

private enum E_Week
{
   Mon = 0,
   Tue,
   . . .
}

What does the following code mean?

E_Week? week= null;

Is it equal to the following code? What is the function of the '?' sign here?

E_Week week= null;

解决方案

Your code is using what's called a nullable type. An enum, much like an int or a DateTime, is what is known as a "value type", which is required to always have some value. Nullable types allow you to treat value types as if they do allow null values.

For example, this code is invalid and won't compile because enums cannot be null:

E_Week week = null;

But this code is valid:

E_Week? week = null;

And it's exactly the same as this:

Nullable<E_Week> week = null;

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