我们可以将空值分配给结构类型的变量吗？
struct MyStruct
{
}
MyStruct var = null;
这在C# .net中是否可能？
如果不是? 那么,C#如何允许Nullable <T >可被分配为无效的固定变量类型?
关于：
C#如何允许可以将Nullable<T>结构类型的变量赋为null？
答案实际上非常简单: C#汇编者知道Nullable<T>,并将其作为维持幻觉的特殊案例处理。
例如，这个：
int? a;
if (a != null) ...
实际上编译成这个：
if (a.HasValue) ...
几乎所有的这种魔法都由编译器处理——.NET主要将Nullable<T>视为另一种ValueType。我记得读过（很可能是来自Eric Lippert的博客）, 在开发过程的后期，他们发现了一个特殊问题，仅靠编译器技巧无法解决。运行时的人同意做出一个改变，将Nullable<T>视为一个特殊情况。不幸的是，我不记得特殊情况是什么了，也找不到参考文献了。
Please review this article for details (especially the second comment which is by Lippert): http://blogs.msdn.com/ericlippert/archive/2008/05/07/covariance-and-contravariance-part-twelve-to-infinity-but-not-beyond.aspx
是的，这是可能的，但你必须将你的变量声明为可为空的：
MyStruct? foo = null;
或者 (huò zhě)
Nullable<MyStruct> foo = null;
在C＃中， struct 是值类型，而 class 是引用类型。
结构体无法变成null，因此您必须使用Nullable
这可以通过在声明中直接使用Nullable<T>类来完成：
Nullable<MyStruct> foo;
或者使用Nullable<T>的语法糖:
MyStruct? foo;
结构体不能为 null，可空类型 用于创建可空结构体的实例。
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