Question

can we assign null value to struct type of variable?

struct MyStruct
{
}

MyStruct var = null;

is this is possible in C# .net?

if not ? then how C# is allowing Nullable < T > struct type of variable can be assigned as null?

Answer 1

On the subject of:

… how C# is allowing Nullable < T > struct type of variable can be assigned as null?

The answer is actually quite simple: The C# compiler knows about Nullable<T> and handles it as a special case to maintain the illusion.

For example, this:

int? a;
if (a != null) ...

Actually compiles into this:

if (a.HasValue) ...

Virtually all of this magic is handled by the compiler — .NET mostly views Nullable<T> as just another ValueType. I remember reading (most likely from Eric Lippert’s blog), that late in the development process they discovered one edge issue that just could not be handled by compiler tricks alone. The runtime folks agreed to make one change that treated Nullable<T> as a special case. Unfortunately, I don’t remember what the special case was and I can’t find the reference anymore.

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