Why am I allowed to compare a non-nullable type with null? [duplicate]

Question

Possible Duplicate:
C# okay with comparing value types to null

If I try to assign null to a non-nullable type in C#:

System.DateTime time = null;

I'll get a compile-time error:

error CS0037: Cannot convert null to 'System.DateTime' because it is a non-nullable value type

which makes sense. But if compare the same type against null:

System.DateTime time = obtainFromSomewhere();
if( time == null ) {
    //whatever;
}

there's no compile-time error. This doesn't make sense to me - if I can't assign null then why would it ever be null?

Why am I allowed to compare a non-nullable type with null?

Answer 1

The reason this works for DateTime, is because DateTime defines it own == operator. Because it does so, it gets a lifted version of the operator which may be used with DateTime?. Since both DateTime and null may be implicitly converted to DateTime?, the comparison compiles but will always evaluate to false at runtime.

Thanks to Matt Ellen for pointing out the fact that my original answer didn't cover the example in the question.

Answer 2

It's due to Boxing.

DateTime can be boxed as object and thus it becomes a reference which can be compared with null (though it will always be false).

However, an object (null) can't be unboxed back to DateTime, thus it can't be assigned to DateTime.

Example: you could do

object now = DateTime.Now;
bool isNull = now == null

EDIT: As Brian Rasmussen pointed out, I was wrong with the boxing theory. Boxing would only occur if explicitly casted to object like in my example or in (object)DateTime.Now == null.