Why does C# (4.0) not allow co- and contravariance in generic class types?

Question

What is the real reason for that limitation? Is it just work that had to be done? Is it conceptually hard? Is it impossible?

Sure, one couldn\'t use

Answer 1

First off, as Tomas says, it is not supported in the CLR.

Second, how would that work? Suppose you have

class C
{ ... how are you planning on using T in here? ... }

T can only be used in output positions. As you note, the class cannot have any field of type T because the field could be written to. The class cannot have any methods that take a T, because those are logically writes. Suppose you had this feature -- how would you take advantage of it?

This would be useful for immutable classes if we could, say, make it legal to have a readonly field of type T; that way we'd massively cut down on the likelihood that it be improperly written to. But it's quite difficult to come up with other scenarios that permit variance in a typesafe manner.

If you have such a scenario, I'd love to see it. That would be points towards someday getting this implemented in the CLR.

UPDATE: See

Why isn't there generic variance for classes in C# 4.0?

for more on this question.