Add a constraint such that T must be a class.
class MyClass<T> where T: class, MyInterface
This gives the compiler enough information to know that T is convertible. You don't need the explicit cast either.
Variance only applies to reference types. T is allowed to be a value type without the constraint which breaks the compilers ability to prove that T is compatible for contravariance.
The reason the second statement works is because as
actually can perform a null conversion. For example:
class SomeClass { }
interface SomeInterface { }
static void Main(string[] args)
{
SomeClass foo = null;
SomeInterface bar = foo as SomeInterface;
}
Foo
is obviously not directly convertable to SomeInterface
, but it still succeeds because a null conversion can still take place. Your MSDN reference may be correct for most scenarios, but the generated IL code is very different which means they are fundamentally different from a technical perspective.