c#4.0: int a real subtype of object? covariance, ienumerable and value types

Answer 1

The simplistic answer is that this is just one of the quirks in the way that variance is implemented in C# and the CLR.

From "Covariance and Contravariance in Generics":

Variance applies only to reference types; if you specify a value type for a variant type parameter, that type parameter is invariant for the resulting constructed type.