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In the following piece of code I expected to be able to implicitly cast from elements to baseElements because TBase is implicitly convertible to IBase . public interface IBase { } public interface IDerived : IBase { } public class VarianceBug { public void Foo<TBase>() where TBase : IBase { IEnumerable<TBase> elements = null; IEnumerable<IDerived> derivedElements = null; IEnumerable<IBase> baseElements; // works fine baseElements = derivedElements; // error CS0266: Cannot implicitly convert type // 'System.Collections.Generic.IEnumerable<TBase>' to // 'System.Collections.Generic.IEnumerable<IBase>'. // An explicit conversion exists (are you missing a cast?) baseElements = elements; } } However, I get the error that is mentioned in the comment. Quoting from the spec: A type T<A1, …, An> is variance-convertible to a type T<B1, …, Bn> if T is either an interface or a delegate type declared with the variant type parameters T<X1, …, Xn> , and for each variant type parameter Xi one of the following holds: Xi is covariant and an implicit reference or identity conversion exists from Ai to Bi Xi is contravariant and an implicit reference or identity conversion exists from Bi to Ai Xi is invariant and an identity conversion exists from Ai to Bi Checking my code, it appears to be consistent with the spec: IEnumerable<out T> is an interface type IEnumerable<out T> is declared with var

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