IEnumerable is not contravariant. This is covariant.
From MSDN (IEnumerable <(Of <(T>)>)) we have that:
Type parameters
out T
The type of objects to enumerate. This type parameter is covariant . That is, you can use either the type you specified or any type that is more derived.
From this post we have the following:
The base class library has been updated to support covariance and contravariance in the various interfaces used. For example, IEnumerable is now a covariant interface - IEnumerable.
Code example:
// Covariant parameters can be used as result types interface IEnumerator<out T> { T Current { get; } bool MoveNext(); } // Covariant parameters can be used in covariant result types interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } // Contravariant parameters can be used as argument types interface IComparer<in T> { bool Compare(T x, T y); }
For more examples, see:
Questions of covariance and contravariance
Covariance and contravariance in C #, part one (Eric Lippert's Big Series of Messages on Covariance and Contravariance)
Understanding C # Covariance and Contravariance (3) Samples