using System.Collections.Generic; interface IA { } interface IB : IA { } static class E { internal static void ToReadOnly<T>(this IEnumerable<T> source) { } internal static void To (this IA i) { } } class C { public static void Main () { } public static void Test (IEnumerable<bool> bindings) { bindings.ToReadOnly(); IB ib = null; ib.To (); } }