// CS1929: Type `System.Collections.IList' does not contain a member `Frobnicate' and the best extension method overload `Extensions.Frobnicate(this Test)' requires an instance of type `Test' // Line: 20 using System; using System.Collections; static class Extensions { public static void Frobnicate (this T foo) where T : IList { } } public class Test { IList mFoo; void Frobnicate () { mFoo.Frobnicate (); } }