using System; using System.Linq; namespace Test { class S { public S () { } public string Where (Func cexpr) { return String.Empty; } } class C { } static class Extension { public static string Is (this C c) { return null; } } class Program { public static void Main () { var value = new S (); var e = from item in value where item.Is () select item; var foo = value.Where (p => p.Is ()); } } }