Consider for example IEnumerable and note its simplicity - there is just one method, yet it is the basis of LINQ more or less. There are many implementions of this interface in Microsoft .NET. Nevertheless, obviously, it would have been burdensome to require each of these implementations to implement the whole series of methods that are defined in the System.Linq namespace to operate on IEnumerables, even though Microsoft has all the source code. Even worse, this would have required everybody besides Microsoft considering to use IEnumerable themselves to also implement all those methods, which would have been very anti-productive seeing the widespread use of this very common interface. Instead, by implementing the one method of this interface, LINQ can be used more or less immediately. Especially seeing in practically most cases IEnumerable's GetEnumerator method is delegated to a private collection, list or array's GetEnumerator implementation.