title: Why doesn't IEnumerable<T> implement Add(T)?
body: <p>Just now find it by chance, Add(T) is defined in <code>ICollection&lt;T&gt;</code>, instead of <code>IEnumerable&lt;T&gt;</code>. And extension methods in Enumerable.cs don't contain Add(T), which I think is really weird. Since an object is enumerable, it must "looks like" a collection of items. Can anyone tell me why?</p>