C# yield return (part 1)

Until recently, I didn’t know that iterator methods in C# (available since version 2.0) could return types other than IEnumerable<T>:

using System.Collections;
using System.Collections.Generic;

class Foo {
  IEnumerable Bar1() { yield return 1; }
  IEnumerator Bar2() { yield return 2; }
  IEnumerable<int> Bar3() { yield return 3; }
  IEnumerator<int> Bar4() { yield return 4; }

  static void Main() { }
}

IEnumerator (not IEnumerable) types are allowed in iterator methods for the convenient implementation of IEnumerable<T> in custom collection types.

10.14.2 Enumerable interfaces The enumerable interfaces are the non-generic interface System.Collections.IEnumerable and all instantiations of the generic interface System.Collections.Generic.IEnumerable<T>. For the sake of brevity, in this chapter these interfaces are referenced as IEnumerable and IEnumerable<T>, respectively.

By the way, an iterator like this:

IEnumerable Bar1() { yield return 1; }

…actually returns a value of type IEnumerable<object>, but this is not documented anywhere in the specification, and it’s not something to rely on, of course…