Why can't iterator methods take either 'ref' or 'out' parameters?

If you want to return both an iterator and an int from your method, a workaround is this:

public class Bar : IFoo
{
    public IEnumerable<int> GetItems( ref int somethingElse )
    {
        somethingElse = 42;
        return GetItemsCore();
    }

    private IEnumerable<int> GetItemsCore();
    {
        yield return 7;
    }
}

You should note that none of the code inside an iterator method (i.e. basically a method that contains yield return or yield break) is executed until the MoveNext() method in the Enumerator is called. So if you were able to use out or ref in your iterator method, you would get surprising behavior like this:

// This will not compile:
public IEnumerable<int> GetItems( ref int somethingElse )
{
    somethingElse = 42;
    yield return 7;
}

// ...
int somethingElse = 0;
IEnumerable<int> items = GetItems( ref somethingElse );
// at this point somethingElse would still be 0
items.GetEnumerator().MoveNext();
// but now the assignment would be executed and somethingElse would be 42

This is a common pitfall, a related issue is this:

public IEnumerable<int> GetItems( object mayNotBeNull ){
  if( mayNotBeNull == null )
    throw new NullPointerException();
  yield return 7;
}

// ...
IEnumerable<int> items = GetItems( null ); // <- This does not throw
items.GetEnumerators().MoveNext();                    // <- But this does

So a good pattern is to separate iterator methods into two parts: one to execute immediately and one that contains the code that should be lazily executed.

public IEnumerable<int> GetItems( object mayNotBeNull ){
  if( mayNotBeNull == null )
    throw new NullPointerException();
  // other quick checks
  return GetItemsCore( mayNotBeNull );
}

private IEnumerable<int> GetItemsCore( object mayNo