My own implementation of Linq SelectMany extension method

Question

What do you think of my own implementation of the extension method SelectMany? Motivating criticism is always welcome.

public static IEnumerable<TResult> MySelectMany<T, TResult>(this IEnumerable<T> source, Func<T, IEnumerable<TResult>> selector)
{
    var theList = new List<TResult>();

    foreach (T item in source)
    {
        foreach (TResult inneritem in selector(item))
        {
            theList.Add(inneritem);
        }
    }

    return theList as IEnumerable<TResult>;
}

Answer 1

The as cast in the return statement is entirely redundant, it doesn’t serve a purpose.

Furthermore, The problem with this implementation is that it’s not lazy. You should use a yield generator instead.

public static IEnumerable<TResult> MySelectMany<T, TResult>(this IEnumerable<T> source, Func<T, IEnumerable<TResult>> selector)
{
    foreach (T item in source)
        foreach (TResult inneritem in selector(item))
            yield return inneritem;
}

If C# already had a yield from statement, this would be even shorter since you wouldn’t need to iterate the inner items explicitly.