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>;
}

• svick: You're doing this just as a learning exercise, right? Otherwise, reimplementing framework code doesn't make much sense.
• HerbalMart: Just to learn indeed. Fooling around with extension methods and delegates etc... :)
• svick: For a detailed explanation about how to implement all of LINQ extension methods, see Jon Skeet's series Edulinq. Specifically, part 9 is about SelectMany().

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.