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>; }

You're doing this just as a learning exercise, right? Otherwise, reimplementing framework code doesn't make much sense. — svick
– svick, Commented Jul 25, 2012 at 10:35
Just to learn indeed. Fooling around with extension methods and delegates etc... :) — HerbalMart
– HerbalMart, Commented Jul 25, 2012 at 10:40
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(). — svick
– svick, Commented Jul 25, 2012 at 11:06

Konrad Rudolph · Accepted Answer · 2012-07-25 10:40:25Z

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.

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