Rx: EnumerableEx.For() vs Enumerable.SelectMany()

Question

System.Interactive.dll includes a For() method with the following implementation:

IEnumerable<TResult> For<TSource, TResult>(
    IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TResult>> resultSelector)
{
    return source.Select<TSource, IEnumerable<TResult>>(resultSelector).Concat<TResult>();
}

Am I missing something or is this equivalent to the existing Enumerable.SelectMany(), minus this?

IEnumerable<TResult> SelectMany<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TResult>> selector)

Answer 1

Good question. They produce the same results but the internal implementations are quite different.

EnumerableEx.For would have been added to System.Interactive to maintain duality between IObservable and IEnumerables. Note that Observable.For and Observable.SelectMany are different:

IObservable<TResult> For<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IObservable<TResult>> resultSelector)

vs.,

IObservable<TResult> SelectMany<TSource, TResult>(this IObservable<TSource> source, Func<TSource, IObservable<TResult>> selector)

Therefore you'd expect that EnumerableEx.For would have this signature, rather than the one it actually has:

IEnumerable<TResult> For<TSource, TResult>(**IObservable**<TSource> source, Func<TSource, IEnumerable<TResult>> resultSelector)

However, it obviously doesn't take an IObservable source. Perhaps it was meant to. I would ask your question in the Rx forums, to see if the Rx team has an answer.

Answer 2

They look like equivalent functionality to me. SelectMany is an extension method on an IEnumerable and .For is written as a static method on EnumerableEx, so they are called differently.

    foreach(var s in list.SelectMany(Filter))
    {
        // ...
    }

    foreach (var s in EnumerableEx.For(list, Filter))
    {
        // ...
    }

I'm sure there are specific reasons for using each one.

Answer 3

My guess is that SelectMany traverses everything dynamically, whereas Concat (in For) traverses all of its outer IEnumerable elements before starting to register with and iterate over them.

In other words, Concat works with a fixed set of IEnumerable even if it receives them as an IEnumerable. Therefore, in For, the the entire set of IEnumerable is created before even returning the first TResult. In SelectMany, you receive a TResult instantly.