Recursive List Flattening

Question

I could probably write this myself, but the specific way I\'m trying to accomplish it is throwing me off. I\'m trying to write a generic extension method similar to the oth

Answer 1

I thought I'd share a complete example with error handling and a single-logic apporoach.

Recursive flattening is as simple as:

LINQ version

public static class IEnumerableExtensions
{
    public static IEnumerable SelectManyRecursive(this IEnumerable source, Func> selector)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (selector == null) throw new ArgumentNullException("selector");

        return !source.Any() ? source :
            source.Concat(
                source
                .SelectMany(i => selector(i).EmptyIfNull())
                .SelectManyRecursive(selector)
            );
    }

    public static IEnumerable EmptyIfNull(this IEnumerable source)
    {
        return source ?? Enumerable.Empty();
    }
}

Non-LINQ version

public static class IEnumerableExtensions
{
    public static IEnumerable SelectManyRecursive(this IEnumerable source, Func> selector)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (selector == null) throw new ArgumentNullException("selector");

        foreach (T item in source)
        {
            yield return item;

            var children = selector(item);
            if (children == null)
                continue;

            foreach (T descendant in children.SelectManyRecursive(selector))
            {
                yield return descendant;
            }
        }
    }
}

Design decisions

I decided to:

• disallow flattening of a null IEnumerable, this can be changed by removing exception throwing and:
  • adding source = source.EmptyIfNull(); before return in the 1st version
  • adding if (source != null) before foreach in the 2nd version
• allow returning of a null collection by the selector - this way I'm removing responsibility from the caller to assure the children list isn't empty, this can be changed by:
  • removing .EmptyIfNull() in the first version - note that SelectMany will fail if null is returned by selector
  • removing if (children == null) continue; in the second version - note that foreach will fail on a null IEnumerable parameter
• allow filtering children with .Where clause on the caller side or within the children selector rather than passing a children filter selector parameter:
  • it won't impact the efficiency because in both versions it is a deferred call
  • it would be mixing another logic with the method and I prefer to keep the logic separated

Sample use

I'm using this extension method in LightSwitch to obtain all controls on the screen:

public static class ScreenObjectExtensions
{
    public static IEnumerable FindControls(this IScreenObject screen)
    {
        var model = screen.Details.GetModel();

        return model.GetChildItems()
            .SelectManyRecursive(c => c.GetChildItems())
            .OfType()
            .Select(c => screen.FindControl(c.Name));
    }
}