Recursive Linq Traversing a Tree. This version traverses a directory structure.

gistfile1.txt

public static IEnumerable<string> Files(
    this string root, Func<string, bool> accept)
{
    return
    Directory.GetFiles(root).Where(accept)
        .Concat(
            Directory.GetDirectories(root)
                .SelectMany(d => d.Files(accept)));
}