/// <summary>
/// /// Returns all the directories in the directory tree below <paramref name="directory"/>
/// </summary>
/// <param name="directory"></param>
/// <returns></returns>
public static IEnumerable <DirectoryInfo> DirectoriesRecursive(this DirectoryInfo directory)
{
Contract.Requires(directory != null);
Contract.Ensures(Contract.Result <IEnumerable <DirectoryInfo> >() != null);
//Contract.Ensures(Contract.Result<IEnumerable<DirectoryInfo>>().None(dir => dir == null));
return(directory.AsHierarchy(dir => dir.GetDirectories()).Flatten().Unwrap());
}