Extension Method to Flatten Any Recursive Collection

Sometimes I wan’t to iterate all items in a recursive collection structure, e.g. the Controls hierarchy of Web Controls or Windows Forms Controls.

Instead of coding a “flatten” method for each collection type, I’ve written a small extension method Flatten that allows you to easily recurse any structure:

/// <summary>
/// Flattens any recursive collection into a single list of all items.
/// </summary>
/// <typeparam name="T">Type of items in collection</typeparam>
/// <param name="collection">Root collection of T items</param>
/// <param name="recursor">Selector that returns the sub collection of a given item</param>
/// <param name="predicate">Predicate to apply to each item before returning it</param>
/// <returns>List of all items</returns>
public static IList<T> Flatten<T>(
  this IEnumerable<T> collection,
  Func<T, IEnumerable<T>> recursor,
  Func<T, bool> predicate)
{
  // Container for results
  var result = new System.Collections.Generic.List<T>();

  // Queue of yet unprocessed collections
  var queue = new Queue<IEnumerable<T>>();

  do
  {
    // Process current collection
    foreach (T control in collection)
    {
      queue.Enqueue(recursor(control));
      if (predicate == null || predicate(control))
        result.Add(control);
    }

    // Go to next or exit if done
    if (queue.Count > 0)
      collection = queue.Dequeue();
    else
      return result;
  } while (true);
}

The usage of it is simple:

IList<Control> allControls = Controls.Cast<Control>().Flatten(c => c.Controls.Cast<Control>(), null);

To avoid the ugly Cast<Control>() calls, you can define overloads of Flatten that works directly on ControlCollection. The reason why Cast is needed is that ControlCollection does not implement IEnumerable<Control>.