Question

I have a method that returns a List<T> of child controls that looks like this:

void GetAllControlsOfType<T>(List<T> lst, Control parent) where T:class
{
    if (parent.GetType() == typeof(T))
        lst.Add(parent as T);

    foreach (Control ch in parent.Controls)
        this.GetAllControlsOfType<T>(lst, ch);
}

But I have to use it like this:

List<WebControl> foo = new List<WebControl>();
GetAllControlsOfType<WebControl>(foo, this); //this = webpage instance

Surely there is some c# magic that will allow me to write a method that I can call like this:

List<WebControl> foo = GetAllControlsOfType<WebControl>(this);

Answer 1

The “magic” is simply declaring another method that returns the List<T> rather than void.

List<T> GetAllControlsOfType<T>(Control parent) where T : class {
    List<T> list = new List<T>();
    GetAllControlsoFType<T>(list, parent);   // Invoke your existing method
    return list;
}

Because you’re using recursion, you can’t simply modify your existing method to return List<T>, since doing so would make it impossible for you to keep track of that list and build on it.

A few other minor points:

• You have:

  if (parent.GetType() == typeof(T))
  

  But it would be more clear to write it as:

  if (parent is T)
  

  Unless, of course, you truly want your method to fail when used with subclasses of T.

• You may want to consider declaring the new method as an extension method by making parent declared as this Control parent (provided it’s declared in a static class)

  This would allow you to invoke the method as this.GetAllControlsOfType<WebControl>()