Generic method to get all controls of type T

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);

The first part of the answer should help: http://stackoverflow.com/questions/253937/recursive-control-search-with-linq — Jan, Aug 23 '12 at 01:10
The second part is equally relevant. Once you have all controls, use `OfType()` to get the ones you're after. — Jay, Aug 23 '12 at 01:17

score 2 · Accepted Answer · answered Aug 23 '12 at 01:09

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>()

Generic method to get all controls of type T

1 Answers1