How to specify type for generic class?

Question

I found some code online that allows me to find controls of a specific type recursively within a Control in ASP.NET Web Forms. It works great. Here's the code:

internal class ControlFinder<T> where T : Control

{

private readonly List<T> _foundControls = new List<T>();

public List<T> FoundControls

{

get { return _foundControls; }

}

public List<T> FindControls(Control control)

{

foreach (Control childControl in control.Controls)

{

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

_foundControls.Add((T)childControl);

else

FindControls(childControl);

}

return FoundControls;

}

}

It can be consumed like this:

var textboxes = new ControlFinder<TextBox>().FindControls(control);

This is exactly what I needed, but I found a need to be more fluid, and allow passing in the type rather than just using "TextBox". In my case, if the input controls needed to be disabled, I have this method:

public static void DisableTextBoxes(this Control control)

{

var controls = new ControlFinder<TextBox>().FindControls(control);

foreach (var textbox in controls)

{

textbox.Text = "";

textbox.Enabled = false;

}

}

As I said, this works great. But I'd like to write an extension method that allows passing in the type when called. The relevant code would be changed to this:

public static void DisableTextBoxes(this Control control, Type type)

{

var controls = new ControlFinder<type>().FindControls(control);

Problem is, this doesn't work. I get an error stating that the type or namespace couldn't be found. Is it not possible to initialize a generic class with a variable?

Vulpes · Answer

Sorry I forgot that (unlike the WF Control class) the ASP.NET Control class does not have Enabled and Text properties :( To answer your original question because of the way that generics work the compiler needs to know what the type parameter actually is when creating a closed generic class and so you can't use a variable for this - you have to use the name of an actual type.

William Snell · Answer

Sorry that code didn't work for me. It's also duplicating work already done in my code. My ControlFinder class recursively searches any Control passed to it to find child Controls of the specified type. This call correctly returns all TextBox Controls in the control passed in: var textboxes = new ControlFinder ().FindControls(control); Also I need the type to be known prior to attempting operations on the collection of controls found. I can't access the Text and Enabled properties of Controls without specifying their type. The DisableTextBoxes method I posted works because the type is specified when I create a new instance of ControlFinder. What I wanted to accomplish was almost exactly what I already have but instead the call to consume the FindControls method would look like this: // Where 'type' is a type of Control passed in. var controls = new ControlFinder ().FindControls(control); Then I could pass the type I wanted into this method call from anywhere. Actually now that I'm thinking about it I don't believe this is a good idea. I'd still have to cast to the required type to operate on unless I'm missing something which negates the benefit of not specifying the type when calling FindControls. I might be trying to refactor this too much.

Vulpes · Answer

I'd expect it to work if you define it like this: static class ControlExtensions // or whatever you want to call it { public static void DisableControls (this Control control) where T : Control { var controls = new ControlFinder ().FindControls(control); foreach (var c in controls) { c.Text = ; c.Enabled = false; } } } If 'cc' is a reference to some container control and you're interested in disabling TextBoxes within that container you could then call with: cc.DisableControls ();