C# ControlCollection Extension GetAllTextboxes

Question

How could I get only the texboxes in a ControlCollection ?

I try :

public static IEnumerable<TextBox> TextBoxes(this ControlCollection controlCollection)
{
    return (IEnumerable<TextBox>)controlCollection.Cast<Control>().Where(c => c is TextBox);
}

But I got the following error : Unable to cast object of type 'WhereEnumerableIterator`1[System.Web.UI.Control]' to type 'System.Collections.Generic.IEnumerable`1[System.Web.UI.WebControls.TextBox]'.

I Use Asp.Net 3.5 with C#

Answer 1

You want OfType():

public static IEnumerable<TextBox> TextBoxes(this ControlCollection controlCollection)
{
    return controlCollection.OfType<TextBox>();
}

Answer 2

You don't actually need a new extension method - there's already one for you that will get this:

controlCollection.OfType<TextBox>();

The OfType method returns a sequence (IEnumerable<T>) subset of the sequence provided. If the type isn't convertible, it's left out. Unlike most of the LINQ extension methods, OfType is available on sequences that aren't strongly-typed:

This method is one of the few standard query operator methods that can be applied to a collection that has a non-parameterized type, such as an ArrayList. This is because OfType<(Of <(TResult>)>) extends the type IEnumerable.

Or if you do want to wrap it in an extension method, it's of course quite simple:

public static IEnumerable<TextBox> TextBoxes(this ControlCollection controls)
{
    return controls.OfType<TextBox>();
}

Answer 3

Here is a recursive extension method to get the Control objects that descend from the specified type, including those that are nested in the control hierarchy.

public static class ControlCollectionExtensions
{
    public static IEnumerable<T> OfTypeRecursive<T>(this ControlCollection controls) where T : Control
    {
        foreach (Control c in controls)
        {
            T ct = c as T;

            if (ct != null)
                yield return ct;

            foreach (T cc in OfTypeRecursive<T>(c.Controls))
                yield return cc;
        }
    }
}

(For Windows Forms instead of ASP.NET, substitute Control.ControlCollection for ControlCollection.)