score:92

Accepted answer

No, IEnumerable doesn't have many extension methods on it: IEnumerable<T> does. They are two separate interfaces, although IEnumerable<T> extends IEnumerable.

The normal LINQ ways of converting are to use the Cast<T>() and OfType<T>() extension methods which do extend the nongeneric interface:

IEnumerable<TextBox> textBoxes = Controls.OfType<TextBox>();
IEnumerable<Control> controls = Controls.Cast<Control>();

The difference between the two is that OfType will just skip any items which aren't of the required type; Cast will throw an exception instead.

Once you've got references to the generic IEnumerable<T> type, all the rest of the LINQ methods are available.

score:2

Linq utilized Generic Collections. ControlsCollection implements IEnumerable not IEnumberable<T>

If you notice this will not work

((IEnumerable)page.Controls).Where(...

However, this does

((IEnumerable<Control>)page.Controls).Where(...

You can either cast to Generic IEnumerable<T> or access an extension method that does, like so:

 page.Controls.OfType<Control>().Where(c => c.ID == "Some ID").FirstOrDefault();

score:5

In addition to the answers provided by Jon Skeet and Dan Tao, you can use query expression syntax by explicitly providing the type.

Control myControl = (from Control control in this.Controls
                    where control.ID == "Some ID"
                    select control).SingleOrDefault();

score:11

This is just because the ControlCollection class came around before generics; so it implements IEnumerable but not IEnumerable<Control>.

Fortunately, there does exist a LINQ extension method on the IEnumerable interface that allows you to generate an IEnumerable<T> through casting: Cast<T>. Which means you can always just do this:

var c = Controls.Cast<Control>().Where(x => x.ID == "Some ID").SingleOrDefault();

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