Why doesn't the Controls collection provide all of the IEnumerable methods?

c# asp.net linq extension-methods ienumerable
Earlz picture Earlz · Jul 21, 2010 · Viewed 12.4k times · Source

I'm not for sure how the ControlCollection of ASP.Net works, so maybe someone can shed some light on this for me.

I recently discovered the magic that is extension methods and Linq. Well, I was very sad to find that this isn't valid syntax

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

However from what I can tell, Controls does implement the IEnumerable interface which provides such methods, so what gives? Why doesn't that just work? I have found a decent work around for this issue at least:

var list = (IEnumerable<Control>)Controls;
var this_item = list.Where(x => x.ID == "Some ID").SingleOrDefault();

Answer

Jon Skeet picture Jon Skeet · Jul 21, 2010

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.

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