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

问题

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();

回答1:

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.

回答2:

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();

回答3:

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();

回答4:

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();

来源：https://stackoverflow.com/questions/3302435/why-doesnt-the-controls-collection-provide-all-of-the-ienumerable-methods

标签

ASP.NET

linq

extension-methods

ienumerable