<p>I want to subclass the built-in DropDownList in ASP.NET so that I can add functionality to it and use it in my pages. I tried doing this with a UserControl but found that it doesn't expose the internal DropDownList (logically, I guess). I've googled for the answer but can't find anything. </p>

<p>I've come as far as writing the actual class, and it's possible to subclass from DropDownList but I'm unable to register the file in my ASP.NET page and use it in the source view. Maybe I'm missing some properties in my class?</p>

<p>Any ideas?</p>

You want to extend DropDownList in a Custom Control... not in a usercontrol.
Create a new Class Library Project called MyLibrary.
Add a class called MyDropDownList.cs
```
```
namespace My.Namespace.Controls
{
[ToolboxData("<{0}:MyDropDownList runat=\"server\"></{0}:MyDropDownList>")]
public class MyDropDownList: DropDownList
{
    // your custom code goes here
    // e.g.
    protected override void  RenderContents(HtmlTextWriter writer)
    {
        //Your own render code
    }
}
}
```
```
Once you compile your library, you can add a reference to it in your web application.
And a tagprefix in your web.config
```
```
<add tagPrefix="my" namespace="My.Namespace.Controls" assembly="MyLibrary" />
```
```
That should allow you to add this to your aspx/ascx's
```
```
<my:MyDropDownList ID="myDDl" runat="server">
    ...
</my:MyDropDownList>
```
```