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>
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