When we want to use a User Control (which is defined in *.ascx) in a web page, we can register the control like this:
<%@ Register Src="~/MyObjectDisplayer.ascx" TagName="MOD" TagPrefix="CustomControl" %>
If we use a Custom Control (which is defined in *.cs), however, the register tag should be a little different:
<%@ Register Namespace="MyControls" TagPrefix="CustomControl" %>
Whenever we want to place this control in the webpage, we write:
<CustomControl:PromptButton id="pb1" runat="server"/>
It assumes that our control classes is PromptButton and it is in MyControls namespace. And note that it has been tested in ASP.NET Web Site project, but not in ASP.NET Web Application project.