Custom Control in App_Code folder compiles ok but returns error in Design Mode

I have a ccTextBox custom control in my App_Code folder to help automatically register javascript for implementing MaxLength on a multi-line TextBox. I added <add tagPrefix="cc" namespace="CustomControls" assembly="App_Code"/> to the Web.Config. The web site compiles and runs just fine; it also works ok in Source Mode. However, when I go to design mode, I get errors "Error Rendering the Control" and "Error Parsing the Theme" unknown server control cc:ccTextBox. What can I do to get this to work in design mode? Note, if I remove the ccTextBox from the skin file, I do not receive these errors.

The ccTextBox code begins with

Namespace CustomControls

Public Class ccTextBox

Inherits TextBox

...

Re: Custom Control in App_Code folder compiles ok but returns error in Design Mode

If possibleplease send your whole projincluding sln fileto v-dedong@microsoft.com for more analysesAnd what's moreyou can entitle your letter with your URL of the issue posted on the forum

Reguards