UserControl type not available if code behind file isn't in app_code directory

I ran into a weird problem with dynamically created ASP.NET 2.0 user controls yesterday. I was using the user control to populate a panel of a tab control which meant I didn’t actually have a reference to the user control object anywhere on my form. The only way I could get a reference to it was by looking at the UserControl property of the tab that it was displaying in.

I’d given the user control a public property that I was using to pass data between the user control and the main page and I wanted to cast the object I got back into the type that I’d created in my user control’s code behind. The problem was that when I tried to cast it the compiler didn’t know about the type that I’d created in code behind and when I looked at the type of the object I was getting it was something weird like ASP.relative_path_usercontrol_aspx.

I eventually figured out that if I moved the user control’s code behind into the App_Code directory my user control suddenly had the type I was expecting at runtime.

The really annoying thing that was I couldn’t actually have my code behind file in the App_Code directory and have the .ascx file linked to the code behind using the CodeFile property. As soon as I linked the two files together I got the following error:
The file ‘/Project/App_Code/MyUserControl.ascx.cs’ is in the special directory ‘App_Code’, which is not allowed

This is annoying because it means that my user control misses out on some of the dynamic class generation stuff that happens in ASP.NET 2.0 and I had to manually add the controls to the top of the user control like we had to in ASP.NET 1.x:

public partial class MyUserControl : System.Web.UI.UserControl
{
   protected Label Label1;
   protected Image Image1;
   protected Label Label2;
   ...
}

I did a bit more searching about the problem and came across a very interesting post about changes to the way pages are compiled in ASP.NET 2.0.