ASHX and "Cannot use a leading .. to exit above the top directory"

Question

I have an ASHX into which I want to load a UserControl from a folder parallel to it. When I try to use a relative path using .., I get the exception, Cannot use a leading .. to exit above the top directory. There are a lot of answers on this error here already, but this situation seems different.

Initially, during development, I loaded the control using ~/MyProject/Controls/MyControl.cs, but in production, I found that the "~" was different (I can't tell why, but it's a very old, very quirky setup, so I'm used to this kind of error).

First, I tried to use a relative path, ../Controls/MyControl.cs. This failed with the error, 'Cannot use a leading .. to exit above the top directory`.

Then, I tried building the absolute path to the file, using Server.MapPath(), but it seems that LoadControl() doesn't like absolute paths.

I'm pretty confused with the .. error, because the entire project is in a subfolder of the webroot, so you'd have to go a couple more levels deeper to go below the webroot. It is as if a generic web handler creates its own application space. I'm clearly misunderstanding something.

My abbreviated web folder structure looks kind of like this:

 / (webroot) 
  + MyProject/
    + Controls/
        MyControl.ascx
        MyControl.ascx.cs
    + WebHandlers/
        MyHandler.ashx

...and I want to load MyControl.ascx into MyHandler.ashx.