title: Open .NET form in designer mode - get "The path is not of a legal form"
body: <p>I attempted to open a C#/VB form in designer mode, and instead of the form, I got an ugly error message saying "The path is not of a legal form".  This form used to work!  What happened?</p>
  
  <hr>
  
  <p><em>Thanks to all who have answered.  This question is a problem I hit a while back, and I struggled with it for a long time, until I found the answer, which I posted below in order to help other people who might have hit this problem.<br>
  Thanks!</em></p>