"The path is not of a legal form."

If you attempt to open your project (standard windows application) and the designer gives you a similar error to the one below....

"One or more errors encountered while loading the designer. The errors are listed below. Some errors can be fixed by rebuilding your project, while others may require code changes."


The the problem is that the form's designer is trying to locate one of the types referenced by your form and is unable.
This means that your project probably wasn't built since the reference was added or that the project file has somehow been changed.

If building the solution doesn't work, try going over the references in the project and looking at where their locations are and verifying that they all actually exist.