That should work, I checked here with a similar path and it worked fine. Could it be there's a typo in one of the filenames/folders? Or the dbf file not actually being there? Maybe something else goes wrong and the error message is a read herring, what happens if you try to open another dbf from the same folder, same error?
Btw, the dialect for the project is set to FoxPro, correct?
Aha! The real problem is that SET DEFAULT TO doesn't validate that the folder name you send in is actually a folder name. In my example the actual folder is C:DevXSharpToDo, not XSharpToDos. GetDefault() returned whatever I sent in. I'd expect it to return either an error or throw an exception. Is this something I should file a bug report for? Download the source, fix, and submit a Pull Request?