This must be one of the stupidest messages in Visual Studio — how glad I am they kept it on board for 2005!
Let’s see: VS knows the file type I just dropped on it, because it can inform me about it. I have no solution open at this point — the internal state is just perfect for me to open a project if I want. I just have to do it in a specific way, and I believe this is the only file type I can open in VS where this ridiculous restriction applies. Please enlighten me if you can: does this restriction have any advantage that I don’t see? Or if you don’t know the answer to that question, how about the name of the girl/guy who decided to do things this way? :-)