"The selected file is not a valid solution file" error when trying to open source safe database

I've had a similar error message after installing Windows 7. Turns out that some features of Visual Studio were not yet installed. It might have been a service pack or EF4. Once that was installed, I was able to open the solution file. Then, I was also getting "The project type is not supported by this installation" until I installed some other updates, like for MVC3.

I presume that the solution file had information that the un-patched Visual Studio did not recognize.

In my case, the message "The selected file is not a valid solution file" occurred when there were file names in my solutions that were too long for the path where i wanted to download the solution. Putting the solution folder directly under the C:\ drive or a folder short to the C-drive solved my problem.