Unity not Opening Solution in Visual Studio--Only File

I have a weird bug with Unity that just started happening today. Basically, when I try to open a code file in Unity, Visual Studio launches as expected. However, it’s just that file, meaning that it doesn’t open the solution like it is supposed to. The editor isn’t attached either, because when I open the same file in Unity it just launches another single-file instance of Visual Studio. Any ideas on why this may be happening, and of course, how I would fix this?

I’m using Visual Studio 2012 Ultimate Update 3 on Windows 7 64 bit. The only thing I can think of is that I updated to Resharper 8 yesterday, but it worked fine yesterday evening right after installation.

I just had the same problem and found the solution to this, just set how unity should open the file and choose visual studio not file type extension.

You can do that by going to:
EditPreferencesExternal ToolsExternal Script Editor → Set your Visual Studio Version

I’m sorry, I don’t have an answer.
I just wanted to mention that the exact same problem is happening to me with the new Unity 2018 version.
All of my older version of Unity work just fine and their solutions are loaded.
It appears that Unity doesn’t even create a solution for VS to load.

This is a Unity issue.

You have to add Visual Studio Editor package to your project, that will fix your problem.