I have recently found out that (merely) installing Visual Studio 2012 on my machine, causes Visual Studio 2010 to use the newer C# compiler. This happens even when setting the language version to 4 (http://ericlippert.com/2013/04/04/what-does-the-langversion-switch-do/).
I wanted to know if it were possible to force Visual Studio to use the old compiler anyway. I think it's not possible because it seems that the compiler is installed in the framework folder which is the same for .NET 4 and .NET 4.5, but I was wondering if someone managed to do it anyway somehow.