As far as I'm aware, Visual Studio 2008 does not support .NET 4.5 (the newest supported version would be .NET 3.5), which is what that version is built against. Consider upgrading to Visual Studio 2013 (the Community Edition is free, after all).
While it's not a canonical resource, this table seems to show what versions of Visual Studio support which frameworks: http://en.wikipedia.org/wiki/Microsoft_Visual_Studio#History
While it's not a canonical resource, this table seems to show what versions of Visual Studio support which frameworks: http://en.wikipedia.org/wiki/Microsoft_Visual_Studio#History
