It is wonderful news for .NET developer that you "can" check out .NET framework 4.5 source code very conveniently at http://referencesource.microsoft.com/. Besides handy code editor functionality like go to definition and find references, you could do "search" very easily and quickly. You will feel browser like light weighted but handy IDE when browsing source code(Note, you can even step into the code).

Blog reference: http://blogs.msdn.com/b/dotnet/archive/2014/02/24/a-new-look-for-net-reference-source.aspx