nrefactory:http://wiki.sharpdevelop.net/NRefactory.ashx
NRefactory is used by SharpDevelop & MonoDevelop for code navigation etc. NRefactory 5 is a complete rewrite of NRefactory 4 and is used by MonoDevelop 3.
a tutorial on nrefactory: http://www.codeproject.com/Articles/408663/Using-NRefactory-for-analyzing-Csharp-code
a similar project by Microsoft Research -- Roslyn: http://msdn.microsoft.com/en-us/roslyn