Seeking .NET opensource library for geometry operations

I believe that NTS, The .NET Topology Suite contains code capable of doing geometry operations on GIS structures. It's a .NET port of JTS.

you might try SharpMap, or more specificially, the Net Topology Suite, which is the .net port of the JTS (java topology suite).

Also checkout DotSpatial

http://dotspatial.codeplex.com/wikipage?title=IGeometry&referringTitle=NTSCompare