Are there any opensource librarys for doing geometry operations (union, intersect, relation...) in .Net?
See MapWindow : www.mapwindow.org
Clipper from Angus Johnson: http://www.angusj.com/delphi/clipper.php http://sourceforge.net/projects/polyclipping/files/
Same library is available as both C#, C++ and Delphi source code.
You may want to try out Topology Framework .NET (TF.NET) - an open source topology manipulation API capable of handling managed objects representation of topological entities based on other popular APIs, exposing it's JTS-based common topology manipulation core to them.
JTS Topology Suite is, in fact, Java API providing spatial object model and fundamental geometric functions, providing a complete, consistent, robust implementation of fundamental 2D spatial algorithms. It implements geometry model defined in the OpenGIS Consortium Simple Features Specification for SQL. JTS port for .NET was named Net Topology Suite (NTS), and is fully conformant to Microsoft .NET 2.0 specification. NTS extends JTS with numerous coordinate transformation and other functions, while TF.NET extends NTS further, including additional Autodesk and Oracle IO functions and generic classes related to geometry graphs.
Another option I haven't seen on here is Gmap.NET (Great Maps). The current version is from April 2015. I haven't personally used it much past creating a layer and viewing it in a WPF application, but it looks like it has great potential.