Proj4Net is a .Net library to transform point coordinates from one geographic coordinate system to another, including datum transformations.
Proj4Net is a direct port of Proj4J
The core of this library is a port of the PROJ.4 C library. The projection algorithms, concepts and coordinate system definitions are all taken directly from PROJ.4. These are wrapped in a C# class model which follows standard Java conventions and OO best practices.
A discussion forum is available for users and developers of Proj4Net.