Blondel et al. invented the very popular "Louvain method" for community detection . This library implements their algorithm in C#. Moreover, the library is parallelized using plinq to maximize speed.
The use of the library is illustrated with the help of a test project. The library also allows construction of the full dendogram.
 V.D. Blondel, J.-L. Guillaume, R. Lambiotte, E. Lefebvre. "Fast unfolding of communities in large networks." J. Stat. Mech., 2008: 1008.