The question was raised in Stackoverflow: http://stackoverflow.com/questions/38772459/connection-to-mongo-db-using-dotnetcore/38772489?noredirect=1#comment65244203_38772489 and there is a workwound for it which I found an added to the thread.

Essentially the MongoClient throws an exception when trying to connect using localhost as part of its connection string. If you however change the name for the ip address it works. So I changed it from localhost to 127.0.0.1 and I could connect successfully.