The C# driver (and other drivers as well) do not currently support connecting to mongos'es deployed behind a load balancer, which is too bad, because deploying mongos'es behind a load balancer has many appealing operational benefits (e.g. the ease of adding/removing mongos instances without changes to the application connection string).

This configuration causes a problem for the drivers, as currently implemented, because for queries that return large result sets the OP_GETMORE messages used to retrieve the next batch are sent on a different connection than the original OP_QUERY. Because the connections are through a load balancer, the OP_GETMORE messages almost invariably end up at the wrong mongos.