The C# driver currently executes the getLastError command as part of the server handshake, in order to get the connectionId from its response. This is useful for logging and monitoring connection use and correlating with server log files. As of 4.2, the server will return the connectionId in the isMaster response, which the driver has to execute anyway on every connection. If the driver detects a connectionId in the isMaster response, it should just use it and not execute getLastError.