Dart.SNMP v4.2.0 throws NotImplementedException under provided scenario. Is this user error or library error?
using System;
using System.Threading;
using Dart.Snmp;
using System.Diagnostics;

namespace ConsoleApplication1
{
    class Program
    {
        ManualResetEvent _snmpCompleteEvent = new ManualResetEvent(false);
        const string IP = "XXX.XXX.X.XX";
        const string USERNAME = "username123";

        static void Main(string[] args)
        {
            new Program().Run();
        }

        private void Run()
        {
            Manager SnmpMgr = new Manager();
            _snmpCompleteEvent.Reset();
            SnmpMgr.Start(SendBadSnmp3RequestWorker, null);
            _snmpCompleteEvent.WaitOne();
            _snmpCompleteEvent.Reset();
            SnmpMgr.Start(SendGoodSnmp3RequestWorker, null);
            _snmpCompleteEvent.WaitOne();
        }

        private void SendBadSnmp3RequestWorker(ManagerSlave slave, object state)
        {
            RequestMessage requestMessage = CreateGetRequestMessage(useValidSecurity: false);
            ResponseMessage responseMessage = slave.GetResponse(requestMessage, IP);
            Debug.Assert(responseMessage.ErrorStatus == ErrorStatus.AuthorizationError, "This should have been an authorization error");
            _snmpCompleteEvent.Set();
        }

        private void SendGoodSnmp3RequestWorker(ManagerSlave slave, object state)
        {
            RequestMessage requestMessage = CreateGetRequestMessage(useValidSecurity: true);
            try
            {
                ResponseMessage responseMessage = slave.GetResponse(requestMessage, IP);
            }
            catch (NotImplementedException exception)
            {
                //Exception is encountered, but why?
                _snmpCompleteEvent.Set();
                throw;
            }
        }

        private RequestMessage CreateGetRequestMessage(bool useValidSecurity)
        {
            const string sysName = "1.3.6.1.2.1.1.5";
            RequestMessage requestMessage = new GetMessage();

            requestMessage.Version = SnmpVersion.Three;
            requestMessage.Variables.Clear();
            requestMessage.Variables.Add(new Variable(sysName));
            if (useValidSecurity)
                SetValidAuthorizationSecurity(requestMessage);
            else
                SetInvalidAuthorizationSecurity(requestMessage);

            return requestMessage;
        }

        private void SetInvalidAuthorizationSecurity(RequestMessage requestMessage)
        {
            requestMessage.Security = new Security
            {
                Username = USERNAME,
                AuthenticationProtocol = AuthenticationProtocol.None,
                AuthenticationPassword = string.Empty,
                PrivacyProtocol = PrivacyProtocol.None,
                PrivacyPassword = string.Empty
            };
        }

        private void SetValidAuthorizationSecurity(RequestMessage requestMessage)
        {
            requestMessage.Security = new Security
            {
                Username = USERNAME,
                AuthenticationProtocol = AuthenticationProtocol.Sha,
                AuthenticationPassword = USERNAME,
                PrivacyProtocol = PrivacyProtocol.Des,
                PrivacyPassword = USERNAME
            };
        }
    }
}