Modbus TCP-Slave Null Exception,Object reference not set to an instance of an object

Question

I am developing a windows form to function as a modbus tcp slave Simuator. I am trying to recieve write multiple coil request from modbus master. I am able to recieve the request correctly, but when I try to do the write operation it's throwing "object reference is not set to an instance of an object" exception. It's saying slave is null, but I have created an instance of slave and initialized it. I am using NModbus Library. Here is my code

        private void Button1_Click(object sender, EventArgs e)
        {

              try
              {

                byte slaveAddress = Convert.ToByte(textBox3.Text);

                IPAddress address = IPAddress.Parse(textBox1.Text);
                int port = Convert.ToInt32(textBox2.Text);

                // create and start the TCP slave
                TcpListener slaveTcpListener = new TcpListener(address, port);
                slaveTcpListener.Start();


                ModbusSlave slave = ModbusTcpSlave.CreateTcp(slaveAddress, slaveTcpListener);


                slave.Listen();
                slave.DataStore = DataStoreFactory.CreateDefaultDataStore();
                DataStore dataStore = slave.DataStore;


                slave.ModbusSlaveRequestReceived += new EventHandler<ModbusSlaveRequestEventArgs>(Modbus_Request_Event);

                slave.DataStore.DataStoreWrittenTo += new EventHandler<DataStoreEventArgs>(DataStoreWrittenTo);



                // prevent the main thread from exiting
                listBox1.Items.Clear();

                Thread.Sleep(Timeout.Infinite);
                //slaveTcpListener.Stop();


              }
              catch
              {
                MessageBox.Show("Error in connection");
              }
        }


 private void DataStoreWrittenTo(object sender, DataStoreEventArgs e)
        {

            switch (e.ModbusDataType)
            {

                case ModbusDataType.HoldingRegister:


                    for (int i = 1; i < e.Data.B.Count; i++)
                    {

               //Line A         slave.DataStore.HoldingRegisters[e.StartAddress + i + 1] = e.Data.B[i];
                    }

                    break;
                case ModbusDataType.Coil:

                    for (int i = 1; i < e.Data.A.Count; i++)
                    {
 //Line B                       slave.DataStore.CoilDiscretes[e.StartAddress + i + 1] = e.Data.A[i];
                    }
                    break;
                    //case Modbus.


            }


        }

I am getting the exception at line A if I try to write Multiple Holding Registers and at line B if I try to write Multiple Coils. Can any one please help me to solve this issue?

Possible duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — ProgrammingLlama, Oct 03 '18 at 05:56
The scope part of [this](https://stackoverflow.com/a/26761773/3181933) answer specifically addresses your problem. It appears that you have a class-level variable `slave`, which you hide by creating a method-level variable `slave` here: `ModbusSlave slave = ModbusTcpSlave.CreateTcp` - prefixing it with the type `ModbusSlave` here is creating a variable that's local to the `Button1_Click` method, and leaves the class-level variable untouched. — ProgrammingLlama, Oct 03 '18 at 06:00

score 1 · Accepted Answer · answered Oct 03 '18 at 06:00

1

It looks like you're not instantiating your slave correctly. You are creating a local variable slave with the scope of Button1_Click.

ModbusSlave slave = ModbusTcpSlave.CreateTcp(slaveAddress, slaveTcpListener);

and you are accessing slave in DataStoreWrittenTo which should have a class scope.

You should change the above instantiation to:

slave = ModbusTcpSlave.CreateTcp(slaveAddress, slaveTcpListener);

answered Oct 03 '18 at 06:00

jegtugado

5,081
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thank you so much,now i realize what a silly mistake it was!!Anyway my code is working now,so thanks again. – pooja Oct 03 '18 at 06:19

Modbus TCP-Slave Null Exception,Object reference not set to an instance of an object

1 Answers1