System.AccessViolationException when trying to use a dll function

[alerico90]

Hello Ihave a problem with the use of a single function of a dll, the rest of the functions works just fine but there is one that keeps giving me a System.AccessVi olationExceptio n

Here is part of my code:

struct TPLU
        {
            public string Name;
            public int LFCode;
            public string Code;
            public int BarCode;
            public int UnitPrice;
            public int WeightUnit;  //4 Kg, 9 PCS
            public int Deptment;
            public double Tare;
            public int ShlefTime;
            public int FixUnit;
            public double FixWeight;
            public int Tolerance;
            public int Message1;
            public int Message2;
            public int MultiLabel;
            public int Rebate;
            public int Account;
            public int IsLock;
            public int Traceid;
        }

        private TPLU[] PPLUCluster;

        private int[] predefinedHotkeys = new int[84]; //84 hotkeys on the scale. 0 to 83

        const string dllfile = @"PBusdrv.dll";

        [DllImport(dllfile)]
        private static extern int PBusConnectEx(string RefLFZKFileName, string RefCFGFileName,string IPAddr);

        [DllImport(dllfile)]
        private static extern int PBusDisConnectEx(string IPAddr);

        [DllImport(dllfile)]
        private static extern int PBusTransferPLUCluster(TPLU[] PPC);

        [DllImport(dllfile)]
        private static extern int PBusClearPLUData();

        [DllImport(dllfile)]
        private static extern int PBusUpTr(string SerialNOStr, string AccountFile);

        [DllImport(dllfile)]
        private static extern int PBusTransferHotkey(int[] HotkeyTable, int TableIndex);

and the part where the error comes out:
  private void button12_Click(object sender, EventArgs e)
        {
            int d;
            int index = 0;
            string ipaddress = textBox1.Text.ToString();
            PPLUCluster = new TPLU[2000];
          //  IntPtr ptrcluster = Marshal.AllocHGlobal(Marshal.SizeOf(PPLUCluster));
            d = PBusConnectEx("", "", ipaddress);
           // Marshal.StructureToPtr(PPLUCluster, ptrcluster, true);
            try
            {
                if (d < 0)
                {
                    MessageBox.Show("connection fail");
                }
                else
                {
                    while (true)
                    {
                      //  int index = 0;
                        d = PBusUpLoadPLU(index, PPLUCluster); //HERE IS WHERE FAILS
                        if (d < 0)
                        {
                            MessageBox.Show("Upload plu fail");
                            break;
                        }
                        if (d == 9) //Upload OK
                        {
                            MessageBox.Show("Upload OK");
                            break;
                        }
                        index++;
                    }
                }
            }
            finally
            {
                PBusDisConnectEx(ipaddress);
            }
        }

    }

}