GetLastError = 998 when calling unmanaged code for the second time, first time works

I am accessing a third party DLL that drives a USB device. The first time I access the DLL I can receive the results of the activity on the device without a problem. If I run the same test again I receive a null value and GetLastError is 998 (error_noaccess).

The first time I run the test after a reboot I am consistently able to access the variable - the next time I run the test I cannot. Rebooting appears to be the only way to resolve the issue - restarting the application or the development environment do not resolve this issue.

I am using .net 3.5 running in SharpDevelop.

Here is the code:

I wrote a wrapper class for all the calls to the USB driver:

Code:

---

public class USBWrapper
{
        [DllImport("USBDriver", CallingConvention= CallingConvention.StdCall)]
        public static extern Boolean InitUSB();
               
        [DllImport("USBDriver", CallingConvention= CallingConvention.StdCall)]
        public static extern Boolean CloseUSB();
               
        [DllImport("USBDriver", CallingConvention= CallingConvention.StdCall)]
        public static extern Boolean USBSendCommand(byte Cmd);
               
        [DllImport("USBDriver", EntryPoint = "USBReadData", CallingConvention= CallingConvention.StdCall, SetLastError = true)]
        Public static extern Boolean USBReadData(byte EPNumber, long BytesToRead, short[] DataBuffer);


        //Codes sent to the device
        public const byte Start_TEST = 0x00;
        public const byte LEGGI_PACCHETTO = 0x01;
        public const byte Cod_VC = 0x02;
        public const byte Cod_MVV = 0x04;
        public const byte Cod_FVC = 0x08;
}

---

Here is the class that calls the wrapper:

Code:

---

public class USBDevice : IDisposable
{
        public bool startUSBDevice(){
                try {
                        return USBWrapper.InitUSB();
                } catch (Exception) {
                        return false;
                }
        }
               
        public byte[] startTest(){
                return startTest(USBWrapper.Start_TEST);
        }
               
               
        public byte[] startTest(byte testCode){
                int lengthOfStructure = 3;
                byte[] test;
                test = sendCommandToDevice(testCode, lengthOfStructure);
                return test;
        }
               
               
        internal byte[] sendCommandToDevice(byte commandToSend, int arrayLength){
                byte[] arrayForReturnValues = new byte[arrayLength];
                if (USBWrapper.USBGetCommand(commandToSend,arrayLength, arrayForReturnValues)) {
                        return arrayForReturnValues;
                } else {
                        throw new Exception("Failed to read command data from device.");
                }               
        }
               
        public void getTestResults(){
                try {
                        readDataFromDevice( 57 );
                } catch (Exception e) {
                        System.Diagnostics.Debug.Print(e.Message);
                }       
        }
       
        internal void readDataFromDevice(long length){
                short[] results = new short[57];               
                if (USBWrapper.USBReadDataWithArray(1, length, results) == true) {
                        System.Diagnostics.Debug.Print("I can read the test results.");
                }else{
                        System.Diagnostics.Debug.Print(Marshal.GetLastWin32Error().ToString());
                        throw new Exception("Could not read test results.");
                }
        }
        public void Dispose(){
                killUSBDevice();
        }

        public bool killUSBDevice(){
                try {
                        return USBWrapper.CloseUSB();
                } catch (Exception) {
                        return false;
                }
        }
}

---

Here is the nominal function that initiates the test (I've collapsed it to make reading easier):

Code:

---

public static void Main(){
        byte[] testStart;
        USBDevice device = new USBDevice();

        device.startUSBDevice();
        byte[] testStart = device.startTest();
//wait until you know that the test is complete in here       
        device.getTestResults();
        device.killUSBDevice();
        device=null;
        GC.Collect();
        GC.WaitForPendingFinalizers();
                       
}

---

The first time I run it I get an output saying: "I can read the test results.". The next time (and every subsequent time) I run it I get: "Could not read test results.".

Any ideas about how to fix this would be greatly appreciated.

Thanks

Please see my response here:

http://forums.codeguru.com/showthrea...43#post2075243

You may have messed up parameter passing by passing types that are not the same size that the DLL expects. Once you do that, then all bets are off as to how your program will behave.

Regards,

Paul McKenzie