Win32 Erro 998 when calling unmanaged code

Hi

I'm making a call to an unmanaged C++ library in order to retrieve data from a device, I have tried numerous different methods but can't seem to get the call to work.

I'm pretty to new to making calls to unmanaged code.

Here's what I've got:

Code:

---


[DllImport("foo", EntryPoint = "USBReadData", CallingConvention= CallingConvention.StdCall, SetLastError = true)]
public static extern Boolean USBReadData(byte EPNumber, long BytesToRead,  MY_STRUCT DataBuffer);


[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
  public struct MY_STRUCT{
  [MarshalAs(UnmanagedType.I2)]
  public Int16 Value1;
  [MarshalAs(UnmanagedType.I2)]
  public Int16 Value2;
  [MarshalAs(UnmanagedType.I2)]
  public Int16 Value3;
}


internal void readDataFromDevice(long length){
  MY_STRUCT results = new MY_STRUCT();
                       
  if (USBReadData(1, length, results) == true) {
    //Great - its working!!
    ;
  }else{
    System.Diagnostics.Debug.Print(Marshal.GetLastWin32Error().ToString());
    throw new Exception("Could not read data.");
  }
}

---

Using this I get a Win32 998 error.

BTW the length of the structure is known in advance

I also tried to do it using a pointer like this:

Code:

---


[DllImport("foo", EntryPoint = "USBGetCommand", CallingConvention= CallingConvention.StdCall, SetLastError = true)]
public static extern Boolean USBGetCommandWithBuffer(byte Cmd, int wLen, IntPtr pData);

internal void readDataFromDevice(long length){
  byte[] results = new byte[6];
  IntPtr ptr = Marshal.AllocHGlobal(results.length);
  Marshal.Copy(results,0, ptr, results.Length);
                       
  if (USBReadData(1, length, ptr) == true) {
    Marshal.FreeHGlobal(ptr);
  }else{
    throw new Exception("Could not read data.");
  }
}

---

When I run this one it freezes at if (USBReadData(1, length, ptr) == true) and occasionally gives me an "Out of memory" error.

Finally I've tried just passing an array like so:

Code:

---

[DllImport("foo", EntryPoint = "USBReadData", CallingConvention= CallingConvention.StdCall, SetLastError = true)]
public static extern Boolean USBReadData(byte EPNumber, long BytesToRead, short[] DataBuffer);

internal void readDataFromDevice(long length){
  short[] results = new short[57];
                       
  if (USBReadData(1, length, results) == true) {
    System.Diagnostics.Debug.Print("We're here!");
  }else{
    System.Diagnostics.Debug.Print(Marshal.GetLastWin32Error().ToString());
    throw new Exception("Could not read test results.");
  }
}

---

The first time I ran this I got "We're here!" as output but every subsequent time I have run it I get the same Win32 998 error.

Any help would be very much appreciated.

Thanks