Hello All,

I've recently come accross a problem when trying to work with a
unmanaged dll compiled in C++ (Visual Studio 6.0). When calling the dll
from my C# application primitive data works okay for converting PLONG,
INT, BOOL to the right function signature. For example:

C++ Function:
LOGGER_API BOOL APIENTRY ConnectToDevice(void);
LOGGER_API LONG APIENTRY GetDeviceVersion(int iTargetMicro);
LOGGER_API LONG APIENTRY GetDeviceDataSize(void);

C#:
[DllImport("Logger.dll")]
private static extern bool ConnectToDevice();
[DllImport("Logger.dll")]
private static extern int GetDeviceVersion(int TargetMicro);
[DllImport("Logger.dll")]
private static extern int GetDeviceDataSize();

All calls invoke the dll perfectly and I get back the right response.
However, there is one particular method that has a method signature:

C++
BOOL GetDeviceData(LPBYTE HrmInputData, PLONG HrmDataSize, LPBYTE
GPSData, PLONG GPSDataSize);

The api docs for this method/function read: 'on successful upload the
API will copy the uploaded device data into the memory space indicated
by the two addresses being passes to this API as parameters. The size
of the uploaded data will also be placed into the variables whose
addresses have been passed.'

So I went ahead and made this method signature in C# to call the
unmanaged code:

[DllImport ("Logger.dll", SetLastError=true)]
public static extern bool GetDeviceData( [In,Out] byte[] array, ref
System.IntPtr hrmDataSize, [In,Out] byte[] gpsData, ref System.IntPtr
gpsDataSize);

C# Calls To UnManaged:
byte[] hrmData = new byte[deviceDataSize];
byte[] gpsData = new byte[deviceDataSize];
System.IntPtr hrmDataSize = new System.IntPtr(0);
System.IntPtr gpsDataSize = new System.IntPtr(0);
TimexHRMGPS.GetDeviceData(hrmData, ref hrmDataSize,  gpsData, ref
gpsDataSize);

The problem resides here, but its rather confusing. The call works
because I see the dll get executed and a progress bar from the dll
showing the status of the upload displays. On return however, I have
the right data for HRM data Size, and GPS data Size BUT NOT any data in
my byte[] array, its just an empty array. I tried using 'ref byte[]' to
simulate what I suspect it wants (a pointer to memory) and even
'Marshal.AllocCoTaskMem(hrmDataSize)' and locking it.

I am probably missing something retarded as I haven't used Interop
before today, but given my success with other functions I am not quite
sure what the method signature should read?

Sorry for the long winded explanation, I do hope you are able to help
as I have hit a wall.

Terrance A. Snyder