Interop: Returning a pointer from C#

daleh@hill-eng.com · #1 09-01-2010, 11:30 PM

All,

My question is:
Is it possible to return a pointer allocated in managed code to unmanged code.

I have an unmanged C++ application that calls into a C# dll. The C# dll is build as a CCW and the unmanaged app is build with /clr. I would like to have the unmanaged app be able to get an unmanaged pointer from the C# dll and then read its content. The problem I am running into is that the pointer gets moved so that the unmanged code asserts.

I've seen articles referring to GCHandle but I can't figure out how to call it. Does the managed code call get the handle or the unmanaged code?

A sample of the code is below.

Any help on this would be greatly appreciated.

TIA,
Dale

Code:

// Unmanaged application
int _tmain(int argc, _TCHAR* argv[])
{
      // Get a pointer to the CCW Object
     GBVssNetInterface::_GBVssDeviceComm *pNetInterface;
     CoInitialize(NULL);

     GBVssNetInterface::_GBVssDeviceCommPtr p(__uuidof   (GBVssNetInterface::GBVssDeviceComm));
     pNetInterface = p;

     __int64 pLuns;
     pNetInterface->GetAllVisableLuns(&size, (LONG*)&pLuns);

     // Blows up
     GlobalFree((HGLOBAL)pLuns);
     return 0;
}


// Managed object
namespace GBVssNetInterface
{
    [Guid("D6F88E95-8A27-4ae6-B6DE-0542A0FC7039")]
    [ComVisible(true)] 
    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    public interface _GBVssDeviceComm
    {
        [DispId(1)]
        //unsafe int GetAllVisableLuns(int* size, char* pLunBuffer);
        unsafe int GetAllVisableLuns(ref int size, ref Int64 pLunBuffer);
    }
 
    [Guid("4CB08BDA-7F12-458a-AE0B-B6C4E4441822")]
    [ComVisible(true)] 
    [ClassInterface(ClassInterfaceType.None)]
    [ProgId("GBVssNetInterface.GBVssDeviceComm")]
    public class GBVssDeviceComm : _GBVssDeviceComm
    {
        public GBVssDeviceComm() {}

        public unsafe int GetAllVisableLuns(ref int Size, ref Int64 pLunBuffer)
        {
            IntPtr lunBuffer = Marshal.AllocHGlobal(size);
            pLunBuffer = lunBuffer.ToInt64();
            return 0;
        }
    }
}