Managed byte[] to unmanaged byte array using ATL/COM

Question

I want to pass some image data from C# code to unmanaged C++ using ATL/COM

From C# code side i do something like this:

void SendFrame([In, MarshalAs(UnmanagedType.SafeArray, SafeArraySubType=VarEnum.VT_UI1)] ref byte[] frameData);

But I'm not sure how should I handle this function in my C++ code.

For now I have something like this:

_ATL_FUNC_INFO OnSendFrameDef = { CC_STDCALL, VT_EMPTY, 1, { VT_SAFEARRAY | VT_UI1 } };

void __stdcall OnSendFrame(SAFEARRAY* ppData)
{
   BYTE* pData;
   SafeArrayAccessData(ppData, (void **) &pData);

   // Doing some stuff with my pData

   SafeArrayUnaccessData(ppData);
}

Can anyone give me some suggestions how I can make this thing work?

Thanks.

Answer 1

I've managed to achieve my goal! For those who are interested:

My event handler descriptor looks like this:

_ATL_FUNC_INFO Stream::OnStreamFrameCallbackDef = { CC_STDCALL, VT_EMPTY, 1, { VT_DISPATCH } };

My C++ function:

void __stdcall Stream::OnStreamFrameCallback(IDispatch* pFrame)
{
   // NOTE that this "IStreamFramePtr" is COM's Ptr of my "IStreamFrame"
   MyCOM::IStreamFramePtr pStreamFrame = pFrame;

   // Thanks casperOne♦ for this:
   CComSafeArray<byte> array;
                   array.Attach(pStreamFrame->GetBuffer());

   // Now I can do stuff that I need...
   byte* pBuffer = &array.GetAt(0);
}

My "IStreamFrame" in my .tlh file looks like this:

struct __declspec(uuid("1f6efffc-0ac7-3221-8175-5272a09cea82"))
IStreamFrame : IDispatch
{
    __declspec(property(get=GetWidth))
    long Width;
    __declspec(property(get=GetHeight))
    long Height;
    __declspec(property(get=GetBuffer))
    SAFEARRAY * Buffer;

    long GetWidth ( );
    long GetHeight ( );
    SAFEARRAY * GetBuffer ( );
};

In my C# code I have something like this:

[ComVisible(true)]
[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIDispatch)]
public interface IStreamFrame
{
    int     Width   { [return: MarshalAs(UnmanagedType.I4)]         get; }
    int     Height  { [return: MarshalAs(UnmanagedType.I4)]         get; }
    byte[]  Buffer  { [return: MarshalAs(UnmanagedType.SafeArray, SafeArraySubType=VarEnum.VT_UI1)]  get; }
};


[ComVisible(false)]
public delegate void StreamFrameCallback(IStreamFrame frame);

[ComVisible(true)]
[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIDispatch)]
public interface IMyCOMStreamEvents
{
    [DispId(1)]
    void OnStreamFrameCallback([In, MarshalAs(UnmanagedType.IDispatch)] IStreamFrame frame);
}

Things seems to work just fine. But if anyone have any suggestions or noticed that I'm doing something wrong please let me know.

Thanks.