How to get IntPtr to a native directx11 struct?

Question

I'm trying to create (and fill) a D3D11Buffer in c++, pass a pointer to the buffers memory via interop to c# in order to read from that buffer using slimdx - no copies of the buffer via cpu should be involved.

here's my code so far

the function export:

extern "C" {__declspec(dllexport) HRESULT getValues(int * debug, ID3D11Buffer* bufOut); }

and the function:

ID3D11Device* d3d11Device;
ID3D11Buffer* triangleVertBuffer;
ID3D11DeviceContext* d3d11DevCon;

struct Vertex    //Overloaded Vertex Structure
{
    Vertex() {}
    Vertex(float x, float y, float z)
        : pos(x, y, z) {}

    XMFLOAT3 pos;
};

HRESULT getValues(int * debug, ID3D11Buffer* bufOut) {


    //Create the vertex buffer
    Vertex v[] =
    {
        Vertex(0.0f, 0.5f, 0.5f),
        Vertex(0.5f, -0.5f, 0.5f),
        Vertex(-0.5f, -0.5f, 0.5f),
    };

    D3D_FEATURE_LEVEL  FeatureLevelsRequested = D3D_FEATURE_LEVEL_11_0;
    UINT               numLevelsRequested = 1;
    D3D_FEATURE_LEVEL  FeatureLevelsSupported;

    //create device
    hr = D3D11CreateDevice(NULL, D3D_DRIVER_TYPE_HARDWARE, NULL, D3D11_CREATE_DEVICE_DEBUG, &FeatureLevelsRequested, numLevelsRequested, D3D11_SDK_VERSION, &d3d11Device, &FeatureLevelsSupported, &d3d11DevCon);
    debug[0] = hr;

    D3D11_BUFFER_DESC vertexBufferDesc;
    ZeroMemory(&vertexBufferDesc, sizeof(vertexBufferDesc));

    vertexBufferDesc.Usage = D3D11_USAGE_DEFAULT;
    vertexBufferDesc.ByteWidth = sizeof(Vertex) * 3;
    vertexBufferDesc.BindFlags = D3D11_BIND_VERTEX_BUFFER;
    vertexBufferDesc.CPUAccessFlags = 0;
    vertexBufferDesc.MiscFlags = 0;

    D3D11_SUBRESOURCE_DATA vertexBufferData;


    ZeroMemory(&vertexBufferData, sizeof(vertexBufferData));
    vertexBufferData.pSysMem = v;
    hr2 = d3d11Device->CreateBuffer(&vertexBufferDesc, &vertexBufferData, &triangleVertBuffer);


    bufOut = triangleVertBuffer;

    debug[1] = hr2;

    return hr;
}

in C#, I import the function like this:

[System.Runtime.InteropServices.DllImport("CreateID3D11Buffer.dll", CallingConvention = CallingConvention.StdCall)]
    private static unsafe extern UInt32 getValues([In, Out] int[] debug, [In, Out] IntPtr bufIn);

and then call it like that:

int[]debug = new int[2];
IntPtr bufPtr;// = IntPtr.Zero;
UInt32 hr = getValues(debug, bufPtr);
DataStream ds = new DataStream(bufPtr, sizeof(Vector3) * 3, true, false);

so far so good....now to my problems (bear with me, still c++ beginner and intermediate in c#...):

• it doesn't work: in c#, the last line, the DataStream constructor, throws an error, as bufPtr, the IntPtr to the unmanaged memory, is null. So, apparently, there's either something wrong with my p/invoke or in my c++ code...therefore, I looked at the hresult of the dx11 functions in c++ ->
• HRESULT: the return value hr of my imported function in c# equals 0, but both elements in the debug array equal -1! On the one hand I don't understand how to interpret negative values here, and on the other hand the return value hr should at least equal debug[0] (I'm aware their type differs)? So, i don't know what to make of this, but it indicates there's a problem with the function D3D11CreateDevice() and then (understandably) also with d3d11Device->CreateBuffer().
• i'm not sure if I understand slimdx's DataStream constructor correctly - the IntPtr to unmanaged memory points to graphics memory, or is this a function to read from unmanaged cpu memory? Unfortunately I can't find any examples using the IntPtr overload...here's the doc: https://slimdx.org/docs/html/M_SlimDX_DataStream__ctor_2.htm

So, am I doing this all wrong? :) If so, please point me in the right direction.

NOTE: most of the cpp code is extracted from https://www.braynzarsoft.net/viewtutorial/q16390-4-begin-drawing

Thank you for any help/hints, been trying to figure this out for days.

Answer 1

ID3D11Buffer* is a pointer. If you want to get back a pointer, you'll have to pass a pointer to a pointer, so define your C++ method like this:

HRESULT getValues(int * debug, ID3D11Buffer** bufOut)
{
    ...

    *bufOut = triangleVertBuffer;

    ...
}

And C# like this:

[DllImport("CreateID3D11Buffer.dll", CallingConvention = CallingConvention.StdCall)]
private static extern int getValues([In, Out] int[] debug, out IntPtr bufIn);

And this part should work.