Retrieve C# .NET image from C++ unmanaged application

Question

I'm using openFrameworks to render a video stream. OpenFrameworks uses the ofImage class, which stores the images as a vector of ofPixel (which stores an unsigned char vector).

I am using a C# .NET dll to retrieve each frame. This library uses Bitmap to return the retrieved frames. So far I know C# .NET works under the Common Language Runtime.

My question is, what .NET tools or functionalities can I use in order to comunicate native C++ code and managed C#?

Answer 1

In order to retrieve the image I used this tutorial. It thoroughly explains how to consume a C# managed library from unmanaged code.

The process I followed is documented here.

Here's an overview

• create a C++/CLI project (CLR Class Library in Visual Studio's New Project dialog)
• add a reference to the C# dll create native classes and/or functions that expose the necessary C# dll functionality to the native world;
• export them using the usual means (__declspec(dllexport) or .def file)
• add the .lib produced by the C++/CLI project as linker input in your native project(s)

About the image information, it can be referred to as a byte array. It has to be pinned in the managed code using a GCHandle. Here is some code to illustrate this concept (Disclaimer, this answer was provided by another author in the original msdn forum post):

#include <unordered_map>

using namespace System;
using namespace System::Runtime::InteropServices;

std::unordered_map<void *, void *> dataHandles;

void *getData() {
    array<Byte>^ arr = gcnew array<Byte>(2);
    arr[0] = 'C';
    arr[1] = '#';

    GCHandle handle = GCHandle::Alloc(arr, GCHandleType::Pinned);
    void *data = (void *)handle.AddrOfPinnedObject();
    dataHandles.insert(std::make_pair(data, (void *)GCHandle::ToIntPtr(handle)));
    return data;
}

void releaseData(void *data) {
    auto i = dataHandles.find(data);

    if (i != dataHandles.end())
        GCHandle::FromIntPtr((IntPtr)i->second).Free();
}

#pragma unmanaged

void native() {
    char *data = (char *)getData();

    for (int i = 0; i < 2; i++)
        printf("%c ", data[i]);

    releaseData(data);
}

#pragma managed

int main(array<System::String ^> ^args) {
    native();
    return 0;
}

More about specific Bitmap to unsigned char * conversion here