How can I use returns std::vector<std::vector<double>> function DLL C ++ to C# marshling?

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I searched in C++, but unfortunately wasn't able to find any information on how I can integrate the following function using the external “C” declspec (dllexport) and call this function in C++. Here's what I tried:

C++

extern "C" declspec(dllexport) std::vector<std::vector<double>> 
cdecl PrepareFaceBank(std::string Path)

But unfortunately, I received the following error. I have no idea where to go from here. Thank you very much for your help in advance.

C++

extern "C" declspec(dllexport) std::vector<std::vector<double>> 
cdecl PrepareFaceBank(std::string Path)
{
    try
    {
        //load model (if it has been loaded anywhere else, just comment it)
        FaceModule = torch::jit::load(FaceModulePath);

        std::cout << "Model loaded successfully" << std::endl;
        //Disable grad for high performance because this is a pretrained model
        torch::NoGradGuard no_grad;
        //an array to save all persons tensor in it
        std::vector<torch::Tensor> embeddings;
        //iterate through facebank directory
        for (const auto& name : get_folderPath(Path))
        {
            std::cout << "name :" << name.filename() << ": " << '\n';
            
            //array to save all of the photos of each person
            std::vector<torch::Tensor> embs;
            //clear before iterating through all photos of each person
            embs.clear();
            //int a = 0;
            for (const auto& fileName : get_filenames(name))
            {
                //std::cout <<'\t' << fileName << std::endl;

                Mat image = imread(fileName);
                
                /* Display if needed:
                cv::namedWindow("Display window");// Create a window for display.
                cv::imshow("Display window", image);
                cv::waitKey(0);
                */
                
                embs.push_back(ProcessFrame(image));
                //empty cache 
                image.release();
                //how many photos each person has
                //std::cout << '\t' << a++ << std::endl;
            }

            //number of photos in database for this person
            //std::cout << "embs size: " << embs.size() << "\n";
            //convert vector to tensorlist and then calculate the Mean value 
            //for all of the photos of each person
            torch::Tensor embedding; // 1*512
            torch::TensorList tensor_list{ embs };
            embedding = torch::mean(torch::cat(tensor_list), 0);
            
            embeddings.push_back(embedding);
        }
        std::vector<double> EachPersonVec;
        std::vector<std::vector<double> > retValue;
        for (const auto& n : embeddings)
        {            
            EachPersonVec.clear();
            for (int i = 0; i < 512; i++)
            {
                EachPersonVec.push_back(n[i].item().to<double>());
            }
            
            retValue.push_back(EachPersonVec);
        }
        //number of persons in database
        std::cout << "embeddings size : " << embeddings.size() << "\n";
        
        return retValue;
    }
    catch (const std::exception& e)
    {
        std::cout << e.what() << std::endl;
    }
}

What I have tried:

C++

extern "C" declspec(dllexport) std::vector<std::vector<double>> cdecl PrepareFaceBank(std::string Path)

Posted 23-Jan-22 23:24pm

Member 14980057

Updated 25-Jan-22 1:39am

Deeksha Shenoy

v3

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Comments

Richard MacCutchan 24-Jan-22 5:29am

What error do you see and where is the C# code that is supposed to use this function?

Member 14980057 24-Jan-22 5:47am

Severity Code Description Project File Line Suppression State
Error C2526 'PrepareFaceBank': C linkage function cannot return C++ class 'std::vector<std::vector<double,std::allocator<double>>,std::allocator<std::vector<double,std::allocator<double>>>>' detect-camera-demo D:\Project\Face\libfacedetection\example\Trian.cpp 233

Richard MacCutchan 24-Jan-22 8:31am

Well that is quite clear. You cannot return C++ class types from standard C code. You also cannot pass those objects to a C# program.

Member 14980057 24-Jan-22 12:09pm

Thanks, so what can I do? I must use std :: vector <std ::="" vector="" <double="">> in C # because my goal is to create a 512 vector and call it in C #. Can I convert a Jason file?

The purpose of this code that I wrote is to come to identify faces for people that the user wants to teach or add people to the folder photos so that I can recognize them later.

Richard MacCutchan 24-Jan-22 12:13pm

The basic issue is that you cannot use C++ classes to pass data back to C# programs. So whatever the application and the library are doing, you need to find a data format that both can understand. You can find more useful information at Platform Invoke (P/Invoke) | Microsoft Docs[^].

Member 14980057 24-Jan-22 12:21pm

thank you . Can't I write a wrapper and use it in C #? If yes, how do I do this?

Richard MacCutchan 24-Jan-22 12:37pm

Yes, you can write anything you want, but you still have to follow the rules. The link I gave you is where you will find the information.

Member 14980057 24-Jan-22 13:07pm

Thanks . If you have a link to write a wrapper for vector, please introduce it to me. Thank you. I am a beginner, I have no idea.

Richard MacCutchan 24-Jan-22 13:13pm

You cannot use a vector (or any other C++ type), as I have explained a number of times. You can only use simple objects that are understood by C# and C, such as byte, integer etc. The link above shows you how to manage them.

merano99 25-Jan-22 9:06am

See my solution for a wrapper that wraps the C++ data for delivery via a C DLL. Someone in the .NET forum probably knows how to unpack it in C#.

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KarstenK · Answer 1 · 2022-01-24T00:16:00

Solution 2

You shouldnt return complex data between the runtimes but plain data types or byte arrays. Read my article with sample code to understand the basics and get it going.

Posted 24-Jan-22 0:16am

KarstenK

Comments

Member 14980057 24-Jan-22 6:34am

@KarstenK Thanks . I've done this before, but I will probably need to return the function I call in C # like std :: vector <std ::="" vector="" <double=""> Can you suggest a way? Thank you

merano99 · Answer 2 · 2022-01-24T14:32:00

As has already been said, simple data types are required for the transfer to a C - DLL.So you have to copy everything.
This is somewhat annoying and you also need a function at the end that cleans up the stuff.

C++

typedef struct FACESTRUCT {
    size_t n;
    FACESTRUCT *next;
    double *dat;
} FACETYPE;

extern "C" __declspec(dllexport) FACETYPE* PrepareFaceBankC(char* path);

std::vector<std::vector<double>> cdecl PrepareFaceBank(std::string Path);

FACETYPE* PrepareFaceBankC(char* path)
{
    std::vector<std::vector<double>> facebank;
    std::string mypath(path);
	FACETYPE *myhead = NULL, *mylst, *tmp;

	// first call the original function
    facebank = PrepareFaceBank(mypath);

	// then copy the result to simple datatypes
    size_t n = (facebank.size());  // number of elements in the vector

    for (size_t i = 0; i < n; i++) {
        // fetch next record 
        std::vector<double> mydat = facebank.at(i);
        // get mem to copy it
        tmp = (FACETYPE*)malloc(sizeof(FACETYPE) * n);
        tmp->next = NULL;
        tmp->n = mydat.size();

        // get mem for array of double
        tmp->dat = (double*)malloc(sizeof(double) * tmp->n);

        for (size_t k = 0; k < tmp->n; k++) {
            tmp->dat[k] = mydat.at(k);   // copy over double
        }

        if (myhead == NULL) {
            myhead = tmp; mylst = tmp;
        }
        else {
            mylst->next = tmp; mylst = tmp;
        }
    }
    return myhead;
}

This is just a rough draft to show how it might be done...