How to return class from c++ code to c#

Question

I'm calling C++ method from C# code. Everything works fine except returning multiple parameters back to C#.

In my case those parameters are: int x, y, width, height;

What I want to do is to return a class or struct from c++ code to c#.

I have provided an example so it would be much more clear what is on my mind. I know that one way to go is to use Marshal class, maybe the only way.

C# code

public class ImageMatch
{
    //method that is used to call pass string parameters and call c++ method
    [System.Runtime.InteropServices.DllImport("ImageComputingWrapper.dll", CallingConvention = System.Runtime.InteropServices.CallingConvention.Cdecl)]
    static extern ImageComputingWrapper.ImageParams ComputeImage(string imgPath, string templPath);

    public  GetImgParams(string imgPath, string templPath)
    {
        //a class from C++ code
        ImageComputingWrapper.ImageParams imgParams;
        //retreive all the data
        imgParams = ComputeImage(imgPath, templPath);
    }
}

C++ code

//ImageComputingWrapper.cpp
extern "C" __declspec(dllexport) ImageComputingWrapper::ImageParams ComputeImage(const char* imgPath, const char* templPath)
{
    computeImage* compImage = new computeImage(imgPath, templPath);
    ImageComputingWrapper::ImageParams imageParams;

    imageParams.x = compImage->x;
    imageParams.y = compImage->y;
    imageParams.width = compImage->width;
    imageParams.height = compImage->height;

    return imageParams;
}

//ImageComputingWrapper.h
//class to return back to c#
public ref class ImageParams
{
    public:
        ImageParams(){}
        int x;
        int y;
        int width;
        int height;
};

I do know that it is not possible to return class from C++ code to C# as it's in this example. It is just to easily understand what I meant.

One thing to point out, I am a C# programmer so there may be something wrong in that C++ code (pointers).

That's a System::Drawing::Rectangle. The point of using C++/CLI is to not have to use [DllImport] in your C# program. Simply add a reference to your C++/CLI project. You'll only see ImageParams now, you already know how to write a real ImageComputingWrapper — Hans Passant
– Hans Passant, Commented Feb 2, 2015 at 21:04

David Heffernan · Accepted Answer · 2015-02-02 21:43:06Z

5

You cannot return a ref class using p/invoke. What you can do is declare a ref class in your C++/CLI assembly and simply consume that from C#.

First of all you need a C++/CLI class library. For instance:

// ClassLibrary1.h

#pragma once

using namespace System;

namespace ClassLibrary1 
{
    public ref class Class1
    {
    public:
        int x;
        int y;
        int width;
        int height;
    public:
        Class1() : x(42), y(666), width(24), height(13) {}
    };
}

You can then consume this class library like any other managed assembly:

using System;
using ClassLibrary1;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Class1 instance = new Class1();
            Console.WriteLine(instance.x);
            Console.WriteLine(instance.y);
            Console.WriteLine(instance.width);
            Console.WriteLine(instance.height);
        }
    }
}

And that's all there is to it.

You ask in comments how to pass string arguments to the C++/CLI code. Use System::String^ on the C++/CLI side. That's the C++/CLI way of referring to the .net string type. So your constructor might become:

public ref class Class1
{
....
public:
    Class1(System::String^ imgPath, System::String^ tempPath)
    {
        ....
    }
};

On the C# side you would create the instance like this:

string imgPath = "...";
string tempPath = "...";
Class1 instance = new Class1(imgPath, tempPath);

edited Feb 2, 2015 at 21:43

answered Feb 2, 2015 at 20:46

David Heffernan

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4 Comments

Mr. Blond Over a year ago

Yes this is really nice solution +1 @David Heffernan. But one reason why I'didn't chose the class constructor is because I couldn't figure out how to pass string parameters imgPath and templPath from c# to c++. The only way that I found was using dllimport. I should highligh this issue more.

David Heffernan Over a year ago

In C++/CLI use System::String^, in C# use string. Both of those refer to the same type, the .net string type. I'll show you in an update.

Mr. Blond Over a year ago

You are dam right man! I always like to make things more complicated than they are, But I searched for this some time and couldn't find. Maybe this post will help someone else to not do the same mistake that I did.

David Heffernan Over a year ago

Thanks. Sometimes it is hard to see the wood for the trees!

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