Question

This is the situation. I am writing a backend application in C++ and my colleagues are using C# for the frontend. Basically, my backend application does image processing on images received from the C# frontend and returns a text file with data about the image.
What we were doing earlier was writing the images to disk and calling the C++ application with the file path of the image as a parameter. However, this writing/reading from disk has become a bottleneck, so we are wanting to convert the C++ application to a DLL.

At the basic level, what I want is to expose a single method from my C++ application to the C# one through a DLL (the C# application is web-based):

int ProcessImage(System::Bitmap^ image, System::String ^results)

How can I go about doing this? I am using VC++ Express 2008 and my C++ code currently compiles with CLR (although I have mixed a lot of native C++ code inside).

I have been looking around on the web, but am still stuck on how to do this. I think a C++/CLI DLL is the best option, but how do I do this?
Writing a DLL in C/C++ for .Net interoperability

Answer 1

I finally figured it out. What you need to do is:

1. In your C++ project, set the option in Visual C++ to compile with clr pure, which makes the project a C++/CLI one

2. In your C++ project, expose methods by creating a public ref class:

   public ref class Interop
   {
   public:
       //! Returns the version of this DLL
       static System::String^ GetVersion() {
           return "3.0.0.0";
       }
   
       //! Processes an image (passed by reference)
       System::Int32^ Process(Bitmap^ image);
   }
   

3. In a C# program, add a reference to the C++ DLL you compiled
4. In the C# program, call these methods (i.e. Interop::Process(myBitmapInCS))

I believe this method is called “implicit P/Invoke”
Hope this helps!