Writing a C++ DLL that a C# .NET Application can call?

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

possible duplicate of C++/CLI Mixed Mode DLL Creation

Hans Passant
– Hans Passant

2012-03-23 00:15:17 +00:00
Commented Mar 23, 2012 at 0:15 — Hans Passant
– Hans Passant, Commented Mar 23, 2012 at 0:15

Nick Whaley · Accepted Answer · 2012-03-23 00:40:11Z

2

If you label your C++ function as a "C" function, then it can be called by PInvoke.

Your C function will look like this:

extern "C" __declspec(dllexport) int Test() {
    return 1;
}

Your C# reference will be like this:

[DllImport("test.dll")]
public static extern int Test();

If you C++ function is an instance member, then you can still do it but it requires some tricks.

answered Mar 23, 2012 at 0:40

Nick Whaley

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4 Comments

Joel Lucsy Over a year ago

You should also add __stdcall to the C++ function, or CallingConvention to the DllImport

01100110 Over a year ago

I believe that extern "C" prevents function/method name mangling.

Ben Voigt Over a year ago

Only functions in the global namespace can be exported in a way that's compatible with p/invoke. Forget about anything having to do with classes, whether you mean member functions of classes (static or not) or class types in parameters or return values.

Amil Over a year ago

Thanks for the response, but I don't think this will work with what I am trying to do. How can I use this method to pass the image by reference and receive results as a string?

Amil · Accepted Answer · 2012-03-29 05:55:38Z

1

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

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

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);
}

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

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

answered Mar 29, 2012 at 5:55

Amil

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