Question

I have this code in a c++ file, with compiles to a dll.

#include "stdafx.h"

#include "WHUU.h"

#include "stdafx.h"

typedef int (__stdcall * Callback)(const int text);

static int x , y= 0;

Callback Handler = 0;

int getX()
{
    return x;
}

void setX(int i)
{
    x = i;
}

void setY(int i)
{
    y = i;
}
int getY()
{
    return y;
}
extern "C" __declspec(dllexport)
void __stdcall SetCallback(Callback handler) {
  Handler = handler;
}


extern "C" __declspec(dllexport)
void __stdcall addX(int x) {
    setX(x);
}

extern "C" __declspec(dllexport)
void __stdcall addY(int y) {
    setY(y);


}


extern "C" __declspec(dllexport)
void __stdcall TestCallback() {
    int z = getX() + getY();
  int retval = Handler(z);
}

My c# application now has to load this dll on runtime. Add to the callback and call the functions. I dont want to use a class. I could load the class and with

 Type[] types = moduleAssembly.GetTypes();

But this overkill! Also c++ is not managed.

I mean its so tiny! (and yes this is an example , but the “real” is just as big as this example).

How do i do that?

Thanks you for your help!

add:

• i dont like frameworks (like pinvoke / assembly)

• the function names / types are fixed and will never change (think of a driver.dll read write)

• this dll is written by customers so it should be as easy as possible!

Answer 1

I used P/Invoke, post my own cause the callback was not included by the others. But they were good answers! The C# Applicationhas use this GUI.

The Code for the Application is this : (mind it is a prototype to answer a technical question)

        public partial class Form1 : Form
        {

        private delegate int Callback(int text);
        private Callback mInstance;   // Ensure it doesn't get garbage collected

        [DllImport(@"C:\Visual Studio 2010\Projects\kalkulatorius\Debug\WHUU1.dll")]
        private static extern void SetCallback(Callback fn);
        [DllImport(@"C:\Visual Studio 2010\Projects\kalkulatorius\Debug\WHUU1.dll")]
        private static extern void addX(int x);
        [DllImport(@"C:\Visual Studio 2010\Projects\kalkulatorius\Debug\WHUU1.dll")]
        private static extern void addY(int y);
        [DllImport(@"C:\\Visual Studio 2010\Projects\kalkulatorius\Debug\WHUU1.dll")]

        private static extern void TestCallback(); 

        private int Handler(int text) 
        {
          textBox3.Text = text.ToString();
          return 42;                       
        }


        private void button1_Click(object sender, System.EventArgs e)
        {
          mInstance = new Callback(Handler); // to set the callback in lib 
          SetCallback(mInstance);            // could also be withhin constructor!

          addX(int.Parse(textBox1.Text)); // other people in this thread posted this correct answer
          addY(int.Parse(textBox2.Text));
          TestCallback();
        }

        public Form1()
        {
          InitializeComponent();
        }

So if you want to use a c++ lib with functions and callbacks (like in the question posted) you can use this.

But how to change the lib?

• The Path to the lib is hardcoded (see [dllImport ……] ). But you
  can swop the lib in filesystem. This can happen AFTER you build this
  application.
• If the lib is not on the given path an exception is thrown. So if
  your program wants to use this lib first check the lib is present on the filesystem.
  (not included in this simple prototype)
• So to switch functionality (swop a driver lib ect) you copy a different lib with the
  same name into the given path. (Copy paste – maybe change name)

This solution has the least overhead and is good if you never change the interface of an dll after the application was build!