Thread: Help with using c++ class called with P/Invoke

Hi,

I've successfully been able to call C++ function into my C# ASP.NET application using P/Invoke. However, calling and using c++ classes from my DLL has been quite troublesome. Here is a small example I'm working with:

Default.aspx.cs:

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Runtime.InteropServices;
 
namespace SampleWebApp{
 
    public partial class _Default : System.Web.UI.Page{
 
        [StructLayout(LayoutKind.Sequential)]
        class ClassWrapper{
 
            [DllImport("MyDll.dll")]
            public static extern Int32 getANumber();
 
            public Int32 getNum(){
 
                return ClassWrapper.getANumber();
            }
 
            [DllImport("MyDll.dll")]
            public static extern ClassWrapper returnByRef();
        };
 
        protected void Page_Load(object sender, EventArgs e){
 
            ClassWrapper c = ClassWrapper.returnByRef(); // seems to work fine
 
            textArea.InnerHtml = c.getNum().ToString();   // Compiler complains no entry point could be found in dll for getANumber()
        }
    }
}

MyDll.h:

Code:

extern "C"{
 
    class __declspec(dllexport) myClass{
 
        public:
 
            myClass(void);
            long getANumber(void);
    };
 
    __declspec(dllexport) class myClass* returnByRef();
}

and MyDll.cpp:

Code:

#include "stdafx.h"
#include "MyDll.h"
 
extern "C"{
 
    __declspec(dllexport) myClass::myClass(){
 
        return;
    }
 
    __declspec(dllexport) long myClass::getANumber(){
 
        return 5;
    }
 
    __declspec(dllexport) class myClass* returnByRef(){
 
        myClass* temp = new myClass();
        return temp;
    }
}

Like I said, functions I've had no problems with, but I really can't figure out what to do about classes( how to intantiate and call member functions, etc.)

Any help/guidance you can offer me is most appreciated.

Thank You

Well for anyone who is interested I've worked one solution out (not to say it's the best solution though)

MyDll.h:

Code:

extern "C"{
 
    class __declspec(dllexport) MyClass{
 
        public:
 
            MyClass( long a, long b );
 
            long getNumA();
            long getNumB();
            long getAvg();
 
        private:
 
            long numA, numB;
    };
 
    __declspec(dllexport) class MyClass* getMyClassRef( long a, long b );
 
    __declspec(dllexport) long getNumberA( class MyClass* refToMyClass );
 
    __declspec(dllexport) long getNumberB( class MyClass* refToMyClass );
 
    __declspec(dllexport) long getAverage( class MyClass* refToMyClass );
}

MyDll.cpp

Code:

#include "stdafx.h"
#include "MyDll.h"
 
extern "C"{
 
    MyClass::MyClass( long a, long b )
    : numA(a_, numB(b){}
 
    long MyClass::getNumA(){
        return numA;
    }
 
    long MyClass::getNumB(){
        return numB;
    }
 
    long MyClass::getAvg(){
        return (numA+numB)/2;
    }
 
    __declspec(dllexport) class MyClass* getMyClassRef( long a, long b ){
 
        MyClass* temp = new MyClass( a, b );
        return temp;
    }
 
    __declspec(dllexport) long getNumberA( class MyClass* refToMyClass ){
 
        return refToMyClass->getNumA();
    }
 
    __declspec(dllexport) long getNumberB( class MyClass* refToMyClass ){
 
        return refToMyClass->getNumB();
    }
 
    __declspec(dllexport) long getAverage( class MyClass* refToMyClass ){
 
        return refToMyClass->getAvg();
    }
}

Default.aspx.cs:

Code:

using ...
using System.Runtime.InteropServices;
 
namespace SampleWebApp{
 
    public partial class _Default : System.Web.UI.Page{
 
        [StructLayout(LayoutKind.Sequential)]
        class MyWrapperClass{
 
            Int32 a, b;
 
            [DllImport("MyDll.dll")]
            public static extern MyWrapperClass getMyClassRef( Int32 x, Int32 y );
 
            [DllImport("MyDll.dll")]
            public static extern Int32 getNumberA( MyWrapperClass mwc );
 
            [DllImport("MyDll.dll")]
            public static extern Int32 getNumberB( MyWrapperClass mwc );
 
 
            [DllImport("MyDll.dll")]
            public static extern Int32 getAverage( MyWrapperClass mwc );
        };
 
        protected void Page_Load(object sender, EventArgs e){
 
            MyWrapperClass m = MyWrapperClass.getMyClassRef( 50, 100 );
            textArea1.InnerHtml = MyWrapperClass.getNumberA( m ).ToString();
            textArea2.InnerHtml = MyWrapperClass.getNumberB( m ).ToString();
            textArea3.InnerHtml = MyWrapperClass.getAverage( m ).ToString();
        }
    }
}

As far as I can tell, there is no way to instantiate an object declared within the DLL ( I assume because of CLR typing - but if you know differently, I'd really really like to know how you do it!)

So I basically had to write wrappers within the DLL to achieve this effect. It certainly doesn't seem perfect to me, so please somebody show me more or give me some pointers, I really need to know everything I can about marshalling data using P/Invoke.

Also, I read through this little gem:
Interop with Native Libraries - Mono

^an excellent site with A LOT of info.

Thank You