C# method calls within Java Program

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Introduction

Have you ever wanted to (for some reason or another) needed to use C# from within a Java program?
Ever wondered how you could possibly make C# calls from within Java? Well, I try to explain what is needed
in this whole process.

Background

I was working on some Keyboard Hooking problems and found it very easy to develop using C#. The problem was,
that I was trying to develop a solution (originally) for a Java based program. After much Internet searching, I finally
stumbled onto a really good article that started to put the pieces together. The article "Experience in integrating
Java with C# and .NET" by Judith Bishop, R. Nigel Horspool and Basil Worrall was the insight I needed to start this
project.

That article does show a method to using C# within a Java program, however I didn't find it was easy to understand the
whole process. After I actually had C# working within a Java program, I realized that the process could become a
Codeproject article (my first article).

Using the code

Figure is taken from the article "Experience in integrating
Java with C# and .NET" by Judith Bishop, R. Nigel Horspool and Basil Worrall

I guess the best way to use this code is to experiment with, use it as a template for a project. I obviously
didn't do anything terribly complicated by just putting up a "Hello World" example, but I wanted people to see
the easiest way possible first. The sample code is simply a console based java program which will show the
"Hello World, from C#" message. I'm also including all the source code and projects in all 4 languages.

Why the need for so many wrappers you may ask? Well, several layers of wrapping are needed because the
DLL produced by compiling a C# program into a library is not compatible with Java's JNI. The double layer of
C++ above is required because the DLL with which the JNI subsystem interacts with has to be static, procedural
code (not object oriented). So, the first layer is essentially C code. Fortunately, static C code will accommodate
(static) pointers to C++ objects. It's doubtful whether the static C code could interact direction with a C# object,
since there are things like garbage collection to consider.

Java Section

I was using Netbeans 4.0 as my main Java IDE at the time, and because of that, the process of using
native code was a little more complicated. The reason of that, is because a Netbeans project wants to
put code inside packages rather than just using the default namespace. The javah console program however
doesn't look at the package and therefore doesn't correctly make the header in the right namespace.
So, as I found out, there's a simple two second fix. Which just requires you edit the generated header
file to correctly make the connection through JNI to the native methods in C++.

Here is the only Java code, which resides in the helloworld package:

public class HelloWorld {
    public native void displayHelloWorld();
    static {
        System.loadLibrary("HelloWorld");
    }
    
    public static void main (String[] args) {
        new HelloWorld().displayHelloWorld();
    }
}

So then, to make this into the header which is needed by the C++ Library, this command needed to be run:

javah -jni helloworld.java

That step produces the following header:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>

/* Header for class HelloWorld */

#ifndef _Included_HelloWorlds
#define _Included_HelloWorld
#ifdef _cplusplus
extern "C" {
#endif
/*
 * Class:        HelloWorld
 * Method:        displayHelloWorld
 * Signature:    ()V
 */
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

However, if we don't change the generated header then the JNI call will fail as the method has a
different signature due to NetBeans and the package. So the JNIEXPORT line is changed to:

JNIEXPORT void JNICALL Java_helloworld_HelloWorld_displayHelloWorld (JNIEnv *, jobject);

C++ Library

The purpose of the C++ library is to become the JNI Wrapper for the call (through the Managed C++ wrapper) to the endpoint C# code.

#include <jni.h>


// This is the java header created using the javah -jni command.

#include "Java\HelloWorld.h"


// This is the Managed C++ header that contains the call to the C#

#include "MCPP\HelloWorld.h"


// This is the JNI call to the Managed C++ Class

// NOTE: When the java header was created, the package name was not included

//       in the JNI call. This naming convention was corrected by adding the 

//         "helloworld" name following the following syntax: 

//       Java_<package name>_<class name>_<method name>

JNIEXPORT void JNICALL Java_helloworld_HelloWorld_displayHelloWorld(JNIEnv *jn, jobject jobj) {

    // Instantiate the MC++ class.

    HelloWorldC* t = new HelloWorldC();

    // The actual call is made. 

    t->callCSharpHelloWorld();
}

Managed C++ Library

#using <mscorlib.dll>
#using "CSharpHelloWorld.netmodule"

using namespace System;

public __gc class HelloWorldC
{
    public:
        CSharpHelloWorld __gc *t; // Provide .NET interop and garbage collecting to the pointer.

        HelloWorldC() {
            t = new CSharpHelloWorld(); // Assign the reference a new instance of the object

        }
        
     // This inline function is called from the C++ Code

        void callCSharpHelloWorld() {
            t->displayHelloWorld();
        }
};

C# Library

The step that is required for the interaction between Managed C++ and C# is that the CSharpHelloWorld
library be compiled as a .netmodule. To do this, the following command must be run from either the
console or as a post-build event within the C# project (I streamlined the process by using post-build):

csc /debug /t:module "$(OutDir)\$(ProjectName).dll"

using System;

public class CSharpHelloWorld
{
    public CSharpHelloWorld() {}

    /// <summary>

    /// displayHelloWorld will be the method called from within java.

    /// </summary>

    public void displayHelloWorld() 
    {
        Console.WriteLine("Hello World, from C#!");
    }
}

Points of Interest

Working with Java JNI is quite a pain, especially by using netbeans as an IDE. Hopefully, I've given you
some insight as to what to expect at the different steps of this process. I must have hit every single
roadblock in this project, and taken the time to look up what was needed.

History

Version 1.0 - First Release!