Java to C# through JNI fails to load C# dll

Question

I'm working on building a Java wrapper around a C# project. I'm using C++ to go in between Java and C#. My problem is that when I run my Java app I'm getting a "Could not load file or assembly 'MyC#DLL, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified." error. If I remove the call in C++ to the C# code things load fine, as far as the Java to C++ part, but when I try to make the call to C# that dll fails to load. I created a C++ test class, external to the wrapper C++ code, to try out the call and everything works fine from there. The C# dll load and runs as expected. So I'm guessing it's a problem with my Java project set up but I can't seem to find it.

I tried using Process Monitor to make sure that the C# dll isn't loading and it isn't. The c++ wrapper does load correctly.

My Java code is executing from the project folder but everything else (C++ and C#) are in the same directory. Any help is appreciated.

Java Code:

public final class MyClass{

static {
    try
    {
        System.load("C:\\Dir\\CppBridge.dll");
    }
    catch(Exception e)
    {
        System.out.println("-!-!- Error loading library -!-!-");
    }

}

private static native String getXML();

public static List<Site> getXMLFromC#()
{
    String xml = MyClass.getXML();
}}

C++ Code (Methods.h):

public ref class Wrapper
{
    public: MyPackage::C#Project^ api;
    public: Wrapper()
    {
        api = gcnew MyPackage::C#Project;
    }

    public: char* getXML()
    {
        char* error;
        try
        {
            System::String^ xml = api->getXML();
            return (char*)Marshal::StringToHGlobalAnsi(xml).ToPointer();
        }
        catch (Exception^ e)
        {
            char* error = (char*)Marshal::StringToHGlobalAnsi("Error:" + e->Message).ToPointer();
        }
        return error;
    }
};

C++ Code (CppBridge.h)

#include <jni.h>
/* Header for class MyC#Class*/

#ifndef _Included_MyC#Class
#define _Included_MyC#Class
#ifdef __cplusplus
extern "C" {
#endif
    /*
    * Class:     MyC#Class
    * Method:    getUserSites
    * Signature: String, String, String, String, String, boolean
    */
    JNIEXPORT jstring JNICALL Java_MyPackage_getXML(JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

C++ Code (CppBridge.cpp)

using namespace System::Runtime::InteropServices; //Marshal

class CppBridge
{
    public: CppBridge() {};
};

JNIEXPORT jstring JNICALL Java_MyPackage_getXML(JNIEnv *jn, jobject jobj)
{
    jstring error;
    try
    {
        Methods::Wrapper^ wrapper = gcnew Methods::Wrapper();
        char* xml = wrapper->getXML();
        jstring result = jn->NewStringUTF(xml);
        return result;
    }
    catch (Exception^ e)
    {
        char* result = (char*)Marshal::StringToHGlobalAnsi("Source: "+ e->Source + " | Message: " + e->Message).ToPointer();
        error = jn->NewStringUTF(result);
    }
    return error;
}

Answer 1

I encountered the same problem a year ago, the reason is simple: java.exe can't not find the C# DLL as java.exe only searches for the C# DLL in the same folder of java.exe and system GAC.

So resolve this problem, there're two way:

1. Put your C# DLL and its dependencies at the same folder of java.exe

2. Sign your C# DLL and make it strong name, then install it to GAC

To find out how java.exe search for the DLL, use the tool procmon from Microsoft