Callback/Delegate to Java for handling some C# event

I am using C# assembly in a Java application via JNA & Unmanaged Exports. Now I would need to implement a Callback/Delegate in order to inform Java that some Event in the C# .dll has occured.

How can this be accomplished? or is there any reading reference you could suggest?

What I tried so far: I am using Java Native Access (JNA) & unmanaged exports by Robert Giesecke in order to call C# methods from Java. - works fine so far.

I pass the Callback-method as a void* pointer from Java to C#. I would like to call this Method somehow.

For verification I added the code I use:

1. Defining JNA Interface (incl. Callback) in Java:

   import com.sun.jna.Callback;
   import com.sun.jna.Library;
       public interface Ijna extends Library {
           interface sig_t extends Callback {
               boolean invoke(int signal);
           }
           String TestCallback(sig_t callbackparam);
       }
   

2. unsafe C# Method exported to unmanaged code, receiving a "any pointer" (void*) as parameter which should be the adress of the method

   [DllExport]
   public unsafe static String TestCallback(void* someMethod) {
       //use someMethod here
       if(someMethod != null)
           return "not null"; // someMethod is not null when running the app
       else
           return "null";
   }
   

3. Load .dll, define Callback function and call it in Java.

   public static void main(String[] args) {
       //load unmanaged .dll
       Ijna.sig_t referenceToFunction = new Ijna.sig_t() {
           @Override
               public boolean invoke(int sig) {
               System.out.println("signal " + sig + " was raised");
               return true;
           }
       };
       String test = INSTANCE.TestCallback(referenceToFunction);//returns "not null"
       System.out.println(test); 
   }
   

I could solve it. Using Marshal.GetDelegateForFunctionPointer(IntPtr, Type) it was possible to convert the void* (any pointer) to a C# delegate.

So the C# code is adapted to the following:

    delegate void Del(int someInt);
    Del csharpfuncptr; //save the Delegate, but not the void* (the void* is not preventing the GB from deleting the method, and could therefore get invalid)
    [DllExport]
    public unsafe static String TestCallback(void* someMethod)
    {
        IntPtr method = new IntPtr(someMethod);
        csharpfuncptr = (Del)Marshal.GetDelegateForFunctionPointer(method, typeof(Del));
        csharpfuncptr(5);
    }