Pass a C++ function pointer to C# with embedded mono

Question

I have a C# method that looks like this:

public delegate void VariableObserver(string variableName, object newValue); public void ObserveVariable(string variableName, VariableObserver observer) { // stuff }

This code is embedded into a c++ application and I want to call the ObserveVariable function from C++, passing a function pointer or something equivalent as the VariableObserver delegate. The intention is then that the C# code can call the delegate and the C++ function will be invoked.

I'm used to using mono_runtime_invoke for calling C# functions, and it works great. However I can't find any useful examples of documentation of how you can pass a function pointer/delegate. Do you need to box it up like you do with some types like strings etc? If so how do you do this? is it even possible?

I found the Marshal.GetDelegateFromFunctionPointer method while searching around, and I figured I could use that in C# to get the delgate and carry on as normal, however these docs says this is now obsolete.

Edit: some more things I discovered. There's this newer Marshal function: Marshal.GetDelegateForFunctionPointer<TDelegate>(IntPtr) which looked promising, but it says explicitly "You cannot use this method with function pointers obtained through C++ or from the GetFunctionPointer method." :(

Edit2: I've tried the suggestion of just using GetDelegateForFunctionPointer, but it doesn't look like the function pointer is marshalled correctly. Take a look:

C# code:

delegate void TestObserver(int num); public void BindObserver(IntPtr observerFuncPtr) { TestObserver obs = Marshal.GetDelegateForFunctionPointer<TestObserver>(observerFuncPtr); obs(1337); }

C++ code:

static void ObserverCallback(int num) { printf("WE GOT A CALLBACK %d", num); } // inside a function I call void* params[1]; params[0] = &ObserverCallback; MonoObject* exception; mono_runtime_invoke(theMonoMethod, theMonoInstance, params, &exception);

However doing this results in this error from C#:

Runtime Exception : System.NullReferenceException: Object reference not set to an instance of an object at (wrapper managed-to-native) System.Object:wrapper_native_8348575608244C89 (int) at InkGlue.GlueStory.BindObserver (System.IntPtr observerFuncPtr) [0x00006] in <78fcd644f8db43978f6f7b1655d0ab2f>:0

Which leads me to believe that the function pointer is not successfully being passed to C#. Any info on how to do this correctly?

Try adding extern “C” to the declaration of ObserverCallback, to ensure it follows C calling conventions. — Zastai
– Zastai, Commented Aug 25, 2018 at 17:34
And you might need [UnmanagedFunctionPointer(CallingConvention.Cdecl)] on the delegate declaration too. — Zastai
– Zastai, Commented Aug 25, 2018 at 17:43
&ObserverCallback is not the correct function pointer. You are missing the function pointer type - see: stackoverflow.com/questions/2298242/callback-functions-in-c — awattar
– awattar, Commented Aug 19, 2021 at 18:43

Behrooz · Accepted Answer · 2018-08-19 17:44:48Z

1

Easy workaround is a C shim. https://stackoverflow.com/a/31232805/179795 and other answers in that question may also help.

answered Aug 19, 2018 at 17:44

Behrooz

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DavidColson Over a year ago

Thanks, but I only have this small use case and I feel adopting this would be too big for such a small problem. Currently considering doing it another way entirely.

catnip · Accepted Answer · 2018-08-18 14:42:28Z

I think Marshal.GetDelegateForFunctionPointer will work if you pass it the address of a function marked static. Pass this as an explicit parameter if necessary.

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