Creating delegates that invoke native callbacks

[esauvisky]

Is it possible to instantiate delegates with their method handlers pointing to a custom NativeCallback instead of an already existent MethodInfo?

Take as an example the following scenario:
– I want to create a System.Action object, called myAction, to pass as the Then callback of a Promise (Promise.Then(myAction)).
– Once the Promise completes, I want it's callback to invoke a normal NativeCallback of mine.
– I can instantiate a System.Action(object, methodHandler) object if I pass a MethodInfo as the methodHandler, but trying to pass the pointer of a NativeCallback doesn't work.

I'm aware I could workaround that by passing any MethodInfo, intercepting it's call and detaching the interceptor immediately after it fires, but my main goal here is understanding exactly how does the pointer of a method info differs from the pointer of a native callback when it comes to the invocation of a delegate.

Even if this might not be feasible in practice, what would the actual solution be? Could I create my own "bogus" MethodInfo just so the game accepts it? Maybe create a delegate with some sort of helper function? Or perhaps this is not even possible given Il2Cpp constructs the array of methodinfos from within the global metadata file?

[vfsfitvnm]

Yes, you got it right. Afaik, there's no way to pass a pointer to a NativeCallback; I tried many times with no success.

I don't actually remember if I already tried this, but you can try cloning a MethodInfo that has the same signature of your callback and then change its field(s) so it points to the NativeCallback.

I'm aware I could workaround that by passing any MethodInfo, intercepting it's call and detaching the interceptor immediately after it fires

This exactly what I do here. I choose Mono.Runtime::GetDisplayName as dummy MethodInfo, and then I create the delegate System.Threading.SendOrPostCallback.

my main goal here is understanding exactly how does the pointer of a method info differs from the pointer of a native callback when it comes to the invocation of a delegate.

I do not have the answer to this question. My understanding is delegates are only allowed to call managed code (i.e. C#), not the unmanaged one. I know this doesn't make sense as everything is transpiled to C++ at the end, but this is the behaviour we have.

[esauvisky]

What about Marshal.GetDelegateForFunctionPointer? Technically that's pretty much what we're looking for, no?

I played a bit with it and can indeed create a delegate by passing a pointer to my NativeCallback and a type System.Action, but invoking crashes as well. Will check it out more deeply, unless you already tried that 😅

[vfsfitvnm]

GetDelegateForFunctionPointer doesn't always "exist" unfortunately, and its icall doesn't exist as well:

Il2Cpp.Api._resolveInternalCall(Memory.allocUtf8String("System.Runtime.InteropServices.Marshal::GetDelegateForFunctionPointerInternal"));

So I didn't really played with it as it can't be used.

[vfsfitvnm]

After taking a closer look, I still believe that workaround is actually the best approach