Why does marshalling a struct of callback delegates cause an AccessViolationException

Answer 1

First of all, kudos for posting clear repro code. Now a couple of issues with your code:

• Most importantly, the cb pointer you're receiving in set_callback (from the C# ref parameter) and storing in cbStruct isn't safe to store. There's no guarantee that the struct it's pointing to will stay around after set_callback returns. If you change your code so a copy of the struct is passed by value instead, I think your errors will go away.

• All three Marshal.GetFunctionPointerForDelegate calls are passed the first delegate.

• If you want to be really sure that the delegates remain valid, insert calls to GC.KeepAlive(cb.c_cb1) etc after the call to exec_callbacks.

Answer 2

I struggled with a similar problem a couple of days ago. Although the pointer to the struct remained valid, the function pointers withhin the struct changed after returning from the first callback. My approach was exactly as shown in your sample code. Instead of passing a copy of the struct by val, i decided to pass a pointer and copy the struct it pointed to in set_callback, it works.

__declspec(dllexport) void __stdcall set_callback(Callback *cb) {
    cbStruct = new Callback(*cb);
    // TODO: Clean up memory e.g. in release_callback()
}

Answer 3

The problem is that cb1, cb2 and cb3 are heap allocated, even though their storage (the struct) is not. Thus they are each subject to GC (compaction/relocation, thus invalidating the pointers originally passed in).

Prior to passing in the struct, each of cb1, cb2 and cb3 should be pinned, ideally immediately after they are new'd. Otherwise they will likely relocate in memory.

Was the decision to use a struct to contruct a classic function map done to avoid this relocation? If so, it's ultimately non-helpful.