P/Invoke - Marshaling delegate as function pointer + void*

Question

A fairly common idiom in C is for functions taking a polymorphic closure to represent this as two arguments, a function pointer and void pointer (which is passed as one of the arguments to the function pointer.

An example taken from the GPGME library:

typedef gpgme_error_t (*gpgme_passphrase_cb_t) (void *hook,
                                                const char *uid_hint,
                                                const char *passphrase_info,
                                                int prev_was_bad, 
                                                int fd);

void gpgme_set_passphrase_cb (gpgme_ctx_t ctx,
                              gpgme_passphrase_cb_t cb, 
                              void *hook_value);

Conceptually, the function pointer plus void pointer represent the same thing as a delegate in C# (a closure). Is there a nice, canonical way to marshal a delegate when making this sort of P/Invoke call?

Answer 1

Is there a nice, canonical way to marshal a delegate when making this sort of P/Invoke call?

You don't need to use the void* parameter because a C# delegate is a closure. Pass IntPtr.Zero as the hook value. Your C# delegate still needs to accept the void* parameter but it can simply ignore it since it does not need it.