How to pass a managed object as a void* parameter

[Stephen Walch]

I am writing a managed C++ app that calls a third party "C" library. One of
the functions I need to call is a standard callback routine: I supply a
"C" style callback function and a optional void* pointer to "private data"
(which gets passed back to the callback function). What I want to do is
pass an instance of a managed class as the "private data" argument and
somehow reconstitute as an instance of a managed class in my callback
function. Can this be done? This is what I want to do, but the compiler
objects:

String* s = new String("Testing...");
String __pin* p1 = s;
void* v = (void*)p1;
// callback occurs
String __pin* p2 = (String __pin*)v;

Any hope of accomplishing what I need here?

Thanks,
Steve