C / C++ / MFC

1. Enable a user-defined message in your UI thread that retrieves the active document (basically move your code to the message handler) and call it from the worker thread with SendMessage. This can pose some deadlock problems, though.
2. Pass the worker thread a pointer to the document on launch time (using the extra parameter at your disposal.)

// Note: if either of the above asserts fire and you are
// writing a multithreaded application, it is likely that
// you have passed a C++ object from one thread to another
// and have used that object in a way that was not intended.
// (only simple inline wrapper functions should be used)
//
// In general, CWnd objects should be passed by HWND from
// one thread to another.  The receiving thread can wrap
// the HWND with a CWnd object by using CWnd::FromHandle.
//
// It is dangerous to pass C++ objects from one thread to
// another, unless the objects are designed to be used in
// such a manner.