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Compiler Error C3724

must #include <windows.h> to use multi-threading with events

The windows.h file is required if you use multi-threading with events. To fix this error, add #include <windows.h> to the top of the file in which event sources and event receivers are defined.

// C3724.cpp
// uncomment the following line to resolve
// #include <windows.h>

[event_source(native), threading(apartment)]
class CEventSrc {
   __event void event1();   // C3724

class CEventRec {
   void handler1() {

   void HookEvents(CEventSrc* pSrc) {
      __hook(CEventSrc::event1, pSrc, CEventRec::handler1);

   void UnhookEvents(CEventSrc* pSrc) {
      __unhook(CEventSrc::event1, pSrc, CEventRec::handler1);

int main() {