Understanding The COM Single-Threaded Apartment Part 1

#define _WIN32_WINNT 0x0400

#include <windows.h>
#include <stdio.h>

#import "..\..\..\..\SimpleCOMObject2\Debug\SimpleCOMObject2.dll"
using namespace SIMPLECOMOBJECT2Lib;





// Simple function that displays the current thread ID.
void DisplayCurrentThreadId()
{
  TCHAR szMessage[256];

  sprintf (szMessage, "Thread ID : 0x%X", GetCurrentThreadId());

  ::MessageBox(NULL, szMessage, "TestMethod1()", MB_OK);
}





// If we enter an STA (using COINIT_APARTMENTTHREADED), 
// all STA objects will live in the same STA as main()'s thread.
//
// If we enter non-STA (using COINIT_MULTITHREADED), STA objects
// created in main()'s thread will live in default STA.
int main()
{
  //::CoInitializeEx(NULL, COINIT_APARTMENTTHREADED); // Use COINIT_APARTMENTTHREADED and the TestMethod1() will run on the same thread as main().
  ::CoInitializeEx(NULL, COINIT_MULTITHREADED);  // Use COINIT_MULTITHREADED and TestMethod1() will run on another thread.

  DisplayCurrentThreadId();

  if (1)
  {
    ISimpleCOMObject2Ptr spISimpleCOMObject2;

    // If a default STA is to be created and used, it will be created
	// right after spISimpleCOMObject2 (an STA object) is created.
    spISimpleCOMObject2.CreateInstance(__uuidof(SimpleCOMObject2));

	spISimpleCOMObject2 -> TestMethod1();  // This will message box display the thread id of the thread in which TestMethod1() is running.
  }

  ::CoUninitialize();

  return 0;
}