Windows prevent multiple instances code not working - c++

Question

I am using CreateEvent to prevent multiple instances of my application:
CreateEvent(NULL, TRUE, FALSE, "MyEvent");
if (GetLastError() == ERROR_ALREADY_EXISTS)
{
// Do Stuff
return FALSE;
}
However, at startup I have noticed that this doesn't work:
After the desktop is shown I automatically run a batch script that attempts to launch multiple instances of my program. The batch script succeeds and I can indeed see multiple instances.
Investigation so far:
OutputDebug shows that each instance does not get ERROR_ALREADY_EXISTS
ProcessExplorer.exe shows that each instance was able to get a handle to the event "MyEvent".
Can anybody think why this might be happening, and how I could solve it?

Answer 1

We use the function below, which is in our common utility DLL. The method is derived from a Microsoft article explaining how to prevent multiple instances in WIN32.
#define STRICT
#include <stdheaders.h>
HANDLE ghSem;
BOOL IExist( LPSTR lpszWindowClass )
{
HWND hWndMe;
int attempt;
for( attempt=0; attempt<2; attempt++ )
{
// Create or open a named semaphore.
ghSem = CreateSemaphore( NULL, 0, 1, lpszWindowClass );
// Close handle and return NULL if existing semaphore was opened.
if( (ghSem != NULL) &&
(GetLastError() == ERROR_ALREADY_EXISTS) )
{ // Someone has this semaphore open...
CloseHandle( ghSem );
ghSem = NULL;
hWndMe = FindWindow( lpszWindowClass, NULL );
if( hWndMe && IsWindow(hWndMe) )
{ // I found the guy, try to wake him up
if( SetForegroundWindow( hWndMe ) )
{ // Windows says we woke the other guy up
return TRUE;
}
}
Sleep(100); // Maybe the semaphore will go away like the window did...
}
else
{ // If new semaphore was created, return FALSE.
return FALSE;
}
}
// We never got the semaphore, so we must
// behave as if a previous instance exists
return TRUE;
}
Just do something like this in your WinMain:
if( IExist("MyWindowClass") )
{
return 1;
}
Of course, you could replace the return with whatever you need to do when you are not the first instance (such as activating the existing instance).