Waiting for entire file to appear before execution

Hello,
I'm trying to wait until an application is downloaded before executing it. Unfortunately, I'm having problems because it will execute the file before the application is fully downloaded.

here's my current code:

char cpath[MAX_PATH];
	GetCurrentDirectory(MAX_PATH, cpath);
	HANDLE  m_hDir = FindFirstChangeNotification(cpath, FALSE, FILE_NOTIFY_CHANGE_FILE_NAME);
WaitForSingleObject(m_hDir, INFINITE);

			Sleep(4000);

			WinExec("app.exe", SW_SHOW);
			

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As you can see, I'm forced to use Sleep() to give it enough time to fully download, but I'd like a more reliable way of doing this. I thought of checking for the full size of the desired file before execution, but the file being downloaded may vary in size a great deal, so that wouldn't be sensible either.

Any input would be helpful


Thanks.