_beginthreadex returns a handle to a thread:
m_hStreamStatsThread = (HANDLE) _beginthreadex( NULL, 0, StreamStatsThread, this, 0, NULL );
This handle may be used if you need to refer to the thread in calls like TerminateThread(..) for example.
According to the MSDN page on _beginthreadex, _beginthreadex won't always return a valid handle - e.g. it may also return -1L on error etc.
When a thread has completed normally, do I have to call CloseHandle on the thread handle, or can I just set its value to NULL / INVALID_HANDLE_VALUE?
Agree with Nemanja Trifunovic.
Even after the thread exited - its handle is valid. You can for instance query its return value.
As a general rule: every Win32 handle must be closed by CloseHandle, unless otherwise specified.
The code sample on the MSDN page you posted a link to includes a call to CloseHandle(). Setting the handle's value to NULL does not decrease the kernel object's internal ref count and is pretty much useless anyway.
来源：https://stackoverflow.com/questions/3959473/must-i-closehandle-on-a-thread-handle