Last time, we discussed briefly the thread affinity rules that govern window handles.

Device contexts (DCs) also have a certain degree of thread affinity. The thread that calls functions such as GetDC must also be the one that calls ReleaseDC, but as with window handles, during the lifetime of the DC, any thread can use it. If you choose to use a DC in a multi-threaded manner, it’s your responsibility to coordinate the consumers of that device context so that only one thread uses it at a time. For example, to host windowless controls across multiple threads, the host obtains a DC on the host thread, then asks each control in sequence to draw itself into that DC. Only one control draws into the DC at a time, even if the control happens to be on a different thread.

The thread affinity of DCs is much more subtle than that of window handles, because if you mess up and release a DC from the wrong thread, things will still seem to be running okay, but the window manager’s internal bookkeeping will be messed up and you may get a bad DC from GetDC a little later down the line.

Next time, the remaining user interface elements.