Question

Is there any way to programmatically check the priority of a window messages in its message queue?

For example: Some of window messages, WM_PAINT and WM_TIMER are known have the lowest priority and be placed after messages with highest priority.

I’m looking for something by which you can confirm that which one of two messages will have the lowest or highest priority or which message will be sent first or last?

Answer 1

That’s just not how it works, Windows messages don’t have a priority attached. It is mostly determined by how the message is generated. A message loop dispatches messages in this order:

• first any messages generated with SendMessage() are dispatched in the order in which the calls were made
• next, any messages generated with PostMessage() and stored in the message queue, in queue order
• next, any messages that are synthesized from the window state. WM_TIMER, WM_PAINT and WM_MOUSEMOVE fit this category.

The ‘synthesized from the window state’ clause is what makes WM_PAINT and WM_TIMER appear to have a low priority. And why moving the mouse rapidly doesn’t flood the message queue with mouse messages. That is however not exclusive, you can for example call UpdateWindow() to force a WM_PAINT message to be sent, making it being dispatched with a ‘high priority’.