WaitForInputIdle doesn’t work for starting mspaint programmatically

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WaitForInputIdle works, but not the way you assume it does. This is largely, because the documentation is misleading (or at least not as explicit as it should be):

Waits until the specified process has finished processing its initial input and is waiting for user input with no input pending, or until the time-out interval has elapsed.

This is almost criminally inaccurate. While the Remarks section notes, that WaitForInputIdle waits at most once per process, it never mentions significant details. Specifically:

  • WaitForInputIdle returns, as soon as the initial startup has come to a point, where any thread in the process is ready to process messages. Those messages need not be user input.
  • WaitForInputIdle was invented to allow a process to communicate with a child process using a message-based protocol. The specific scenario addressed was DDE, which no one1) uses anymore.

WaitForInputIdle cannot be used as a reliable solution to your problem: Waiting for a child process’ UI to show up. You really need to wait for the UI show up.

The system offers two solutions you can use:

  1. A global CBT hook, and wait for the HCBT_CREATEWND callback. You can inspect the CREATESTRUCT‘s lpszClass and/or lpszName members to filter out the window you are interested in.
  2. Use WinEvents and respond to the EVENT_OBJECT_CREATE event.

The global CBT hook is called, whenever a window is about to be created. The kernel structures that the HWND references have been fully populated, but the client calling CreateWindow[Ex] may still terminate window creation. In contrast, the WinEvent is issued, after the window has been fully constructed, and is ready for interaction.

In general, a solution based on a CBT hook is used, when an application needs to update certain aspects of a window before the caller of CreateWindowEx gets to see the HWND for the first time. WinEvents, instead, are usually the tool of choice when implementing accessibility or UI automation solutions.

Additional resources:

1) Yes, I know, some applications might still use DDE. But if Raymond Chen suggested in 2007, that we should feel free to stop using DDE, I’ll just pass that on as authoritative guidance.

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