[Nice3point]

Summary of the Pull Request

What is this about:

Completes OnShutdownAsync of an asynchronous external application after Revit shuts down the WPF dispatcher of its thread.

Description:

What changed

• AsyncExternalApplication and AsyncExternalCommand wait for their task through one internal MessagePump.Run.
• The wait depends on the state of the dispatcher of the calling thread:

Dispatcher at the call	Before	After
Running	Pumps a DispatcherFrame	Unchanged
Shutdown posted and not yet executed	The frame exits before the task completes, and the blocking wait deadlocks	The frame is created with exitWhenRequested: false and continues until the task completes
Shut down	Dispatcher.PushFrame throws InvalidOperationException	The continuations run on the calling thread

• For a dispatcher that has shut down, the pump installs a SynchronizationContext that queues the continuations, and restores the previous context when the task completes.
• During that wait, the pump dispatches the window messages of the thread with MsgWaitForMultipleObjectsEx and PeekMessage.
• A WM_QUIT message removed during the wait is posted again with its exit code.
• The remarks of OnShutdownAsync, the changelog, and the revit-toolkit-internals skill describe both states of the dispatcher.

Where to look

• Revit shuts down the dispatcher itself: UIFramework.MainWindow.MainWindow_Unloaded calls Dispatcher.BeginInvokeShutdown. In five exits of Revit 2026, the shutdown had finished before ApplicationClosing.
• A blocking wait was the alternative for a dispatcher that has shut down. The Revit thread has a WindowsFormsSynchronizationContext, which delivers a continuation as a window message, and a blocking wait never receives it.
• The pump dispatches window messages with a raw message loop, without the message preprocessing of WPF. A queue of continuations without that loop was the alternative. It never completes a shutdown that awaits a callback delivered as a window message.
• The frame ignores Dispatcher.ExitAllFrames() as well as the shutdown request. The dispatcher finishes its shutdown when the frame exits.
• The queue context throws NotSupportedException for a Send from another thread, and never runs a callback posted after the asynchronous method completes.
• The changelog opens the 2027.0.5 section. The build settings still state 2027.0.4.

How verified

• Build: the library on Release.R20, Release.R24, and Release.R27, and the tests on Debug.R24, Debug.R26, and Debug.R27, zero errors and no new warnings.
• Revit 2027: the whole test project, 144 passed.
• Revit 2026 and Revit 2024: AsyncExternalApplicationTests, 9 passed.
• With the previous wait restored, 8 of the 9 tests fail on Revit 2026: the six tests of a dispatcher that has shut down throw the exception of the issue, and the two tests of a posted shutdown time out.
• A probe in an ApplicationClosing handler of Revit 2026 ran the same wait after the dispatcher shut down: the previous wait threw the exception of the issue, and the pump completed with the continuations on the Revit thread.
• Not covered: an exit of Revit with an add-in built on this branch, and a test run on Revit 2020 to 2023 and 2025.

Open questions

• A WM_QUIT message that arrives during the frame of a running dispatcher ends the frame before the task completes, and the wait after it blocks. No session showed this case, and the change leaves it as it is.

Quality Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings