Bug: uithread-init-contract-residuals-784-787-788 #809

• Work Mode: full-bug

Summary

Consolidates three findings on one file, UtilitiesCS/Threading/UiThread.cs, that were filed separately as #784, #787, and #788 after the #781 and #782 reviews. (1) Init() accepts a non-STA caller and installs that worker's non-pumping dispatcher and context into set-once process-global state (#787). (2) Init() consumes its single-shot latch before Initialize() runs, so a failed first attempt can never be retried, and the naive re-arm was measured to regress in #782 (#788). (3) SynchronizationContextAwaiter.IsCompleted compares contexts by reference, so any context captured inside a WPF dispatcher operation always posts instead of continuing inline on the UI thread (#784). All three touch the same initialization and awaiter code and should ship as one change with one test suite.

Environment

• OS/version: Windows 11 Pro 10.0.26200
• Runtime: .NET Framework 4.8 VSTO add-in hosted by Outlook desktop; main at 04a54e68
• Command/flags used: vstest.console.exe <test assemblies> /InIsolation; runtime probe in the Bug: breadcrumb-ui-boundary-guard-rejects-dispatcher-built-viewers #781 feature folder (evidence/other/dispatcher-synccontext-probe.2026-09-05T10-40.md)
• Data source or fixture: QuickFiler.Test/Controllers/QfcHomeControllerRunAsyncTests.cs:329 (MTA caller of UiThread.Init(false))

Steps to Reproduce

1. Bug: uithread-init-accepts-non-sta-callers #787: call UiThread.Init(false) from an MTA thread (the in-repo instance is the test at QfcHomeControllerRunAsyncTests.cs:329). It returns normally and every later UiThread.Dispatcher / UiSyncContext / UiThreadId read marshals onto a thread with no message loop.
2. Bug: uithread-init-latch-not-rearmed-after-failed-initialize #788: arrange for Initialize() to throw (headless or non-STA), call Init(), fix the condition, call Init() again. The second call is a no-op because _loaded.CheckAndSetFirstCall at UiThread.cs:36 was consumed before Initialize() ran.
3. Bug: uithread-synccontext-awaiter-always-posts-for-dispatcher-built-viewers #784: construct an ItemViewer through ItemViewerQueue.Dequeue (inside UiThread.Dispatcher.Invoke, so UiSyncContext is a DispatcherSynchronizationContext), then on the UI thread evaluate viewer.UiSyncContext.GetAwaiter().IsCompleted. It is false, so the continuation posts instead of running inline.

Expected Behavior

• Init() rejects a non-STA caller with a named InvalidOperationException before capturing anything.
• A failed Initialize() leaves the latch re-armed so a later Init() retries, without reintroducing the regression Refactor: pr-778-post-merge-review-residuals #782 measured.
• IsCompleted is true when the caller already runs on the owning UI thread, regardless of which SynchronizationContext instance is ambient.

Actual Behavior

See the three reproduction steps. #787 succeeds silently and poisons the globals for the process lifetime; #788 leaves UiThread.Dispatcher throwing an exception that names Init() as the remedy while Init() is a no-op; #784 adds one queued hop per await and changes ordering relative to already-queued UI work.

Logs / Screenshots

• Attached minimal logs or screenshot
• Snippet: UtilitiesCS/Threading/UiThread.cs line 100 (verified 2026-09-05): public bool IsCompleted => _context == SynchronizationContext.Current; (reference comparison). Probe result: Invoke ctx == outer ambient : False on .NET Framework 4.8 STA. Bug: uithread-init-accepts-non-sta-callers #787 and Bug: uithread-init-latch-not-rearmed-after-failed-initialize #788 are missing-precondition and ordering defects with no diagnostic output.

Impact / Severity

• Blocker
• High
• Medium
• Low

Medium, carried from #787: in production ThisAddIn.cs:35-40 is the only Init() caller and runs on the Outlook STA, so the hazards are reachable today only from test code, but a worker-thread read of the lazy accessors before startup completes would poison the process. #784 and #788 are Low individually.

Source

From: docs/features/potential/2026-09-07-uithread-init-contract-residuals-784-787-788.md