Fix XamlMessageBox STA Threading Crash from Background Threads#2400
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brianrob merged 3 commits intomicrosoft:mainfrom Mar 27, 2026
Merged
Fix XamlMessageBox STA Threading Crash from Background Threads#2400brianrob merged 3 commits intomicrosoft:mainfrom
brianrob merged 3 commits intomicrosoft:mainfrom
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XamlMessageBox.Show() now auto-dispatches to the UI thread when called from a background thread, matching the old System.Windows.MessageBox behavior. This fixes the SecurityCheck delegate crash during symbol resolution, where the delegate is invoked on Task.Run threads. The fix checks Application.Current.Dispatcher.CheckAccess() and uses synchronous Dispatcher.Invoke to marshal the call, preserving the return value for callers. Existing UI-thread call sites are unaffected (CheckAccess returns true, no dispatch needed). Fixes microsoft#2300 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
…ingleton, guard RegisterClassHandler - XamlMessageBox.cs: Use owner?.Dispatcher ?? Application.Current?.Dispatcher so the dispatch targets the correct UI thread when an owner window is provided. Simplify with Dispatcher.Invoke(Func<T>) to return result directly. - XamlMessageBoxTests.cs: Guard Application creation with Application.Current ?? to prevent InvalidOperationException in shared AppDomains. - XamlMessageBoxTests.cs: Add static guard for RegisterClassHandler (permanent, AppDomain-wide registration) and use XamlMBTest_ prefix for unique captions. - XamlMessageBoxTests.cs: Use indexer assignment for theme resources (idempotent). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
…spatcher in test Replace app.Dispatcher.BeginInvoke with Dispatcher.CurrentDispatcher.BeginInvoke to ensure work is queued to the dispatcher being pumped by Dispatcher.Run(). If Application.Current was reused from a prior test on a different thread, app.Dispatcher would target that thread's dispatcher, causing the test to hang. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Mar 27, 2026
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Summary
Fixes #2300 —
XamlMessageBox.Show()now auto-dispatches to the UI thread when called from a background thread, matching the oldSystem.Windows.MessageBoxbehavior.Problem
PR #2276 replaced
System.Windows.MessageBox(Win32 API, no thread affinity) withXamlMessageBox(WPFShowDialog(), requires STA/UI thread). Several call sites invokeXamlMessageBox.Show()from background threads — most notably theSecurityCheckdelegate inApp.cs, which runs during symbol resolution onTask.Runthreads. This causes:Fix
Added auto-dispatch logic to the core
XamlMessageBox.Show()overload (all 10+ overloads funnel through this single method):owner?.Dispatcher ?? Application.Current?.Dispatcher!CheckAccess()), uses synchronousDispatcher.Invoketo marshal the callTesting
XamlMessageBoxTests.cswith a regression test that exercises both UI-thread and background-threadShow()callsEventManager.RegisterClassHandlerto auto-close dialogs, and verifies no exceptions are thrown