WPF 的 Application.Current.Dispatcher 中，为什么 Current 可能为 null

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/// <summary> /// The Current property enables the developer to always get to the application in /// AppDomain in which they are running. /// </summary> static public Application Current { get { // There is no need to take the _globalLock because reading a // reference is an atomic operation. Moreover taking a lock // also causes risk of re-entrancy because it pumps messages. return _appInstance; } }

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/// <summary> /// Application constructor /// </summary> /// <SecurityNote> /// Critical: This code posts a work item to start dispatcher if in the browser /// PublicOk: It is ok because the call itself is not exposed and the application object does this internally. /// </SecurityNote> SecurityCritical public Application() { // 省略了一部分代码。 lock(_globalLock) { // set the default statics // DO NOT move this from the begining of this constructor if (_appCreatedInThisAppDomain == false) { Debug.Assert(_appInstance == null, "_appInstance must be null here."); _appInstance = this; IsShuttingDown = false; _appCreatedInThisAppDomain = true; } else { //lock will be released, so no worries about throwing an exception inside the lock throw new InvalidOperationException(SR.Get(SRID.MultiSingleton)); } } // 省略了一部分代码。 }

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/// <summary> /// DO NOT USE - internal method /// </summary> ///<SecurityNote> /// Critical: Calls critical code: Window.InternalClose /// Critical: Calls critical code: HwndSource.Dispose /// Critical: Calls critical code: PreloadedPackages.Clear() ///</SecurityNote> SecurityCritical internal virtual void DoShutdown() { // 省略了一部分代码。 // Event handler exception continuality: if exception occurs in ShuttingDown event handler, // our cleanup action is to finish Shuttingdown. Since Shuttingdown cannot be cancelled. // We don't want user to use throw exception and catch it to cancel Shuttingdown. try { // fire Applicaiton Exit event OnExit(e); } finally { SetExitCode(e._exitCode); // By default statics are shared across appdomains, so need to clear lock (_globalLock) { _appInstance = null; } _mainWindow = null; _htProps = null; NonAppWindowsInternal = null; // 省略了一部分代码。 } }

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/// <summary> /// This method gets called on dispatch of the Shutdown DispatcherOperationCallback /// </summary> ///<SecurityNote> /// Critical: Calls critical code: DoShutdown, Dispatcher.CritcalInvokeShutdown() ///</SecurityNote> SecurityCritical private void ShutdownImpl() { // Event handler exception continuality: if exception occurs in Exit event handler, // our cleanup action is to finish Shutdown since Exit cannot be cancelled. We don't // want user to use throw exception and catch it to cancel Shutdown. try { DoShutdown(); } finally { // Quit the dispatcher if we ran our own. if (_ownDispatcherStarted == true) { Dispatcher.CriticalInvokeShutdown(); } ServiceProvider = null; } }

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//<SecurityNote> // Critical - as it accesses security critical data ( window handle) //</SecurityNote> SecurityCritical private void ShutdownImplInSecurityContext(Object state) { // 省略了一部分代码。 // Now that the queue is off-line, abort all pending operations, // including inactive ones. DispatcherOperation operation = null; do { lock(_instanceLock) { if(_queue.MaxPriority != DispatcherPriority.Invalid) { operation = _queue.Peek(); } else { operation = null; } } if(operation != null) { operation.Abort(); } } while(operation != null); // 省略了一部分代码。 }

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using System; using System.Threading; using System.Threading.Tasks; using System.Windows; namespace Walterlv.Demo.ApplicationDispatcher { class Program { STAThread static void Main(string[] args) { var app = new Application(); Task.Delay(1000).ContinueWith(t => { app.Dispatcher.InvokeAsync(() => app.Shutdown()); }); Task.Delay(2000).ContinueWith(t => { Application.Current.Dispatcher.InvokeAsync(() => { }); }); app.Run(); Thread.Sleep(2000); } } }

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private void OnUsbDeviceChanged(object sender, EventArgs e) { // 记得这里需要判空，因为此上下文可能在非 UI 线程。 Application.Current?.InvokeAsync(() => { }); }