WPF 的 Application.Current.Dispatcher 中，Dispatcher 属性一定不会为 null

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using System; using System.Windows; using System.Threading; using MS.Internal.WindowsBase; // FriendAccessAllowed namespace System.Windows.Threading { /// <summary> /// A DispatcherObject is an object associated with a /// <see cref="Dispatcher"/>. A DispatcherObject instance should /// only be access by the dispatcher's thread. /// </summary> /// <remarks> /// Subclasses of <see cref="DispatcherObject"/> should enforce thread /// safety by calling <see cref="VerifyAccess"/> on all their public /// methods to ensure the calling thread is the appropriate thread. /// <para/> /// DispatcherObject cannot be independently instantiated; that is, /// all constructors are protected. /// </remarks> public abstract class DispatcherObject { /// <summary> /// Returns the <see cref="Dispatcher"/> that this /// <see cref="DispatcherObject"/> is associated with. /// </summary> System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Advanced) public Dispatcher Dispatcher { get { // This property is free-threaded. return _dispatcher; } } // This method allows certain derived classes to break the dispatcher affinity // of our objects. FriendAccessAllowed // Built into Base, also used by Framework. internal void DetachFromDispatcher() { _dispatcher = null; } // Make this object a "sentinel" - it can be used in equality tests, but should // not be used in any other way. To enforce this and catch bugs, use a special // sentinel dispatcher, so that calls to CheckAccess and VerifyAccess will // fail; this will catch most accidental uses of the sentinel. FriendAccessAllowed // Built into Base, also used by Framework. internal void MakeSentinel() { _dispatcher = EnsureSentinelDispatcher(); } private static Dispatcher EnsureSentinelDispatcher() { if (_sentinelDispatcher == null) { // lazy creation - the first thread reaching here creates the sentinel // dispatcher, all other threads use it. Dispatcher sentinelDispatcher = new Dispatcher(isSentinel:true); Interlocked.CompareExchange<Dispatcher>(ref _sentinelDispatcher, sentinelDispatcher, null); } return _sentinelDispatcher; } /// <summary> /// Checks that the calling thread has access to this object. /// </summary> /// <remarks> /// Only the dispatcher thread may access DispatcherObjects. /// <p/> /// This method is public so that any thread can probe to /// see if it has access to the DispatcherObject. /// </remarks> /// <returns> /// True if the calling thread has access to this object. /// </returns> System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never) public bool CheckAccess() { // This method is free-threaded. bool accessAllowed = true; Dispatcher dispatcher = _dispatcher; // Note: a DispatcherObject that is not associated with a // dispatcher is considered to be free-threaded. if(dispatcher != null) { accessAllowed = dispatcher.CheckAccess(); } return accessAllowed; } /// <summary> /// Verifies that the calling thread has access to this object. /// </summary> /// <remarks> /// Only the dispatcher thread may access DispatcherObjects. /// <p/> /// This method is public so that derived classes can probe to /// see if the calling thread has access to itself. /// </remarks> System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never) public void VerifyAccess() { // This method is free-threaded. Dispatcher dispatcher = _dispatcher; // Note: a DispatcherObject that is not associated with a // dispatcher is considered to be free-threaded. if(dispatcher != null) { dispatcher.VerifyAccess(); } } /// <summary> /// Instantiate this object associated with the current Dispatcher. /// </summary> protected DispatcherObject() { _dispatcher = Dispatcher.CurrentDispatcher; } private Dispatcher _dispatcher; private static Dispatcher _sentinelDispatcher; } }

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protected DispatcherObject() { _dispatcher = Dispatcher.CurrentDispatcher; }

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public static Dispatcher CurrentDispatcher { get { // Find the dispatcher for this thread. Dispatcher currentDispatcher = FromThread(Thread.CurrentThread);; // Auto-create the dispatcher if there is no dispatcher for // this thread (if we are allowed to). if(currentDispatcher == null) { currentDispatcher = new Dispatcher(); } return currentDispatcher; } }

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// This method allows certain derived classes to break the dispatcher affinity // of our objects. FriendAccessAllowed // Built into Base, also used by Framework. internal void DetachFromDispatcher() { _dispatcher = null; } // Make this object a "sentinel" - it can be used in equality tests, but should // not be used in any other way. To enforce this and catch bugs, use a special // sentinel dispatcher, so that calls to CheckAccess and VerifyAccess will // fail; this will catch most accidental uses of the sentinel. FriendAccessAllowed // Built into Base, also used by Framework. internal void MakeSentinel() { _dispatcher = EnsureSentinelDispatcher(); }

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internal static readonly DependencyObject SentinelContainer = new DependencyObject(); internal static readonly DependencyObject UnresolvedContainer = new DependencyObject(); internal static readonly DependencyObject KeyContainer = new DependencyObject(); internal static readonly DependencyObject RemovedContainer = new DependencyObject(); static ItemInfo() { // mark the special DOs as sentinels. This helps catch bugs involving // using them accidentally for anything besides equality comparison. SentinelContainer.MakeSentinel(); UnresolvedContainer.MakeSentinel(); KeyContainer.MakeSentinel(); RemovedContainer.MakeSentinel(); }