Replace the Dictionary with HashSet in AppDomainShutdownMonitor

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/InterOp/D3DImage.cs

@@ -85,7 +85,7 @@ public D3DImage(double dpiX, double dpiY)
             _dpiX = dpiX;
             _dpiY = dpiY;
 
-            _listener = new WeakReference(this);
+            _listener = new WeakReference<IAppDomainShutdownListener>(this);
             AppDomainShutdownMonitor.Add(_listener);
         }
 
@@ -944,7 +944,7 @@ void IAppDomainShutdownListener.NotifyShutdown()
         private EventHandler _sendPresentDelegate;
 
         // WeakReference to this used to listen to AddDomain shutdown
-        private WeakReference _listener;
+        private WeakReference<IAppDomainShutdownListener> _listener;
 
         // Keeps track of how many times the user has nested Lock
         private uint _lockCount;

src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/AppDomainShutdownMonitor.cs

@@ -28,59 +28,57 @@ internal static class AppDomainShutdownMonitor
     {
         static AppDomainShutdownMonitor()
         {
+            _listeners =
+                new HashSet<WeakReference<IAppDomainShutdownListener>>();
+
             AppDomain.CurrentDomain.DomainUnload += OnShutdown;
             AppDomain.CurrentDomain.ProcessExit += OnShutdown;
-
-            _dictionary = new Dictionary<WeakReference, WeakReference>();
         }
-        
-        public static void Add(WeakReference listener)
+
+        public static void Add(WeakReference<IAppDomainShutdownListener> listener)
         {
-            Debug.Assert(listener.Target != null);
-            Debug.Assert(listener.Target is IAppDomainShutdownListener);
+            Debug.Assert(listener.TryGetTarget(out _));
 
-            lock (_dictionary)
+            lock (_listeners)
             {
                 if (!_shuttingDown)
                 {
-                    _dictionary.Add(listener, listener);
+                    _listeners.Add(listener);
                 }
             }
         }
 
-        public static void Remove(WeakReference listener)
+        public static void Remove(WeakReference<IAppDomainShutdownListener> listener)
         {
-            Debug.Assert(listener.Target == null || listener.Target is IAppDomainShutdownListener);
-
-            lock (_dictionary)
+            lock (_listeners)
             {
                 if (!_shuttingDown)
                 {
-                    _dictionary.Remove(listener);
+                    _listeners.Remove(listener);
                 }
             }
         }
 
         private static void OnShutdown(object sender, EventArgs e)
         {
-            lock (_dictionary)
-            {     
+            lock (_listeners)
+            {
                 // Setting this to true prevents Add and Remove from modifying the list. This
                 // way we call out without holding a lock (which would be bad)
                 _shuttingDown = true;
             }
-            
-            foreach (WeakReference value in _dictionary.Values)
+
+            foreach (WeakReference<IAppDomainShutdownListener> weakReference in _listeners)
             {
-                IAppDomainShutdownListener listener = value.Target as IAppDomainShutdownListener;
-                if (listener != null)
+                if (weakReference.TryGetTarget(out var listener))
                 {
                     listener.NotifyShutdown();
                 }
             }
         }
-
-        private static Dictionary<WeakReference, WeakReference> _dictionary;
+
+        private static readonly HashSet<WeakReference<IAppDomainShutdownListener>> _listeners;
+
         private static bool _shuttingDown;
     }
 }