Fix register/unregister issues of WeakReferenceMessenger

Microsoft.Toolkit.Mvvm/Messaging/WeakReferenceMessenger.cs

@@ -406,6 +406,8 @@ public bool MoveNext()
 
                     while (node is not null)
                     {
+                        LinkedListNode<WeakReference<TKey>>? nextNode = node.Next;
+
                         // Get the key and value for the current node
                         if (node.Value.TryGetTarget(out TKey? target) &&
                             this.owner.table.TryGetValue(target!, out TValue? value))
@@ -421,7 +423,7 @@ public bool MoveNext()
                             this.owner.keys.Remove(node);
                         }
 
-                        node = node.Next;
+                        node = nextNode;
                     }
 
                     return false;

UnitTests/UnitTests.Shared/Mvvm/Test_Messenger.cs

@@ -511,6 +511,32 @@ void Test()
             messenger.Cleanup();
         }
 
+        // See https://github.com/windows-toolkit/WindowsCommunityToolkit/issues/4081
+        [TestCategory("Mvvm")]
+        [TestMethod]
+        [DataRow(typeof(StrongReferenceMessenger))]
+        [DataRow(typeof(WeakReferenceMessenger))]
+        public void Test_Messenger_RegisterMultiple_UnregisterSingle(Type type)
+        {
+            var messenger = (IMessenger)Activator.CreateInstance(type);
+
+            var recipient1 = new object();
+            var recipient2 = new object();
+
+            int handler1CalledCount = 0;
+            int handler2CalledCount = 0;
+
+            messenger.Register<object, MessageA>(recipient1, (r, m) => { handler1CalledCount++; });
+            messenger.Register<object, MessageA>(recipient2, (r, m) => { handler2CalledCount++; });
+
+            messenger.UnregisterAll(recipient2);
+
+            messenger.Send(new MessageA());
+
+            Assert.AreEqual(1, handler1CalledCount);
+            Assert.AreEqual(0, handler2CalledCount);
+        }
+
         public sealed class RecipientWithNoMessages
         {
             public int Number { get; set; }
@@ -550,4 +576,4 @@ public sealed class MessageB
         {
         }
     }
-}
+}