Fix register/unregister issues of WeakReferenceMessenger (#4082)

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## Fixes #4081
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## PR Type
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Bugfix
<!-- - Feature -->
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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
The attached message handlers of a WeakReferenceMessenger do not get called anymore correctly for all registered objects after unregistering one of them.

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
The attached message handlers get called for all registered objects.

There was an issue with the enumerator of the ConditionalWeakTable. The linked list nodes were not iterated correctly when a key gets removed while enumerating. In this case the "Next"-property of the node was set to null and the while loop stopped immediately (without checking the remaining nodes). 

As a fix the next node is now kept as a locale variable so that it does not get lost when removing the key.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [x] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->
- [ ] Sample in sample app has been added / updated (for bug fixes / features)
    - [ ] Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https:/windows-toolkit/WindowsCommunityToolkit-design-assets)
- [ ] New major technical changes in the toolkit have or will be added to the [Wiki](https:/windows-toolkit/WindowsCommunityToolkit/wiki) e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
- [x] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [ ] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [x] Contains **NO** breaking changes

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## Other information

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:/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
  {
  }
  }
-}
+}