Fix app domain unloading issue on .NET Framework

Port of CommunityToolkit/dotnet@e2b3361

Microsoft.Toolkit.Mvvm/Messaging/Internals/System/Gen2GcCallback.cs

@@ -41,6 +41,16 @@ private Gen2GcCallback(Action<object> callback, object target)
         /// <param name="target">The target object to pass as argument to <paramref name="callback"/>.</param>
         public static void Register(Action<object> callback, object target)
         {
+#if NETSTANDARD2_0
+            if (RuntimeInformation.FrameworkDescription.StartsWith(".NET Framework"))
+            {
+                // On .NET Framework using a GC callback causes issues with app domain unloading,
+                // so the callback is not registered if that runtime is detected and just ignored.
+                // Users on .NET Framework will have to manually trim the messenger, if they'd like.
+                return;
+            }
+#endif
+
             _ = new Gen2GcCallback(callback, target);
         }
 

Microsoft.Toolkit.Mvvm/Messaging/WeakReferenceMessenger.cs

@@ -30,6 +30,7 @@ namespace Microsoft.Toolkit.Mvvm.Messaging
     /// The <see cref="WeakReferenceMessenger"/> type will automatically perform internal trimming when
     /// full GC collections are invoked, so calling <see cref="Cleanup"/> manually is not necessary to
     /// ensure that on average the internal data structures are as trimmed and compact as possible.
+    /// Note: this is not supported when running on .NET Framework, due to app domain unloading issues.
     /// </para>
     /// </remarks>
     public sealed class WeakReferenceMessenger : IMessenger