Fix ChildChanged event not being triggered for BusinessListBase objects in Blazor ViewModel (#4747)

* Initial plan

* Fix ChildChanged event hook to support BusinessListBase objects

Co-authored-by: rockfordlhotka <2333134+rockfordlhotka@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rockfordlhotka <2333134+rockfordlhotka@users.noreply.github.com>

Author: Copilot

Source/Csla.Blazor/ViewModel.cs

@@ -114,7 +114,7 @@ protected void UnhookChangedEvents(T? model)
       if (model is INotifyPropertyChanged npc)
         npc.PropertyChanged -= OnModelPropertyChanged;
 
-      if (model is IBusinessBase ncc)
+      if (model is INotifyChildChanged ncc)
         ncc.ChildChanged -= OnModelChildChanged;
 
       if (model is INotifyCollectionChanged cc)
@@ -130,7 +130,7 @@ private void HookChangedEvents(T? model)
       if (model is INotifyPropertyChanged npc)
         npc.PropertyChanged += OnModelPropertyChanged;
 
-      if (model is IBusinessBase ncc)
+      if (model is INotifyChildChanged ncc)
         ncc.ChildChanged += OnModelChildChanged;
 
       if (model is INotifyCollectionChanged cc)

Source/tests/Csla.Blazor.Test/ViewModelChildChangedEventTests.cs

@@ -0,0 +1,101 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Csla.Blazor.Test.Fakes;
+using Csla.TestHelpers;
+using System.Threading.Tasks;
+using Csla.Core;
+
+namespace Csla.Blazor.Test
+{
+  [TestClass]
+  public class ViewModelChildChangedEventTests
+  {
+    private static TestDIContext _testDIContext;
+
+    [ClassInitialize]
+    public static void ClassInitialize(TestContext context)
+    {
+      _testDIContext = TestDIContextFactory.CreateDefaultContext();
+    }
+
+    [TestCleanup]
+    public void CleanupTests()
+    {
+      FakeDataStorage.ClearDataStorage();
+    }
+
+    [TestMethod]
+    public void ViewModel_WithBusinessBase_ShouldHookChildChangedEvent()
+    {
+      // Arrange
+      var appContext = TestDIContextExtensions.CreateTestApplicationContext(_testDIContext);
+      var vm = new ViewModel<FakePerson>(appContext);
+      bool childChangedCalled = false;
+      vm.ModelChildChanged += (sender, e) => childChangedCalled = true;
+
+      // Act - Create and set a FakePerson model (BusinessBase)
+      IDataPortal<FakePerson> dataPortal = _testDIContext.CreateDataPortal<FakePerson>();
+      var person = dataPortal.Create();
+      vm.Model = person;
+
+      // Trigger child changed by modifying email addresses
+      var emailAddress = person.EmailAddresses.AddNew();
+      emailAddress.EmailAddress = "test@example.com";
+
+      // Assert
+      Assert.IsTrue(childChangedCalled, "ModelChildChanged event should be raised for BusinessBase");
+    }
+
+    [TestMethod]
+    public void ViewModel_WithBusinessListBase_ShouldHookChildChangedEvent()
+    {
+      // Arrange
+      var appContext = TestDIContextExtensions.CreateTestApplicationContext(_testDIContext);
+      var vm = new ViewModel<FakePersonEmailAddresses>(appContext);
+      bool childChangedCalled = false;
+      vm.ModelChildChanged += (sender, e) => childChangedCalled = true;
+
+      // Act - Create and set a FakePersonEmailAddresses model (BusinessListBase)
+      IChildDataPortal<FakePersonEmailAddresses> dataPortal = _testDIContext.CreateChildDataPortal<FakePersonEmailAddresses>();
+      var emailAddresses = dataPortal.CreateChild();
+      vm.Model = emailAddresses;
+
+      // Trigger child changed by adding an item
+      var emailAddress = emailAddresses.AddNew();
+      emailAddress.EmailAddress = "test@example.com";
+
+      // Assert
+      Assert.IsTrue(childChangedCalled, "ModelChildChanged event should be raised for BusinessListBase");
+    }
+
+    [TestMethod]
+    public void ViewModel_WhenModelChanged_ShouldUnhookOldAndHookNewChildChangedEvent()
+    {
+      // Arrange
+      var appContext = TestDIContextExtensions.CreateTestApplicationContext(_testDIContext);
+      var vm = new ViewModel<FakePersonEmailAddresses>(appContext);
+      int childChangedCount = 0;
+      vm.ModelChildChanged += (sender, e) => childChangedCount++;
+
+      IChildDataPortal<FakePersonEmailAddresses> dataPortal = _testDIContext.CreateChildDataPortal<FakePersonEmailAddresses>();
+      var emailAddresses1 = dataPortal.CreateChild();
+      var emailAddresses2 = dataPortal.CreateChild();
+
+      // Act - Set first model
+      vm.Model = emailAddresses1;
+      var email1 = emailAddresses1.AddNew();
+      email1.EmailAddress = "test1@example.com";
+
+      // Change model
+      vm.Model = emailAddresses2;
+      var email2 = emailAddresses2.AddNew();
+      email2.EmailAddress = "test2@example.com";
+
+      // Modify old model - should NOT trigger event
+      var email3 = emailAddresses1.AddNew();
+      email3.EmailAddress = "test3@example.com";
+
+      // Assert
+      Assert.AreEqual(2, childChangedCount, "ModelChildChanged should only be raised for current model");
+    }
+  }
+}