Fix ComboBox not showing Text property validation (#20978)

src/Avalonia.Controls/ComboBox.cs

@@ -85,7 +85,8 @@ namespace Avalonia.Controls
         /// Defines the <see cref="Text"/> property
         /// </summary>
         public static readonly StyledProperty<string?> TextProperty =
-            TextBlock.TextProperty.AddOwner<ComboBox>(new(string.Empty, BindingMode.TwoWay));
+            TextBlock.TextProperty.AddOwner<ComboBox>(new(string.Empty, BindingMode.TwoWay,
+                enableDataValidation: true));
 
         /// <summary>
         /// Defines the <see cref="SelectionBoxItemTemplate"/> property.
@@ -236,6 +237,14 @@ namespace Avalonia.Controls
             UpdateFlowDirection();
         }
 
+        protected override void UpdateDataValidation(AvaloniaProperty property, BindingValueType state, Exception? error)
+        {
+            base.UpdateDataValidation(property, state, error);
+
+            if (property == TextProperty)
+                DataValidationErrors.SetError(this, error);
+        }
+
         protected internal override Control CreateContainerForItemOverride(object? item, int index, object? recycleKey)
         {
             return new ComboBoxItem();

tests/Avalonia.Controls.UnitTests/ComboBoxTests.cs

@@ -391,6 +391,28 @@ namespace Avalonia.Controls.UnitTests
             
         }
 
+        [Fact]
+        public void Text_Validation()
+        {
+            using (UnitTestApplication.Start(TestServices.MockThreadingInterface))
+            {
+                var target = new ComboBox
+                {
+                    Template = GetTemplate(),
+                };
+
+                target.ApplyTemplate();
+                target.Presenter!.ApplyTemplate();
+                
+                var exception = new System.InvalidCastException("failed validation");
+                var textObservable = new BehaviorSubject<BindingNotification>(new BindingNotification(exception, BindingErrorType.DataValidationError));
+                target.Bind(ComboBox.TextProperty, textObservable);
+
+                Assert.True(DataValidationErrors.GetHasErrors(target));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(target));
+            }
+        }
+
         [Fact]
         public void Close_Window_On_Alt_F4_When_ComboBox_Is_Focus()
         {