Fix MultiValidationBehavior overwrites external BindingContext bindings

src/CommunityToolkit/Xamarin.CommunityToolkit/Behaviors/BaseBehavior.shared.cs

@@ -21,6 +21,30 @@ static readonly FieldInfo? bindingField
 
 		protected TView? View { get; private set; }
 
+		internal bool TrySetBindingContext(Binding binding)
+		{
+			if (!IsBound(BindingContextProperty))
+			{
+				SetBinding(BindingContextProperty, defaultBindingContextBinding = binding);
+				return true;
+			}
+
+			return false;
+		}
+
+		internal bool TryRemoveBindingContext()
+		{
+			if (defaultBindingContextBinding != null)
+			{
+				RemoveBinding(BindingContextProperty);
+				defaultBindingContextBinding = null;
+
+				return true;
+			}
+
+			return false;
+		}
+
 		protected virtual void OnViewPropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
 		}
@@ -30,28 +54,17 @@ protected override void OnAttachedTo(TView bindable)
 			base.OnAttachedTo(bindable);
 			View = bindable;
 			bindable.PropertyChanged += OnViewPropertyChanged;
-
-			if (!IsBound(BindingContextProperty))
+			TrySetBindingContext(new Binding
 			{
-				defaultBindingContextBinding = new Binding
-				{
-					Path = BindingContextProperty.PropertyName,
-					Source = bindable
-				};
-				SetBinding(BindingContextProperty, defaultBindingContextBinding);
-			}
+				Path = BindingContextProperty.PropertyName,
+				Source = bindable
+			});
 		}
 
 		protected override void OnDetachingFrom(TView bindable)
 		{
 			base.OnDetachingFrom(bindable);
-
-			if (defaultBindingContextBinding != null)
-			{
-				RemoveBinding(BindingContextProperty);
-				defaultBindingContextBinding = null;
-			}
-
+			TryRemoveBindingContext();
 			bindable.PropertyChanged -= OnViewPropertyChanged;
 			View = null;
 		}

src/CommunityToolkit/Xamarin.CommunityToolkit/Behaviors/Validators/MultiValidationBehavior.shared.cs

@@ -92,7 +92,7 @@ void OnChildrenCollectionChanged(object? sender, NotifyCollectionChangedEventArg
 			{
 				foreach (var child in e.NewItems.OfType<ValidationBehavior>())
 				{
-					child.SetBinding(BindingContextProperty, new Binding
+					child.TrySetBindingContext(new Binding
 					{
 						Path = BindingContextProperty.PropertyName,
 						Source = this
@@ -103,7 +103,7 @@ void OnChildrenCollectionChanged(object? sender, NotifyCollectionChangedEventArg
 			if (e.OldItems != null)
 			{
 				foreach (var child in e.OldItems.OfType<ValidationBehavior>())
-					child.RemoveBinding(BindingContextProperty);
+					child.TryRemoveBindingContext();
 			}
 		}
 	}