Add IAddChild implementation too StyleBase

src/Avalonia.Base/Styling/StyleBase.cs

@@ -11,7 +11,7 @@ namespace Avalonia.Styling
     /// <summary>
     /// Base class for <see cref="Style"/> and <see cref="ControlTheme"/>.
     /// </summary>
-    public abstract class StyleBase : AvaloniaObject, IStyle, IResourceProvider
+    public abstract class StyleBase : AvaloniaObject, IStyle, IResourceProvider, IAddChild, IAddChild<IStyle>, IAddChild<SetterBase>
     {
         private IResourceHost? _owner;
         private StyleChildren? _children;
@@ -72,6 +72,25 @@ namespace Avalonia.Styling
         public void Add(SetterBase setter) => Setters.Add(setter);
         public void Add(IStyle style) => Children.Add(style);
 
+        void IAddChild.AddChild(object child)
+        {
+            switch (child)
+            {
+                case SetterBase setter:
+                    Setters.Add(setter);
+                    break;
+                case IStyle style:
+                    Children.Add(style);
+                    break;
+                default:
+                    throw new InvalidOperationException($"Cannot add {child.GetType()} to a style.");
+            }
+        }
+
+        void IAddChild<SetterBase>.AddChild(SetterBase setter) => Setters.Add(setter); 
+
+        void IAddChild<IStyle>.AddChild(IStyle style) => Children.Add(style);
+
         public event EventHandler? OwnerChanged;
 
         public bool TryGetResource(object key, ThemeVariant? themeVariant, out object? result)