WPF Generics in XAML

public class MyList : List<String> { };
//или
public class MyList : List<Object> { };

    [ContentProperty("Items")]
    [MarkupExtensionReturnType(typeof(IList))]
    public class ListExtension : MarkupExtension
    {
        private Type elementType = typeof(object);
 
        private IList list = new List<object>();
 
        [ConstructorArgument("elementType")]
        public Type ElementType
        {
            get => elementType;
            set
            {
                elementType = value ?? typeof(object);
 
                Type lt = typeof(List<>).MakeGenericType(elementType);
 
                var constructor = lt.GetConstructor(Array.Empty<Type>());
                IList _list = (IList)constructor!.Invoke(Array.Empty<object>());
                foreach (var item in list)
                {
                    _list.Add(item);
                }
                list = _list;
            }
        }
 
        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            return list;
        }
 
        public ListExtension()
        {
        }
 
        public ListExtension(Type? elementType)
        {
            ElementType = elementType ?? typeof(object);
        }
 
 
        /// <summary>
        /// Called to Add an object as a new array item. This will append the
        /// object to the end of the array.
        /// </summary>
        /// <param name="value">Object to add to the end of the array.</param>
        public void AddChild(object value) => list.Add(value);
 
        /// <summary>
        /// Called to Add a text as a new array item. This will append the
        /// object to the end of the array.
        /// </summary>
        /// <param name="text">Text to Add to the end of the array.</param>
        public void AddText(string text) => AddChild(text);
 
        /// <summary>
        /// An IList accessor to the contents of the array
        /// </summary>
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
        public IList Items => list;
    }