您需要将您的属性声明为 Dependency Properties

You need to declare your property as Dependency Properties

namespace MyProject.Controls
{
    public partial class TestControl : UserControl
    {
        //Register Dependency Property

        public static readonly DependencyProperty TestMeDependency = DependencyProperty.Register("MyProperty", typeof(string), typeof(TestControl));

        public string MyCar
        {
            get
            {

                return (string)GetValue(TestMeDependency);

            }
            set
            {
                SetValue(TestMeDependency, value);
            }
        }

        public TestControl()
        {
            InitializeComponent();
        }
    }
}