class SomeControl : TestControl
{
    public string Test
    {
        get { return (string)GetValue(TestProperty); }
        set { SetValue(TestProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Test.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty TestProperty =
        DependencyProperty.Register("Test", typeof(string), typeof(SomeControl), new PropertyMetadata(default));
}