public class MyControl : Control
    {
        public GridLength ContentArea
        {
            get { return (GridLength)GetValue(ContentAreaProperty); }
            set { SetValue(ContentAreaProperty, value); }
        }

        public static readonly DependencyProperty ContentAreaProperty = DependencyProperty.Register(
            "ContentArea", typeof(GridLength), typeof(MyControl),
            new PropertyMetadata(new GridLength(0.0f)));
    }