RequiredFieldAdorner.cs

public class RequiredFieldAdorner : Adorner
    {
        public RequiredFieldAdorner(UIElement adornedElement)
            : base(adornedElement)
        {
        }

        protected override void OnRender(DrawingContext drawingContext)
        {
            var adornedElementRect = new Rect(AdornedElement.RenderSize);

            var renderBrush = new SolidColorBrush(Colors.Red);
            var renderPen = new Pen(new SolidColorBrush(Colors.Red), 1);

            var geometry = Geometry.Parse("M 0,0 L 7,0 7,7 0,0");
            var transform = new TranslateTransform(adornedElementRect.Width - 8, 1);

            drawingContext.DrawGeometry(renderBrush, renderPen, Geometry.Combine(geometry, Geometry.Empty, GeometryCombineMode.Union, transform));
        }
    }