关键代码：

protected override Geometry DefiningGeometry {
        get {
            Geometry result = new RectangleGeometry(new Rect(0, 0, base.Width, base.Height), RadiusX, RadiusY);
            double halfWidth = base.Width / 2;
            double halfHeight = base.Height / 2;

            if (!RoundTopLeft)
                result = new CombinedGeometry(GeometryCombineMode.Union, result, new RectangleGeometry(new Rect(0, 0, halfWidth, halfHeight)));
            if (!RoundTopRight)
                result = new CombinedGeometry(GeometryCombineMode.Union, result, new RectangleGeometry(new Rect(halfWidth, 0, halfWidth, halfHeight)));
            if (!RoundBottomLeft)
                result = new CombinedGeometry(GeometryCombineMode.Union, result, new RectangleGeometry(new Rect(0, halfHeight, halfWidth, halfHeight)));
            if (!RoundBottomRight)
                result = new CombinedGeometry(GeometryCombineMode.Union, result, new RectangleGeometry(new Rect(halfWidth, halfHeight, halfWidth, halfHeight)));

            return result;
        }
    }