我有一个Windows窗体应用程序，我在其中添加不同的图形（矩形，圆形等）到主窗体。这个图形是一个UserControl，它的形状我用GraphicsPath定义。增加新图形的方法：

void AddElement(ShapeType shape, string guid)
    {
        Shape newShape = new Shape();
        newShape.Name = guid;
        newShape.Size = new Size(100, 100);           
        newShape.Type = shape;
        newShape.Location = new Point(100, 100);

        newShape.MouseDown += new MouseEventHandler(Shape_MouseDown);
        newShape.MouseMove += new MouseEventHandler(Shape_MouseMove);
        newShape.MouseUp += new MouseEventHandler(Shape_MouseUp);
        newShape.BackColor = this.BackColor;

        this.Controls.Add(newShape);
    }

在Shape（Figure）类中：

private ShapeType shape;
 private GraphicsPath path = null;
 public ShapeType Type
    {
        get { return shape; }
        set
        {
            shape = value;
            DrawElement();
        }
    } 

 private void DrawElement()
     {
        path = new GraphicsPath();
        switch (shape)
        {
            case ShapeType.Rectangle:
                path.AddRectangle(this.ClientRectangle);
                break;

            case ShapeType.Circle:
                path.AddEllipse(this.ClientRectangle);
                break;

            case ShapeType.Line:
                path.AddLine(10,10,20,20);                   
                break;
        }
        this.Region = new Region(path);
    }

protected override void OnPaint(System.Windows.Forms.PaintEventArgs e)
    {
        if (path != null)
        {              
            e.Graphics.DrawPath(new Pen(Color.Black, 4), path);
        }
    }

当我重新绘制图形时，我重新绘制它：

protected override void OnResize(System.EventArgs e)
    {
        DrawElement();
        this.Invalidate();
    }

当我添加像矩形和圆形这样的形状时，一切都很好。但是当我选择Line时，表单上什么也没有显示。断点显示程序在所有方法中执行步骤，也包括this.Controls.Add(newShape);。
我不明白为什么这行不通。我很感激任何建议。