C#非矩形窗体实现方法

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Drawing.Drawing2D;
namespace WindowsApplication1
{
  public partial class Form3 : Form
  {
    Point downPoint = Point.Empty;
    public Form3()
    {
      InitializeComponent();
    }
    void Set()
    {
      Rectangle rect = this.ClientRectangle;
      using (GraphicsPath path = new GraphicsPath())
      {
        path.AddEllipse(rect);
        this.Region = new Region(path);
      }
    }
    private void Form3_Load(object sender, EventArgs e)
    {
      Set();
    }
    private void Form3_MouseDown(object sender, MouseEventArgs e)
    {
      if (e.Button != MouseButtons.Left) return;
      downPoint = new Point(e.X, e.Y);
    }
    private void Form3_MouseMove(object sender, MouseEventArgs e)
    {
      if (downPoint == Point.Empty) return;
      Point location = new Point(this.Left + e.X - downPoint.X, this.Top + e.Y - downPoint.Y);
      this.Location = location;
    }
    private void Form3_MouseUp(object sender, MouseEventArgs e)
    {
      if (e.Button != MouseButtons.Left) return;
      downPoint = Point.Empty;
    }
  }
}