WinForm限制窗体不能移到屏幕外的方法

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Drawing;
using System.Runtime.InteropServices;
namespace AppForm
{
 /// <summary>
 /// WinForm限制窗体不能移到屏幕外
 /// </summary>
 public class FrmBase : Form
 {
 private Point _mouseDownPos;
 private bool _move;
 protected override void WndProc(ref Message m)
 {
  RECT nativeRect;
  switch (m.Msg)
  {
  case 0x20:
   int lp = m.LParam.ToInt32();
   if ((lp & 0xFFFF) == 2 &&
   ((lp >> 0x10) & 0xFFFF) == 0x201)
   {
   _mouseDownPos = Control.MousePosition;
   _move = true;
   }
   break;
  case 0x231:
   if (_move)
   {
   Rectangle rect = Screen.GetWorkingArea(this);
   nativeRect = new RECT(
    _mouseDownPos.X - Location.X,
    _mouseDownPos.Y - Location.Y,
    rect.Right - (Bounds.Right - _mouseDownPos.X),
    rect.Bottom - (Bounds.Bottom - _mouseDownPos.Y));
   ClipCursor(ref nativeRect);
   }
   break;
  case 0x0232:
   if (_move)
   {
   nativeRect = new RECT(Screen.GetWorkingArea(this));
   ClipCursor(ref nativeRect);
   _move = false;
   }
   break;
  }
  base.WndProc(ref m);
 }
 [DllImport("user32.dll")]
 public static extern bool ClipCursor(ref RECT lpRect);
 [StructLayout(LayoutKind.Sequential)]
 public struct RECT
 {
  public int Left;
  public int Top;
  public int Right;
  public int Bottom;
  public RECT(int left, int top, int right, int bottom)
  {
  Left = left;
  Top = top;
  Right = right;
  Bottom = bottom;
  }
  public RECT(Rectangle rect)
  {
  Left = rect.Left;
  Top = rect.Top;
  Right = rect.Right;
  Bottom = rect.Bottom;
  }
  public Rectangle Rect
  {
  get
  {
   return new Rectangle(
   Left,
   Top,
   Right - Left,
   Bottom - Top);
  }
  }
  public Size Size
  {
  get
  {
   return new Size(Right - Left, Bottom - Top);
  }
  }
  public static RECT FromXYWH(int x, int y, int width, int height)
  {
  return new RECT(x,
    y,
    x + width,
    y + height);
  }
  public static RECT FromRectangle(Rectangle rect)
  {
  return new RECT(rect.Left,
     rect.Top,
     rect.Right,
     rect.Bottom);
  }
 }
 }
}