再winform中，怎么让控件移动时移不出父容器。

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;



namespace tc

{

    public partial class Form4 : Form

    {

        //将被拖动的控件 

        private Control control; 

        public Form4()

        {

            InitializeComponent();

            //this.Paint += new System.Windows.Forms.PaintEventHandler(this.Form4_Paint);

            this.Paint += new System.Windows.Forms.PaintEventHandler(this.panel1_Paint);



            control = new Button();

            control.MouseDown += new MouseEventHandler(button1_MouseDown);

            control.MouseMove += new MouseEventHandler(button1_MouseMove);

            control.MouseUp += new MouseEventHandler(button1_MouseUp);

            this.Controls.Add(control);   

        }





        //鼠标按下坐标（control控件的相对坐标） 

        Point mouseDownPoint = Point.Empty;

        //显示拖动效果的矩形 

        Rectangle rect = Rectangle.Empty;

        //是否正在拖拽 

        bool isDrag = false;





        //重置变量 

        private void reset()

        {

            mouseDownPoint = Point.Empty;

            rect = Rectangle.Empty;

            isDrag = false;

        } 











        //窗体重绘 

        private void Form4_Paint(object sender, PaintEventArgs e)

        {

            if (rect != Rectangle.Empty)

            {

                if (isDrag)

                {//画一个和Control一样大小的黑框 

                    e.Graphics.DrawRectangle(Pens.Black, rect);

                }

                else

                {

                    e.Graphics.DrawRectangle(new Pen(this.BackColor), rect);

                }

            }

        }

        //把相对与control控件的坐标，转换成相对于窗体的坐标。 

        private Point getPointToForm(Point p)

        {

            return this.PointToClient(control.PointToScreen(p));

        }



         void button1_MouseDown(object sender, MouseEventArgs e)

        {

            if (e.Button == MouseButtons.Left)

            {

                mouseDownPoint = e.Location;

                //记录控件的大小 

                rect = control.Bounds;

            } 



        }



         void button1_MouseMove(object sender, MouseEventArgs e)

        {



            if (e.Button == MouseButtons.Left)

            {

                    isDrag = true;

                    //重新设置rect的位置，跟随鼠标移动 

                    rect.Location = getPointToForm(new Point(e.Location.X - mouseDownPoint.X, e.Location.Y - mouseDownPoint.Y));

                    this.Refresh();

                    

               // }

            } 



        }



         void button1_MouseUp(object sender, MouseEventArgs e)

        {

            if (e.Button == MouseButtons.Left)

            {

                if (isDrag)

                {

                    isDrag = false;

                    //移动control到放开鼠标的地方 

                    control.Location = rect.Location;

                    this.Refresh();

                }

                reset();



            } 



        }



        private void panel1_Paint(object sender, PaintEventArgs e)

        {

            if (rect != Rectangle.Empty)

            {

                if (isDrag)

                {//画一个和Control一样大小的黑框 

                    e.Graphics.DrawRectangle(Pens.Black, rect);

                }

                else

                {

                    e.Graphics.DrawRectangle(new Pen(this.BackColor), rect);

                }

            }

        } 





    }

}