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实现方式一：

const int HTLEFT = 10;
        const int HTRIGHT = 11;
        const int HTTOP = 12;
        const int HTTOPLEFT = 13;
        const int HTTOPRIGHT = 14;
        const int HTBOTTOM = 15;
        const int HTBOTTOMLEFT = 0x10;
        const int HTBOTTOMRIGHT = 17;
        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case 0x0084:
                    base.WndProc(ref m);
                    Point vPoint = new Point((int)m.LParam & 0xFFFF,
                        (int)m.LParam >> 16 & 0xFFFF);
                    vPoint = PointToClient(vPoint);
                    if (vPoint.X <= 5)
                        if (vPoint.Y <= 5)
                            m.Result = (IntPtr)HTTOPLEFT;
                        else if (vPoint.Y >= ClientSize.Height - 5)
                            m.Result = (IntPtr)HTBOTTOMLEFT;
                        else m.Result = (IntPtr)HTLEFT;
                    else if (vPoint.X >= ClientSize.Width - 5)
                        if (vPoint.Y <= 5)
                            m.Result = (IntPtr)HTTOPRIGHT;
                        else if (vPoint.Y >= ClientSize.Height - 5)
                            m.Result = (IntPtr)HTBOTTOMRIGHT;
                        else m.Result = (IntPtr)HTRIGHT;
                    else if (vPoint.Y <= 5)
                        m.Result = (IntPtr)HTTOP;
                    else if (vPoint.Y >= ClientSize.Height - 5)
                        m.Result = (IntPtr)HTBOTTOM;
                    break;
                case 0x0201://鼠标左键按下的消息 
                    m.Msg = 0x00A1;//更改消息为非客户区按下鼠标 
                    m.LParam = IntPtr.Zero;//默认值 
                    m.WParam = new IntPtr(2);//鼠标放在标题栏内 
                    base.WndProc(ref m);
                    break;
                default:
                    base.WndProc(ref m);
                    break;
            }
        }

实现方式二：

 #region 更改窗体大小
        private bool isMouseDown = false; //表示鼠标当前是否处于按下状态，初始值为否 
        private MouseDirection direction = MouseDirection.None;//表示拖动的方向，起始为None，表示不拖动
        private void QueueForm_MouseDown(object sender, MouseEventArgs e)
        {
            isMouseDown = true;//鼠标按下 
        }

        private void QueueForm_MouseUp(object sender, MouseEventArgs e)
        {
            isMouseDown = false;// 鼠标弹起，
            direction = MouseDirection.None; //既然鼠标弹起了，那么就不能再改变窗体尺寸，拖拽方向置 none
        }

        private void QueueForm_MouseMove(object sender, MouseEventArgs e)
        {
            //鼠标移动过程中，坐标时刻在改变 
            //当鼠标移动时横坐标距离窗体右边缘5像素以内且纵坐标距离下边缘也在5像素以内时，要将光标变为倾斜的箭头形状，同时拖拽方向direction置为MouseDirection.Declining 
            if (e.Location.X >= this.Width - 5 && e.Location.Y > this.Height - 5)
            {
                this.Cursor = Cursors.SizeNWSE;
                direction = MouseDirection.Declining;
            }
            //当鼠标移动时横坐标距离窗体右边缘5像素以内时，要将光标变为倾斜的箭头形状，同时拖拽方向direction置为MouseDirection.Herizontal
            else if (e.Location.X >= this.Width - 5)
            {
                this.Cursor = Cursors.SizeWE;
                direction = MouseDirection.Herizontal;
            }
            //同理当鼠标移动时纵坐标距离窗体下边缘5像素以内时，要将光标变为倾斜的箭头形状，同时拖拽方向direction置为MouseDirection.Vertical
            else if (e.Location.Y >= this.Height - 5)
            {
                this.Cursor = Cursors.SizeNS;
                direction = MouseDirection.Vertical;
            }
            //否则，以外的窗体区域，鼠标星座均为单向箭头（默认）             
            else this.Cursor = Cursors.Arrow;
            //设定好方向后，调用下面方法，改变窗体大小  
            ResizeWindow();
        }

        private void ResizeWindow()
        {
            //这个判断很重要，只有在鼠标按下时才能拖拽改变窗体大小，如果不作判断，那么鼠标弹起和按下时，窗体都可以改变 
            if (!isMouseDown) return;
            //MousePosition的参考点是屏幕的左上角，表示鼠标当前相对于屏幕左上角的坐标this.left和this.top的参考点也是屏幕，属性MousePosition是该程序的重点
            if (direction == MouseDirection.Declining)
            {
                //此行代码在mousemove事件中已经写过，在此再写一遍，并不多余，一定要写
                this.Cursor = Cursors.SizeNWSE;
                //下面是改变窗体宽和高的代码，不明白的可以仔细思考一下
                this.Width = MousePosition.X - this.Left;
                this.Height = MousePosition.Y - this.Top;
            }
            //以下同理
            if (direction == MouseDirection.Herizontal)
            {
                this.Cursor = Cursors.SizeWE;
                this.Width = MousePosition.X - this.Left;
            }
            else if (direction == MouseDirection.Vertical)
            {
                this.Cursor = Cursors.SizeNS;
                this.Height = MousePosition.Y - this.Top;
            }
            //即使鼠标按下，但是不在窗口右和下边缘，那么也不能改变窗口大小
            else this.Cursor = Cursors.Arrow;
        }
        
        #endregion
   
    }

    /// <summary>
    /// 定义一个枚举，表示拖动方向
    /// </summary>
    public enum MouseDirection
    {
        Herizontal,//水平方向拖动，只改变窗体的宽度        
        Vertical,//垂直方向拖动，只改变窗体的高度 
        Declining,//倾斜方向，同时改变窗体的宽度和高度        
        None//不做标志，即不拖动窗体改变大小 
    }