C#窗体越界时鼠标还能回到初始坐标位置

对窗体加越界限制后，鼠标拖动窗体越界时，窗体不能动，鼠标位置可动，但窗体不再越界时，鼠标位置还能回到鼠标按下时相对窗体的坐标：
1、首先创建一个窗体Form1，然后在窗体上拖一个button1按钮（主要通过这个按钮来拖动窗体）

2、然后对按钮添加鼠标按下事件、鼠标移动事件和鼠标抬起事件，事件里面是对窗体和鼠标坐标的操作，整体代码如下：
using System.Drawing;
using System.Windows.Forms;
namespace 窗体越界鼠标还能回到初始坐标
{
    public partial class Form1 : System.Windows.Forms.Form
    {
        #region 变量定义
        public Point mainFormPosition;//主窗口左上角位置
        int workingAreaHeight = SystemInformation.WorkingArea.Height;
        int workingAreaWidth = SystemInformation.WorkingArea.Width;
        private Point relativetMousePosition;//鼠标相对窗体的相对坐标
        bool FirstLeft = false;//是否左边第一次按下时坐标
        bool FirstRight = false;//是否右边第一次按下时坐标
        bool FirstUp = false;//是否上边第一次按下时坐标
        bool FirstDown = false;//是否下边第一次按下时坐标
        bool isFirstDown = true;//是否第一次按下，鼠标越界纠正时判断是否是第一次按下，不是按下后又拖动了
        bool isMoveOut = false;//是否拖出
        //这两个变量做差，差值为拖出的距离时软件弹出侧边
        int oldMove_X = 0;//侧边停靠拖出时起始坐标
        //上一个时刻、鼠标的位置
        private Point oldMousePosition;
        //当前时刻、鼠标的位置
        private Point currentMousePosition;
        //鼠标事件
        [System.Runtime.InteropServices.DllImport("user32")]
        private static extern int mouse_event(int dwFlags, int dx, int dy, int cButtons, int dwExtraInfo);
        //移动鼠标
        const int MOUSEEVENTF_MOVE = 0x0001;
        //模拟鼠标左键按下
        const int MOUSEEVENTF_LEFTDOWN = 0x0002;
        //模拟鼠标左键抬起
        const int MOUSEEVENTF_LEFTUP = 0x0004;
        //模拟鼠标右键按下
        const int MOUSEEVENTF_RIGHTDOWN = 0x0008;
        //模拟鼠标右键抬起
        const int MOUSEEVENTF_RIGHTUP = 0x0010;
        //模拟鼠标中键按下
        const int MOUSEEVENTF_MIDDLEDOWN = 0x0020;
        //模拟鼠标中键抬起
        const int MOUSEEVENTF_MIDDLEUP = 0x0040;
        //标示是否采用绝对坐标
        const int MOUSEEVENTF_ABSOLUTE = 0x8000;
        #endregion

#region 窗体初始化
        public Form1()
        {
            InitializeComponent();
        }
        #endregion

//-------------------------按钮button1鼠标事件---------------------------------------------------------------
        #region 鼠标左键按下Button1按钮事件
        private void Button1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                currentMousePosition = Control.MousePosition;
                oldMousePosition = currentMousePosition;   //鼠标按下时、两个时刻的坐标变成一样，防止错误移动            
                mainFormPosition = Location;
                if (isFirstDown)//鼠标按下时，记录第一次鼠标按下相对主窗体的坐标
                {
                    relativetMousePosition.X = currentMousePosition.X - Location.X;
                    relativetMousePosition.Y = currentMousePosition.Y - Location.Y;
                    isFirstDown = false;
                }
                if (isMoveOut)//鼠标按下时，记录拖出时的初始坐标
                {
                    oldMove_X = currentMousePosition.X;
                }
            }
        }
        #endregion

#region 鼠标左键按住Button1按钮移动事件
        private void Button1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                currentMousePosition = Control.MousePosition;
                if (currentMousePosition.X < relativetMousePosition.X && currentMousePosition.Y > relativetMousePosition.Y && currentMousePosition.Y < workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y)//左边窗体越界坐标纠正
                {
                    mainFormPosition.X = 0;
                    mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.Y = currentMousePosition.Y;
                    FirstLeft = true;
                }
                else if (currentMousePosition.X > workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X && currentMousePosition.Y > relativetMousePosition.Y && currentMousePosition.Y < workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y)//右边窗体越界坐标纠正
                {
                    mainFormPosition.X = workingAreaWidth - this.ClientSize.Width;
                    mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.Y = currentMousePosition.Y;
                    FirstRight = true;
                }
                else if (currentMousePosition.Y < relativetMousePosition.Y && currentMousePosition.X > relativetMousePosition.X && currentMousePosition.X < workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X)//上边窗体越界坐标纠正
                {
                    mainFormPosition.Y = 0;
                    mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.X = currentMousePosition.X;
                    FirstUp = true;
                }
                else if (currentMousePosition.Y > workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y && currentMousePosition.X > relativetMousePosition.X && currentMousePosition.X < workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X)//下边窗体越界坐标纠正
                {
                    mainFormPosition.Y = workingAreaHeight - this.ClientSize.Height;
                    mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.X = currentMousePosition.X;
                    FirstDown = true;
                }
                else if (currentMousePosition.X < relativetMousePosition.X && currentMousePosition.Y < relativetMousePosition.Y)//左上角窗体越界坐标纠正
                {
                    mainFormPosition.X = 0;
                    mainFormPosition.Y = 0;
                    mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                    mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.X = currentMousePosition.X;
                    oldMousePosition.Y = currentMousePosition.Y;
                    FirstLeft = true;
                    FirstUp = true;
                }
                else if (currentMousePosition.X > workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X && currentMousePosition.Y < relativetMousePosition.Y)//右上角窗体越界坐标纠正
                {
                    mainFormPosition.X = workingAreaWidth - this.ClientSize.Width;
                    mainFormPosition.Y = 0;
                    mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                    mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.X = currentMousePosition.X;
                    oldMousePosition.Y = currentMousePosition.Y;
                    FirstRight = true;
                    FirstUp = true;
                }
                else if (currentMousePosition.X < relativetMousePosition.X && currentMousePosition.Y > workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y)//左下角窗体越界坐标纠正
                {
                    mainFormPosition.X = 0;
                    mainFormPosition.Y = workingAreaHeight - this.ClientSize.Height;
                    mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                    mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.X = currentMousePosition.X;
                    oldMousePosition.Y = currentMousePosition.Y;
                    FirstLeft = true;
                    FirstDown = true;
                }
                else if (currentMousePosition.X > workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X && currentMousePosition.Y > workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y)//右下角窗体越界坐标纠正
                {
                    mainFormPosition.Y = workingAreaHeight - this.ClientSize.Height;
                    mainFormPosition.X = workingAreaWidth - this.ClientSize.Width;
                    mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                    mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                    //保存上一个时刻的鼠标位置
                    oldMousePosition.X = currentMousePosition.X;
                    oldMousePosition.Y = currentMousePosition.Y;
                    FirstRight = true;
                    FirstDown = true;
                }
                if (FirstLeft)
                {
                    currentMousePosition.X = relativetMousePosition.X;
                    FirstLeft = false;
                }
                if (FirstRight)
                {
                    currentMousePosition.X = workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X;
                    FirstRight = false;
                }
                if (FirstUp)
                {
                    currentMousePosition.Y = relativetMousePosition.Y;
                    FirstUp = false;
                }
                if (FirstDown)
                {
                    currentMousePosition.Y = workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y;
                    FirstDown = false;
                }
                if (FirstLeft && FirstUp)
                {
                    currentMousePosition.X = relativetMousePosition.X;
                    currentMousePosition.Y = relativetMousePosition.Y;
                    FirstLeft = false;
                    FirstUp = false;

}
                if (FirstRight && FirstUp)
                {
                    currentMousePosition.X = workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X;
                    currentMousePosition.Y = relativetMousePosition.Y;
                    FirstUp = false;
                    FirstRight = false;
                }
                if (FirstLeft && FirstDown)
                {
                    currentMousePosition.X = relativetMousePosition.X;
                    currentMousePosition.Y = workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y;
                    FirstLeft = false;
                    FirstDown = false;
                }
                if (FirstRight && FirstDown)
                {
                    currentMousePosition.X = workingAreaWidth - this.ClientSize.Width + relativetMousePosition.X;
                    currentMousePosition.Y = workingAreaHeight - this.ClientSize.Height + relativetMousePosition.Y;
                    FirstRight = false;
                    FirstDown = false;
                }
                //对窗体进行移动
                mainFormPosition.X += currentMousePosition.X - oldMousePosition.X;
                mainFormPosition.Y += currentMousePosition.Y - oldMousePosition.Y;
                //保存上一个时刻的鼠标位置
                oldMousePosition = currentMousePosition;

#region 越界处理
                if (mainFormPosition.X < 0)//左边越界
                {
                    mainFormPosition.X = 0;
                }
                else if (mainFormPosition.Y < 0)//上边越界
                {
                    mainFormPosition.Y = 0;
                }
                else if (mainFormPosition.X > workingAreaWidth - this.ClientSize.Width)//右边越界
                {
                    mainFormPosition.X = workingAreaWidth - this.ClientSize.Width;
                }
                else if (mainFormPosition.Y > workingAreaHeight - this.ClientSize.Height)//下边越界
                {
                    mainFormPosition.Y = workingAreaHeight - this.ClientSize.Height;
                }
                //更新主窗口位置
                Location = mainFormPosition;
                #endregion
            }
        }
        #endregion

#region 鼠标在按钮Button1抬起事件
        private void Button1_MouseUp(object sender, MouseEventArgs e)
        {
            isFirstDown = true;
            //如果主窗体在这个位置，就判断允许主窗体在鼠标抬起后执行移动事件
            if (mainFormPosition.X == workingAreaWidth - this.ClientSize.Width)
            {
                isMoveOut = false;
            }
            if (mainFormPosition.X == 0)
            {
                isMoveOut = false;
            }
        }
        #endregion
    }
}

动画效果如下：

这是本人在一个项目中遇到的问题，所以特意总结出来分享给大家。