在 form上可以实现，在panel实现不了，为什么啊？
通过大家的帮助，我已经能够实现图形随着鼠标绘制。
但是问题又来了：

public partial class Form2 : Form
    {
        Point Form2_MouseMove;

        public Form2()
        {
            InitializeComponent();
        }

        private void Form2_Paint(object sender, PaintEventArgs e)
        {
            Graphics dc = e.Graphics;
            Pen blue = new Pen(Color.Blue, 3);
            dc.DrawRectangle(blue, Form2_MouseMove.X, Form2_MouseMove.Y, 50, 50);      
        }

        private void Form2_MouseMove_1(object sender, MouseEventArgs e)
        {
            Form2_MouseMove.X = e.X;
            Form2_MouseMove.Y = e.Y;
            this.Text = string.Format("鼠标位置：({0},{1})", Form2_MouseMove.X, Form2_MouseMove.Y);
            this.Invalidate();
        }

      

以上可以实现。
以下不能实现：

public partial class Form1 : Form
    {
        Point pt;

        public Form1()
        {
            InitializeComponent();
            
        }

        private void panel1_Paint(object sender, PaintEventArgs e)
        {
            Graphics dc = e.Graphics;
            Pen blue = new Pen(Color.Blue, 3);
            dc.DrawRectangle(blue, pt.X, pt.Y, 50, 50);  
        }

        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
            pt.X = e.X;
            pt.Y = e.Y;
            this.Text = string.Format("鼠标位置：({0},{1})", pt.X, pt.Y);
            this.Invalidate();
        }

  
    }

是不是panel需要新的坐标体系和新的绘制方法？

谢谢！！
[解决办法]

public partial class Form1 : Form
    {
        Point pt;
 
        public Form1()
        {
            InitializeComponent();
             
        }
 
        private void panel1_Paint(object sender, PaintEventArgs e)
        {
            Graphics dc = e.Graphics;
            Pen blue = new Pen(Color.Blue, 3);
            dc.DrawRectangle(blue, pt.X, pt.Y, 50, 50);  
        }
 
        private void panel1_MouseMove(object sender, MouseEventArgs e) 
 
        {
            pt.X = e.X;
            pt.Y = e.Y;
            this.Text = string.Format("鼠标位置：({0},{1})", pt.X, pt.Y);
            panel1.Invalidate();
        }
 
   
    }