程序代码：

using System;
using System.Collections.Generic;
using using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace _5zq
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
           

        }
            private void Form_paint(object sender, PaintEventArgs e)
        {   
             Graphics g = e.Graphics;
            Pen pen = new Pen(Color.Red, 5);
            Point point1 = new Point(300, 100);
            Point point2=new Point(750,100);
            Point point3 = new Point(300,600);
            Point point4 = new Point(750, 600);
            g.DrawLine(pen, point1,point2);
            g.DrawLine(pen, point2, point3);
            g.DrawLine(pen, point3, point4);
            g.DrawLine(pen, point4, point1);

           
            }
       
        }

        }