มีปัญหาเรื่องการเขียนโปรแกรมใช้เมาส์ลากเส้นเหมือนเราวากรูปด้วยดินสอ

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

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        private bool isMouseDownLeftState = false;
        private Point beginLocation;
        private Point endLocation;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                beginLocation = new Point(e.X, e.Y);
                isMouseDownLeftState = true;
            }
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (isMouseDownLeftState)
            {
                endLocation = new Point(e.X, e.Y);
                DrawLine();
            }
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            endLocation = new Point(e.X, e.Y);
            isMouseDownLeftState = false;
            DrawLine();
        }

        private void DrawLine()
        {
            using (Graphics g = this.CreateGraphics())
            {
                g.Clear(this.BackColor);
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                g.DrawLine(Pens.Red, beginLocation, endLocation);
            }
        } 
    }
}