private string CheckCode()
        {
            int number;
            char code;
            string checkCode = String.Empty;
            Random random = new Random();
            for (int i = 0; i < 4; i++)
            {
                number=random.Next();
                if(number%2==0)
                    code=(char)('0'+(char)(number%10));
                else
                    code=(char)('A'+(char)(number%26));
                checkCode += " " + code.ToString();
            }
            return checkCode;
        }

        private void CodeImage(string checkCode)
        {
            if (checkCode == null || checkCode.Trim() == String.Empty)
            {
                return;
            }
            System.Drawing.Bitmap image=new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length*50.0)),50);
            Graphics g=Graphics.FromImage(image);
            try
            {
                Random random = new Random();
                g.Clear(Color.White);
                for (int i = 0; i < 3; i++)
                {
                    int x1 = random.Next(image.Width);
                    int x2 = random.Next(image.Width);
                    int y1 = random.Next(image.Height);
                    int y2 = random.Next(image.Height);
                    g.DrawLine(new Pen(Color.Black), x1, y1, x2, y2);
                }
                Font font = new System.Drawing.Font("Arial", 30, (System.Drawing.FontStyle.Bold));
                g.DrawString(checkCode, font, new SolidBrush(Color.Red), 2, 2);
                for (int i = 0; i < 200; i++)
                {
                    int x = random.Next(image.Width);
                    int y = random.Next(image.Height);
                    image.SetPixel(x, y, Color.FromArgb(random.Next()));
                }
                g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);
                this.pictureBox1.Width = image.Width;
                this.pictureBox1.Height = image.Height;
                this.pictureBox1.BackgroundImage = image;
            }
            catch
            {

            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            CodeImage(CheckCode());
        }
        private void button1_Click(object sender, EventArgs e)
        {
            CodeImage(CheckCode());
        }