登錄頁面的數字+字母圖片驗證碼自動生成

c#自動生成數字+字母組成的驗證碼圖片，可以自動設置驗證碼的位數。

代碼如下：

using System.Drawing;
public ActionResult Image()
        {
            string checkCode = GetValidation(4);
            Session["Code"] = checkCode;
            Bitmap image = new Bitmap(75, 34); //圖片大小尺寸
            Graphics g = Graphics.FromImage(image);

            try
            {
                Random random = new Random(); //生成隨機生成器
                g.Clear(Color.White); //清空圖片背景顏色
                //畫背景噪音線
                for (int i = 0; i < 25; 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(GetRandomColor()), x1, y1, x2, y2);
                }
                //設置字體格式
                Font font = new Font("Arial", 12, FontStyle.Bold);
                System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(
                    0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2F, true);
                g.DrawString(checkCode, font, brush, 12, 8);
                //畫前景噪音點
                g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);
                System.IO.MemoryStream ms = new System.IO.MemoryStream();
                image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
                return File(ms.ToArray(), "image/Gif");
            }
            finally
            {
                g.Dispose();
                image.Dispose();
            }
        }

        private Color GetRandomColor()
        {
            int r, g, b;
            Random ra = new Random();
            r = ra.Next(0, 256);
            g = ra.Next(0, 256);
            b = ra.Next(0, 256);
            Color _color = Color.FromArgb(r, g, b);
            return _color;
        }

        public string GetValidation(int num)
        {
            string str = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxzy";
            string validatecode = "";
            Random rd = new Random();
            for (int i = 0; i < num; i++)
            {
                validatecode += str.Substring(rd.Next(0, str.Length), 1);
            }
            return validatecode;
        }