生成验证码

StringBuilder sb = new StringBuilder();
Random random = new Random();
for (int i = 0; i < 4; i++)
{
    sb.Append(random.Next(0, 9));
}

Bitmap img = new Bitmap((int)Math.Ceiling((sb.ToString().Length * 12.5)), 22);
Graphics gc = Graphics.FromImage(img);
gc.Clear(Color.White);
gc.DrawRectangle(new Pen(Color.Silver), 0, 0, img.Width - 1, img.Height - 1);
Font font = new Font("Arial", 12, (FontStyle.Bold | FontStyle.Italic));
LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, img.Width, img.Height), Color.Blue, Color.DarkRed, 1.2f, true);
gc.DrawString(sb.ToString(), font, brush, 2, 2);

MemoryStream ms = new MemoryStream();
img.Save(ms, ImageFormat.Jpeg);
Response.ClearContent();
Response.ContentType = "image/gif";
Response.BinaryWrite(ms.ToArray());
gc.Dispose();
img.Dispose();
Response.End();