.netcore 3.1 验证码

public class VerifyCodeHelper
    {
        public static Bitmap CreateVerifyCode(out string code)
        {
            //建立Bitmap对象，绘图
            Bitmap bitmap = new Bitmap(200, 60);
            Graphics graph = Graphics.FromImage(bitmap);
            graph.FillRectangle(new SolidBrush(Color.White), 0, 0, 200, 60);
            Font font = new Font(FontFamily.GenericSerif, 48, FontStyle.Bold, GraphicsUnit.Pixel);
            Random r = new Random();
            string letters = "ABCDEFGHIJKLMNPQRSTUVWXYZ0123456789";

            StringBuilder sb = new StringBuilder();

            //添加随机的五个字母
            for (int x = 0; x < 5; x++)
            {
                string letter = letters.Substring(r.Next(0, letters.Length - 1), 1);
                sb.Append(letter);
                graph.DrawString(letter, font, new SolidBrush(Color.Black), x * 38, r.Next(0, 15));
            }
            code = sb.ToString();

            //混淆背景
            Pen linePen = new Pen(new SolidBrush(Color.Black), 2);
            for (int x = 0; x < 6; x++)
                graph.DrawLine(linePen, new Point(r.Next(0, 199), r.Next(0, 59)), new Point(r.Next(0, 199), r.Next(0, 59)));
            return bitmap;
        }
    }

public ActionResult VerifyCode()
{
    string code = "";
    Bitmap bitmap = VerifyCodeHelper.CreateVerifyCode(out code);
    base.HttpContext.Session.SetString("CheckCode", code);
    MemoryStream stream = new MemoryStream();
    bitmap.Save(stream, ImageFormat.Gif);
    return File(stream.ToArray(), "image/gif");
}