from:https://blog.csdn.net/qq_29011299/article/details/81137980

using(Bitmap bmp=new Bitmap(300,50))
     {
         using(Graphics g=Graphics.FromImage(bmp))
         {
             g.Clear(Color.WhiteSmoke);
             g.DrawString("字串", new Font("隸書", 40), Brushes.Red, new PointF(50, 0));
             g.DrawEllipse(new Pen(Color.Green, 2), 10, 10, 10, 10);
         }
         using(System.IO.MemoryStream ms=new System.IO.MemoryStream())
         {
             bmp.Save(ms,System.Drawing.Imaging.ImageFormat.Png);
             string base64 = Convert.ToBase64String(ms.ToArray());
             Image1.ImageUrl = "data:image/png;base64,"+base64;
         }
     }