理解C#生成验证码的过程

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Text;

namespace Controllers.Core.Util
{
  /// <summary>
  /// 验证码
  /// </summary>
  public class VerifyCodeHelper : AdminBaseController
  {
    #region 变量
    /// <summary>
    /// 颜色表
    /// </summary>
    private static Color[] colors = new Color[]{
      Color.FromArgb(220,20,60),
      Color.FromArgb(128,0,128),
      Color.FromArgb(65,105,225),
      Color.FromArgb(70,130,180),
      Color.FromArgb(46,139,87),
      Color.FromArgb(184,134,11),
      Color.FromArgb(255,140,0),
      Color.FromArgb(139,69,19),
      Color.FromArgb(0,191,255),
      Color.FromArgb(95,158,160),
      Color.FromArgb(255,20,147),
      Color.FromArgb(255,165,0)};

    /// <summary>
    /// 字体表
    /// </summary>
    private static string[] fonts = new string[] { 
      "Arial",
      "Verdana", 
      "Georgia", 
      "黑体" };

    /// <summary>
    /// 字体大小
    /// </summary>
    private static int fontSize = 22;
    #endregion

    #region 生成验证码图片
    /// <summary>
    /// 生成验证码图片
    /// </summary>
    public static Bitmap CreateVerifyCodeBmp(out string code)
    {
      int width = 120;
      int height = 40;
      Bitmap bmp = new Bitmap(width, height);
      Graphics g = Graphics.FromImage(bmp);
      Random rnd = new Random();

      //背景色
      g.FillRectangle(new SolidBrush(Color.White), new Rectangle(0, 0, width, height));

      //文字
      StringBuilder sbCode = new StringBuilder();
      for (int i = 0; i < 4; i++)
      {
        string str = GetChar(rnd);
        Font font = GetFont(rnd);
        Color color = GetColor(rnd);
        g.DrawString(str, font, new SolidBrush(color), new PointF((float)(i * width / 4.0), 0));
        sbCode.Append(str);
      }
      code = sbCode.ToString();

      //噪音线
      for (int i = 0; i < 10; i++)
      {
        int x1 = rnd.Next(bmp.Width);
        int x2 = rnd.Next(bmp.Width);
        int y1 = rnd.Next(bmp.Height);
        int y2 = rnd.Next(bmp.Height);

        Pen p = new Pen(GetColor(rnd), 1);
        g.DrawLine(p, x1, y1, x2, y2);
      }

      //扭曲
      bmp = TwistImage(bmp, true, 3, rnd.NextDouble() * Math.PI * 2);
      g = Graphics.FromImage(bmp);

      //噪点
      for (int i = 0; i < 100; i++)
      {
        int x1 = rnd.Next(bmp.Width);
        int y1 = rnd.Next(bmp.Height);

        Pen p = new Pen(GetColor(rnd), 1);
        g.DrawRectangle(p, x1, y1, 1, 1);
      }

      //边框
      g.DrawRectangle(new Pen(new SolidBrush(Color.FromArgb(153, 153, 153))), new Rectangle(0, 0, width - 1, height - 1));

      return bmp;
    }
    #endregion

    #region 获取随机字符
    /// <summary>
    /// 获取随机字符
    /// </summary>
    private static string GetChar(Random rnd)
    {
      int n = rnd.Next(0, 61);
      if (n <= 9)
      {
        return ((char)(48 + n)).ToString();
      }
      else if (n <= 35)
      {
        return ((char)(65 + n - 10)).ToString();
      }
      else
      {
        return ((char)(97 + n - 36)).ToString();
      }
    }
    #endregion

    #region 获取随机字体
    /// <summary>
    /// 获取随机字体
    /// </summary>
    private static Font GetFont(Random rnd)
    {
      return new Font(fonts[rnd.Next(0, fonts.Length)], fontSize, FontStyle.Bold);
    }
    #endregion

    #region 获取随机颜色
    /// <summary>
    /// 获取随机颜色
    /// </summary>
    private static Color GetColor(Random rnd)
    {
      return colors[rnd.Next(0, colors.Length)];
    }
    #endregion

    #region 正弦曲线Wave扭曲图片
    /// <summary>  
    /// 正弦曲线Wave扭曲图片（Edit By 51aspx.com）  
    /// </summary>  
    /// <param name="srcBmp">图片路径</param>  
    /// <param name="bXDir">如果扭曲则选择为True</param>  
    /// <param name="nMultValue">波形的幅度倍数，越大扭曲的程度越高，一般为3</param>  
    /// <param name="dPhase">波形的起始相位，取值区间[0-2*PI)</param>  
    private static System.Drawing.Bitmap TwistImage(Bitmap srcBmp, bool bXDir, double dMultValue, double dPhase)
    {
      System.Drawing.Bitmap destBmp = new Bitmap(srcBmp.Width, srcBmp.Height);

      // 将位图背景填充为白色  
      System.Drawing.Graphics graph = System.Drawing.Graphics.FromImage(destBmp);
      graph.FillRectangle(new SolidBrush(System.Drawing.Color.White), 0, 0, destBmp.Width, destBmp.Height);
      graph.Dispose();

      double dBaseAxisLen = bXDir ? (double)destBmp.Height : (double)destBmp.Width;

      for (int i = 0; i < destBmp.Width; i++)
      {
        for (int j = 0; j < destBmp.Height; j++)
        {
          double dx = 0;
          dx = bXDir ? (Math.PI * 2 * (double)j) / dBaseAxisLen : (Math.PI * 2 * (double)i) / dBaseAxisLen;
          dx += dPhase;
          double dy = Math.Sin(dx);

          // 取得当前点的颜色  
          int nOldX = 0, nOldY = 0;
          nOldX = bXDir ? i + (int)(dy * dMultValue) : i;
          nOldY = bXDir ? j : j + (int)(dy * dMultValue);

          System.Drawing.Color color = srcBmp.GetPixel(i, j);
          if (nOldX >= 0 && nOldX < destBmp.Width
           && nOldY >= 0 && nOldY < destBmp.Height)
          {
            destBmp.SetPixel(nOldX, nOldY, color);
          }
        }
      }

      return destBmp;
    }
    #endregion

  }
}

public ActionResult VerifyCode()
{
  string code;
  Bitmap bmp = VerifyCodeHelper.CreateVerifyCodeBmp(out code);
  Bitmap newbmp = new Bitmap(bmp, 108, 36);
  HttpContext.Session["VerifyCode"] = code;

  Response.Clear();
  Response.ContentType = "image/bmp";
  newbmp.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Bmp);

  return View();
}

$(function () {
  //刷新验证码
  $("#refreshVerifyCode").click(function () {
    refreshVerifyCode(); //刷新验证码
  });
  $("#verifyCode").click(function () {
    refreshVerifyCode(); //刷新验证码
  });
  refreshVerifyCode();
});

//刷新验证码
function refreshVerifyCode() {
  $("#verifyCode").attr("src", "VerifyCode?t=" + new Date().valueOf());
}
 

public ActionResult CheckVCode(string vcode)
{
  if (HttpContext.Session["VerifyCode"].ToString().ToLower() == vcode.ToLower())
  {
    Dictionary<string, object> dic = new Dictionary<string, object>();
    dic["ok"] = true;
    return Content(JsonConvert.SerializeObject(dic));
  }
  else
  {
    Dictionary<string, object> dic = new Dictionary<string, object>();
    dic["ok"] = false;
    return Content(JsonConvert.SerializeObject(dic));
  }
}