asp.net验证码生成和刷新及验证

验证码技术是为了防止暴力破解等而设定的。现在一般的网站注册等都提供验证码功能，特别是腾讯更是长长的一串。文中参考了http://www.cnblogs.com/netflu/archive/2007/01/10/242038.html的代码。有了就没有必要再写了。可以读一下。不过我测试时发现了两次PageLoad的问题。注释了两句即可。同时修改了namespaces。同时提供完整的验证说明：
1 新建VerifyCode.aspx
cs文件代码如下：

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Text;

/**///// <summary>
/// 页面验证码程序
/// 使用：在页面中加入HTML代码 <img src="VerifyCode.aspx">
/// </summary>
public partial class VerifyCode : System.Web.UI.Page
...{


    static string[] FontItems  = new string[] ...{   "Arial", 
                                                  "Helvetica", 
                                                  "Geneva", 
                                                  "sans-serif", 
                                                  "Verdana"
                                              };

    static Brush[] BrushItems = new Brush[] ...{     Brushes.OliveDrab,
                                                  Brushes.ForestGreen,
                                                  Brushes.DarkCyan,
                                                  Brushes.LightSlateGray,
                                                  Brushes.RoyalBlue,
                                                  Brushes.SlateBlue,
                                                  Brushes.DarkViolet,
                                                  Brushes.MediumVioletRed,
                                                  Brushes.IndianRed,
                                                  Brushes.Firebrick,
                                                  Brushes.Chocolate,
                                                  Brushes.Peru,
                                                  Brushes.Goldenrod
                                            };

    static string[] BrushName = new string[] ...{    "OliveDrab",
                                                  "ForestGreen",
                                                  "DarkCyan",
                                                  "LightSlateGray",
                                                  "RoyalBlue",
                                                  "SlateBlue",
                                                  "DarkViolet",
                                                  "MediumVioletRed",
                                                  "IndianRed",
                                                  "Firebrick",
                                                  "Chocolate",
                                                  "Peru",
                                                  "Goldenrod"
                                             };

    private static Color BackColor    =  Color.White;
    private static Pen   BorderColor  =  Pens.DarkGray;
    private static int   Width        =  52;
    private static int   Height       =  21;

    private Random _random;
    private string _code;
    private int    _brushNameIndex;

    
    override protected void OnInit(EventArgs e)
    ...{
        //
        // CODEGEN: This call is required by the ASP.NET Web Form Designer.
        //
        //InitializeComponent();
        //base.OnInit(e);
    }
    
    /**//**//**//// <summary>
    /// Required method for Designer support - do not modify
    /// the contents of this method with the code editor.
    /// </summary>
    private void InitializeComponent()
    ...{    
        //this.Load += new System.EventHandler(this.Page_Load);
    }



    /**//// <summary>
    /// 
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    public void Page_Load(object sender, System.EventArgs e)
    ...{
        if (!IsPostBack)
        ...{
            //
            // TODO : initialize
            //
            this._random = new Random();
            this._code = GetRandomCode();

            //
            // TODO : use Session["code"] save the VerifyCode
            //
            Session["code"] = this._code;

            //
            // TODO : output Image
            //
            this.SetPageNoCache();
            this.OnPaint();
        }
    }




    /**//**//**//// <summary>
    /// 设置页面不被缓存
    /// </summary>
    private void SetPageNoCache()
    ...{
        Response.Buffer = true;
        Response.ExpiresAbsolute = System.DateTime.Now.AddSeconds(-1);
        Response.Expires = 0;
        Response.CacheControl = "no-cache";
        Response.AppendHeader("Pragma","No-Cache");
    }

    /**//**//**//// <summary>
    /// 取得一个 4 位的随机码
    /// </summary>
    /// <returns></returns>
    private string GetRandomCode()
    ...{
        return Guid.NewGuid().ToString().Substring(0, 4);
    }

    /**//**//**//// <summary>
    /// 随机取一个字体
    /// </summary>
    /// <returns></returns>
    private Font GetFont()
    ...{
        int fontIndex = _random.Next(0, FontItems.Length);
        FontStyle fontStyle = GetFontStyle(_random.Next(0, 2));
        return new Font(FontItems[fontIndex], 12, fontStyle);
    }

    /**//**//**//// <summary>
    /// 取一个字体的样式
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    private FontStyle GetFontStyle(int index)
    ...{
        switch (index)
        ...{
            case 0:
                return FontStyle.Bold;
            case 1:
                return FontStyle.Italic;
            default:
                return FontStyle.Regular;
        }
    }

    /**//**//**//// <summary>
    /// 随机取一个笔刷
    /// </summary>
    /// <returns></returns>
    private Brush GetBrush()
    ...{
        int brushIndex = _random.Next(0, BrushItems.Length);
        _brushNameIndex = brushIndex;
        return BrushItems[brushIndex];
    }



    /**//**//**//// <summary>
    /// 绘画事件
    /// </summary>
    private void OnPaint()
    ...{
        Bitmap objBitmap = null;
        Graphics g = null;

        try
        ...{
            objBitmap = new Bitmap(Width, Height);
            g = Graphics.FromImage(objBitmap);

            Paint_Background(g);
            Paint_Text(g);
            Paint_TextStain(objBitmap);
            Paint_Border(g);

            objBitmap.Save(Response.OutputStream, ImageFormat.Gif);
            Response.ContentType = "image/gif";
        }
        catch ...{}
        finally
        ...{
            if (null != objBitmap)
                objBitmap.Dispose();
            if (null != g)
                g.Dispose();
        }
    }

    /**//**//**//// <summary>
    /// 绘画背景颜色
    /// </summary>
    /// <param name="g"></param>
    private void Paint_Background(Graphics g)
    ...{
        g.Clear(BackColor);
    }

    /**//**//**//// <summary>
    /// 绘画边框
    /// </summary>
    /// <param name="g"></param>
    private void Paint_Border(Graphics g)
    ...{
        g.DrawRectangle(BorderColor, 0, 0, Width - 1, Height - 1);
    }

    /**//**//**//// <summary>
    /// 绘画文字
    /// </summary>
    /// <param name="g"></param>
    private void Paint_Text(Graphics g)
    ...{
        g.DrawString(_code, GetFont(), GetBrush(), 3, 1);
    }

    /**//**//**//// <summary>
    /// 绘画文字噪音点
    /// </summary>
    /// <param name="g"></param>
    private void Paint_TextStain(Bitmap b)
    ...{
        for (int n=0; n<30; n++)
        ...{
            int x = _random.Next(Width);
            int y = _random.Next(Height);
            b.SetPixel(x, y, Color.FromName(BrushName[_brushNameIndex]));
        }

    }


} 

 2 页面引用：

<asp:Image ID="getcode" src="VerifyCode.aspx" runat="server" />

一般需要同时提供刷新功能(看不清楚换一张)，代码如下

    <asp:Image ID="getcode" src="VerifyCode.aspx" runat="server" />&nbsp;&nbsp;<A href="javascript:getimgcode()">刷新验证码</A>
 
超链接对应的javascript如下：

 <script language="javascript" type="text/javascript">
function getimgcode()
{
     var getimagecode = document.getElementById("getcode");
     getimagecode.src="VerifyCode.aspx";
}
</script>

3 判断验证

   上述代码是将验证码存贮在Session中，用code来标志。读取代码Session["code"].ToString();
   使用中，我们只需要比较Session["code"].ToString()和文本框输入的串(TextBoxCode.Text)是否相同即可进行判断。

        if(Session["code"].ToString().Trim().Equals(TextBoxCode.Text.Trim()))
        ...{

            Response.Write("Success");
        }

测试通过!