asp.net验证码的生成(vb)

         为了防止机器人的自动攻击，几乎每个asp.net程序中都要用到验证码，特别是需要用户登陆的地方，找了些资料，模仿网上的源程序做了一个VB版的验证码程序，分享一下．
　
　　1．先新建一个类CreateImage.vb，将下面的代码Copy进去；

　　2．新建一个页面Image.aspx，在Page_load中添加类CreateImage.vb的引用：
　　　　 Dim myimage As New CreateImage
      　　　 myimage.DrawImage()

        3.　在需要验证码的页面，添加一个Image控件，其中属性：ImageUrl="Image.aspx"
　　　　页面再添加一个TextBox控件txtCode，用于输入验证码的值．

　　4.　在提交按钮（如登陆）的代码中，加上一个＂与＂的条件，txtCode.Text=Session("CheckCode")

    　 5.   这个代码有些缺点，希望大家能提出来一起解决，期待......

Imports System.Drawing
Public Class CreateImage

    Public Shared Sub DrawImage()
        Dim img As New CreateImage()
        HttpContext.Current.Session("CheckCode") = img.RndNum(4)
        img.CreateImages(HttpContext.Current.Session("CheckCode").ToString())
    End Sub 'DrawImage

    '/ <summary>
    '/ 生成验证图片
    '/ </summary>
    '/ <param name="checkCode">验证字符</param>
    Private Sub CreateImages(ByVal checkCode As String)
        Dim iwidth As Integer = CInt(checkCode.Length * 13)
        Dim image As New System.Drawing.Bitmap(iwidth, 23)
        Dim g As Graphics = Graphics.FromImage(image)
        g.Clear(Color.White)
        '定义颜色
        Dim c As Color() = {Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple}
        '定义字体
        Dim font As String() = {"Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体"}
        Dim rand As New Random()
        '随机输出噪点
        Dim i As Integer
        For i = 0 To 49
            Dim x As Integer = rand.Next(image.Width)
            Dim y As Integer = rand.Next(image.Height)
            g.DrawRectangle(New Pen(Color.LightGray, 0), x, y, 1, 1)
        Next i

        '输出不同字体和颜色的验证码字符
        For i = 0 To checkCode.Length - 1
            Dim cindex As Integer = rand.Next(7)
            Dim findex As Integer = rand.Next(5)

            Dim f = New System.Drawing.Font(font(findex), 10, System.Drawing.FontStyle.Bold)
            Dim b = New System.Drawing.SolidBrush(c(cindex))
            Dim ii As Integer = 4
            If (i + 1) Mod 2 = 0 Then
                ii = 2
            End If
            g.DrawString(checkCode.Substring(i, 1), f, b, 3 + i * 12, ii)
        Next i
        '画一个边框
        g.DrawRectangle(New Pen(Color.Black, 0), 0, 0, image.Width - 1, image.Height - 1)

        '输出到浏览器
        Dim ms As New System.IO.MemoryStream()
        image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg)
        HttpContext.Current.Response.ClearContent()
        'Response.ClearContent();
        HttpContext.Current.Response.ContentType = "image/Jpeg"
        HttpContext.Current.Response.BinaryWrite(ms.ToArray())
        g.Dispose()
        image.Dispose()
    End Sub 'CreateImages

    '/ <summary>
    '/ 生成随机的字母
    '/ </summary>
    '/ <param name="VcodeNum">生成字母的个数</param>
    '/ <returns>string</returns>
    Private Function RndNum(ByVal VcodeNum As Integer) As String
        Dim Vchar As String = "0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k"
        Dim VcArray As String() = Vchar.Split(","c)
        Dim VNum As String = "" '由于字符串很短，就不用StringBuilder了
        Dim temp As Integer = -1 '记录上次随机数值，尽量避免生产几个一样的随机数
        '采用一个简单的算法以保证生成随机数的不同
        Dim rand As New Random()
        Dim i As Integer
        For i = 1 To (VcodeNum + 1) - 1
            If temp <> -1 Then
                'rand = New Random(i * temp * Convert.ToInt64(Int(DateTime.Now.Ticks)))
                rand = New Random(i * temp * 1000000)
                '
                'ToDo: Error processing original source shown below
                '-------------------------^--- GenCode(token): unexpected token type
            End If
            Dim t As Integer = rand.Next(VcArray.Length)
            If temp <> -1 And temp = t Then
                Return RndNum(VcodeNum)
            End If
            temp = t
            VNum += VcArray(t)
        Next i
        Return VNum
    End Function 'RndNum