asp.net(VB)验证码

<%@ import namespace="System.Drawing.Imaging"%>
<%@ import namespace="System.Drawing"%>
<%@ import namespace="System.io"%>
<%@ import namespace="System"%>
<script language="vb" runat="server">
Sub Page_Load(Sender as object,e as eventargs)
'RndNum是一个自定义函数
dim VNum as string=RndNum(4)
Session("VNum")=VNum
ValidateCode(VNum)
End Sub
'生成图象验证码函数
Sub ValidateCode(VNum)
Dim Img as System.Drawing.Bitmap
Dim g as Graphics
Dim ms as MemoryStream
dim gheight as integer=Int(Len(VNum)*11.5)
'gheight为图片宽度,根据字符长度自动更改图片宽度
img=new BitMap(Gheight,20)
g=Graphics.FromImage(img)
g.DrawString(VNum,(New Font("Arial",10)),(New SolidBrush(color.blue)),3,3)'在矩形内绘制字串（字串，字体，画笔颜色，左上x.左上y）
ms=New MemoryStream()
img.Save(ms,ImageFormat.Png)
Response.ClearContent() '需要输出图象信息 要修改HTTP头
Response.ContentType="image/Png"
Response.BinaryWrite(ms.ToArray())
g.Dispose()
img.Dispose()
Response.End()
End Sub
'--------------------------------------------
'函数名称:RndNum
'函数参数:VcodeNum--设定返回随机字符串的位数
'函数功能:产生数字和字符混合的随机字符串
Function RndNum(VcodeNum)
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,L,M,N,O,P,Q,R,S,T,U,W,X,Y,Z"
dim VcArray() as string=split(Vchar,",") '将字符串生成数组
dim VNum as string=""
dim i as byte
For i=1 to VcodeNum
Randomize
VNum=VNum & VcArray(Int(35*Rnd)) '数组一般从0开始读取，所以这里为35*Rnd
Next
Return VNum
End Function
</script>