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Captcha – VB.NET

Email form

  <td><img style="WIDTH: 119px; HEIGHT: 34px" alt="" src="Captcha.aspx" /> 
<a href="">Can't Read?</a></td>
   <td><asp:textbox runat="server" ID="txtcaptcha"></asp:textbox></td>
     <asp:ImageButton ID="ContactSeller" CausesValidation="True" runat="server" 
     ImageURL="/images/buttons/btn_submit.gif" /></td>

Email page code behind

 Dim captchaStr As String = Session("randomStr")
 Dim captchaTxt As TextBox = ContactSeller.FindControl("txtcaptcha")

If captchaStr <> captchaTxt.Text.ToString Then
   MsgCheck.Text = "<span style='color: red; font-weight: bold;'>
   Incorrect captcha.</span>"
'your email code
End If

Captcha markup page

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Captcha.aspx.vb" 
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Captcha </title>

    <form id="form1" runat="server">

Captcha code behind

Imports System.Drawing
Imports System.Drawing.Imaging
Imports System.Drawing.Text
Partial Class Captcha
    Inherits System.Web.UI.Page
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) 
                                                    Handles Me.Load
        'create object of Bitmap Class and set its width and height.
        Dim objBMP As Bitmap = New Bitmap(180, 51)
        'Create Graphics object and assign bitmap object to graphics' object.
        Dim objGraphics As Graphics = Graphics.FromImage(objBMP)
        objGraphics.TextRenderingHint = TextRenderingHint.AntiAlias
        Dim objFont As Font = New Font("arial", 30, FontStyle.Regular)
        'genetating random 6 digit random number
        Dim randomStr As String = GeneratePassword()
        'set this random number in session
        Session.Add("randomStr", randomStr)
        objGraphics.DrawString(randomStr, objFont, Brushes.White, 2, 2)
        Response.ContentType = "image/GIF"
        objBMP.Save(Response.OutputStream, ImageFormat.Gif)
    End Sub
    Public Function GeneratePassword() As String
    'Below code describes how to create random numbers.some of the digits and letters
    'are ommited because they look same like "i","o","1","0","I","O".
        Dim allowedChars As String = "a,b,c,d,e,f,g,h,j,k,m,n,p,q,r,s,t,u,v,w,x,y,z,"
        allowedChars += "A,B,C,D,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z,"
        allowedChars += "2,3,4,5,6,7,8,9"
        Dim sep() As Char = {","c}
        Dim arr() As String = allowedChars.Split(sep)
        Dim passwordString As String = ""
        Dim temp As String
        Dim rand As Random = New Random()
        Dim i As Integer
        For i = 0 To 6 - 1 Step i + 1
            temp = arr(rand.Next(0, arr.Length))
            passwordString += temp
        Return passwordString
    End Function
End Class