Numeric Randomized CAPTCHA Field Control - By ncToolControls
 
Default.aspx  (Sample ASP.NET page)
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="TestCaptcha.aspx.cs" Inherits="TestCaptcha" %>

<%@ Register Assembly="ncToolControls" Namespace="ncToolControls" TagPrefix="nc" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Test Captcha Field</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <table style="border: 1px solid black; background-color: InfoBackground; width: 15%;" id="table1">        
        <tr>
            <td colspan="2" align="center">                            
                <nc:CaptchaField ID="captc" Width="80" Height="30px" Font_Size=14 Font_Family="Verdana" runat="server" />
            </td>
        </tr>
        <tr>
            <td align="right">Code:</td>
            <td align="left"><asp:TextBox ID="txtCatpcha" runat="server"></asp:TextBox></td>
        </tr>
        <tr><td colspan="2">&nbsp;</td></tr>
        <tr>
            <td colspan="2" align="center"><asp:Button ID="btnSubmit" Text="Submit" OnClick="btnSubmit_Click" runat="server" /></td>
        </tr>
        <tr><td colspan="2">&nbsp;</td></tr>
        </table>
        <p align="center"><asp:Label ID="lblMessage" BackColor="Red" runat="server"></asp:Label></p>        
    </div>
    </form>         
</body>
</html>
Default.aspx.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class TestCaptcha : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            
        }
    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        if (this.txtCatpcha.Text.Trim() == Session["random_captcha_field"].ToString())
        {
            this.lblMessage.Text = "Successfully Passed";            
        }
        this.txtCatpcha.Text = "";
    }    
}

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