Asp.Net Captcha C#

This article will show you how you  can create a captcha code in asp.net and perform validation weather user have entered correct captcha or not.

Now for this article fist we will create a new asp.net application and add two pages. First page we will use to generate the captcha and second to display the captcha and validation.

So first we will see the page which we will use to generate the captcha.

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Captcha.aspx.cs" Inherits="Shoping_Cart.Captcha" %>

In this we will put only page tag and remove rest of the tags from the page. Now check the code to generate the Captcha.

using System; using System.Collections.Generic; using System.Drawing; using System.Linq; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; namespace Shoping_Cart {     public partial class Captcha : System.Web.UI.Page     {         protected void Page_Load(object sender, EventArgs e)         {             string[] strArray = new string[36];             strArray = new string[] { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };             Random autoRand = new Random();             string strCaptcha = string.Empty;             for (int i = 0; i < 6; i++)             {                 int j = Convert.ToInt32(autoRand.Next(0, 62));                 strCaptcha += strArray[j].ToString();             }             Session["Captcha"] = strCaptcha;             ImageConverter converter = new ImageConverter();             Response.BinaryWrite((byte[])converter.ConvertTo(CaptchaGeneration(strCaptcha), typeof(byte[])));         }         public Bitmap CaptchaGeneration(string captchatxt)         {             Bitmap bmp = new Bitmap(133, 48);             using (Graphics graphics = Graphics.FromImage(bmp))             {                 Font font = new Font("Tahoma", 14);                 graphics.FillRectangle(new SolidBrush(Color.Gray), 0, 0, bmp.Width, bmp.Height);                 graphics.DrawString(captchatxt, font, new SolidBrush(Color.Gold), 25, 10);                 graphics.Flush();                 font.Dispose();                 graphics.Dispose();             }             return bmp;         }     } }

In above code I have generated the random number and character combination. And then I have converted the string into image.

One of the most important things just stores the captcha string into session. This will help use while validation by entered captcha.

Now we will see how to display the captcha and perform validation. For this if user enter correct captcha he will be redirected to the success page.

<form id="form1" runat="server">         <div>             <asp:Image runat="server" ImageUrl="~/Captcha.aspx"></asp:Image><br />             <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>             <asp:Button ID="Button1" runat="server" Text="Submit" OnClick="Button1_Click" />             <br />             <asp:Label ID="lblmessage" runat="server" style="color: #FF3300" Text=""></asp:Label>         </div>     </form>

Now check the validation code.

protected void Button1_Click(object sender, EventArgs e)         {             string captcha = Session["Captcha"].ToString();             if (captcha == TextBox1.Text)             {                 Response.Redirect("SuccessPage.aspx");             }             else             {                 lblmessage.Text = "Please enter a valid captcha.";             }         }

Now we have done run the application to check the output.