Code for create Captcha in ASP.NET website

Step 1: Add a generic handler file in your web application and name it "CaptchaHandler.ashx". It will be used to create CAPTCHA image as a bitmap. Add following code in this file:

<%@ WebHandler Language="C#" Class="CaptchaHandler" %>
using System;
using System.Web;
using System.Drawing;
using System.Drawing.Text;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;
using System.IO;
using System.Web.SessionState;

namespace CustomControls
{

public class CaptchaHandler : IHttpHandler, IReadOnlySessionState
{
  public void ProcessRequest (HttpContext context)
  {
    Bitmap objBmp = new Bitmap(75, 25);

    Graphics objGraphics = Graphics.FromImage(objBmp);
    objGraphics.SmoothingMode = SmoothingMode.AntiAlias;
    objGraphics.Clear(Color.Gray);
    objGraphics.TextRenderingHint = TextRenderingHint.AntiAlias;


    Font objFont = new Font("Arial", 12, System.Drawing.FontStyle.Bold);

    string strCaptcha = string.Empty;
    if (context.Session["Captcha"].ToString() != null)
    {
      strCaptcha = context.Session["Captcha"].ToString();
    }
    objGraphics.DrawString(strCaptcha, objFont, Brushes.White, 3, 3);

    MemoryStream ms = new MemoryStream();
    objBmp.Save(ms, ImageFormat.Png);

    byte[] bmpBytes = ms.GetBuffer();
    context.Response.ContentType = "image/png";
    context.Response.BinaryWrite(bmpBytes);

    objBmp.Dispose();
    objFont.Dispose();
    objGraphics.Dispose();
    ms.Close();
    context.Response.End();
  }

  public bool IsReusable
  {
    get
    {
      return false;
    }
  }
}

}

Step2: Now a class file in your web application and name it "Captcha.cs". This class file implements a custom web control which displays the bitmap CAPTCHA image created by the "CaptchaHandler.ashx" generic handler. Add following code in this file:

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CustomControls
{

public class Captcha : Control
{
  Image _imgCaptcha;

  public string Text
  {
    get
    {
      if (HttpContext.Current.Session["Captcha"] != null)
      {
        return HttpContext.Current.Session["Captcha"].ToString();
      }
      else
      {
        return null;
      }
    }
  }

  protected override void OnLoad(EventArgs e)
  {
    base.OnLoad(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();
    }

    HttpContext.Current.Session.Add("Captcha", strCaptcha);
   
    _imgCaptcha = new Image();
    _imgCaptcha.ImageUrl = "~/CaptchaHandler.ashx";//Image URL is set to the generic handler created in Step1
    this.Controls.Add(_imgCaptcha);
  }
}
}

Step3: Now define a tag prefix for the created CAPTCHA web control. To do this, locate the <pages><controls> tags in the web.config file and add a tagprefix tag within these tags as below:
<pages>
<controls>
<add namespace="CustomControls" tagPrefix="cwc"/>
</controls>
</pages>


Step4: Now add the custom web control (created in step2) in the web page where CAPTCHA image needs to be displayed. Following code snippet can be used:

<cwc:Captcha ID="Captcha1" runat="Server" />
Use following code snippet to compare the CAPTCHA image text with the user entered text:

if(Captcha1.Text != UserTextBox.Text )
{
--Display error message--

}
else
{

--proceed to next--
}