AntiBotImage.ashx

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<%@ WebHandler Language="C#" Class="AntiBotImage" %>

//===============================================================================================
//
// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Permissive License.
// See http://www.microsoft.com/resources/sharedsource/licensingbasics/sharedsourcelicenses.mspx.
// All other rights reserved.
//
//===============================================================================================

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.SessionState;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Text;

/// <summary>
/// This Handler renders out a so called Anti-Bot image. It is used to prevent bots from making automatic entries in the guestbook.
/// The random text for the image is taken out of the session-variable "antibotimage". If this value is not present in the session, a empty gif is returned.
/// </summary>
public class AntiBotImage : IHttpHandler, IRequiresSessionState
{
    #region IHttpHandler Members

    public bool IsReusable
    {
        get { return true; }
    }

    public void ProcessRequest(HttpContext context)
    {
        if(context.Session["antibotimage"] == null)
        {
            context.Session["antibotimage"] = generateRandomString(4).ToUpper();
        }
        
        GenerateImage(context.Session["antibotimage"].ToString(), 100, 20, "Arial").Save(context.Response.OutputStream, ImageFormat.Jpeg);
    }
    
    #endregion

    private Bitmap GenerateImage(string text, int width, int height, string fontFamily)
    {
        Random random = new Random();

        // Create a new 32-bit bitmap image.
        Bitmap bitmap = new Bitmap(width, height, PixelFormat.Format32bppArgb);

        // Create a graphics object for drawing.
        Graphics g = Graphics.FromImage(bitmap);