CAPTCHA: A best technique to restrict automatic form submission when developing a web page is to add some kind of verification.
HANDLER
<%@ WebHandler Language="C#" Class="Handler" %>
using System;
using System.Web;
using System.Web.SessionState;
using System.Drawing;
using System.Drawing.Imaging;
public class Handler : IHttpHandler,IRequiresSessionState
{
public void ProcessRequest(HttpContext context)
{
using (Bitmap b = new Bitmap(250, 50))
{
Font f = new Font("Arial", 10F);
Graphics g = Graphics.FromImage(b);
SolidBrush whiteBrush = new SolidBrush(Color.Blue);
SolidBrush blackBrush = new SolidBrush(Color.White);
RectangleF canvas = new RectangleF(0, 0, 250, 50);
g.FillRectangle(whiteBrush, canvas);
context.Session["Captcha"] = GetRandomString();
g.DrawString(context.Session["Captcha"].ToString(), f, blackBrush, canvas);
context.Response.ContentType = "image/gif";
b.Save(context.Response.OutputStream, ImageFormat.Gif);
}
}
public bool IsReusable
{
get
{
return false;
}
}
private string GetRandomString()
{
string []arrStr = "A,B,C,D,1,2,3,4,5,6,7,8,9,0".Split(",".ToCharArray());
string strDraw = string.Empty;
Random r = new Random();
for(int i = 0; i < 5 ; i++)
{
strDraw += arrStr[r.Next(0,arrStr.Length-1)];
}
return strDraw;
}
}
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!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>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<img src="Handler.ashx" />
<br />
Please type above text here. To identify your self as a human
<br />
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><br />
<br />
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label></div>
</form>
</body>
</html>
CODE BEHIND
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
if (Session["Captcha"].ToString() == TextBox1.Text)
{
Label1.Text = " Code Matched!";
}
else
{
Label1.Text = "Please try again";
TextBox1.Text = "";
}
}
}
