How To create Captacha Image in C#

How to create Captacha 

Captacha is One Type of Generated random number and alphabetic  image  then how to generate

first of all add using namespace for bitmap 

using System.Drawing.Imaging; using System.Drawing.Text; using System.Drawing;

First of Write code in Page_load Event to create Bitmap image

Bitmap objBMP = new Bitmap(180, 51); Graphics objGraphics = Graphics.FromImage(objBMP); objGraphics.Clear(Color.OrangeRed); objGraphics.TextRenderingHint = TextRenderingHint.AntiAlias; Font objFont = new Font("arial", 30, FontStyle.Regular); string randomStr = GeneratePassword(); Session.Add("randomStr", randomStr); objGraphics.DrawString(randomStr, objFont, Brushes.White, 2, 2); //Response.ContentType = "image/GIF"; //Response.ContentType = "image/GIF"; objBMP.Save(Response.OutputStream, ImageFormat.Gif); objFont.Dispose(); objGraphics.Dispose(); objBMP.Dispose();

In this Generate Bitmap image one Function GeneratePassword is generate image and return string

value to page_load.

public string GeneratePassword() { string allowedChars = "a,b,c,d,e,f,g,h,j,k,m,n,p,q,r,s,t,u,v,w,x,y,z,"; allowedChars += "A,B,C,D,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,U,V,W,X,Y,Z,"; allowedChars += "2,3,4,5,6,7,8,9"; char[] sep ={ ',' }; string[] arr = allowedChars.Split(sep); string passwordString = ""; string temp; Random rand = new Random(); for (int i = 0; i < 6; i++) { temp = arr[rand.Next(0, arr.Length)]; passwordString += temp; } return passwordString; }

<table border="1" width="70%"> <tr> <td align="center" colspan="2"> Registration of Captcha Details </td> </tr> <tr> <td> Captcha </td> <td> <asp:Image ID="imgresult" runat="server" /> </td> </tr> </table> <table border="1" width="70%"> <tr> <td> Captcha Match </td> <td> <asp:TextBox ID="txtcaptcha" runat="server"></asp:TextBox> </td> </tr> <tr> <td colspan="2" align="center"> <asp:Button ID="btnsubmit" runat="server" Text="Submit" OnClick="btnsubmit_Click" /> </td> </tr> <tr> <td colspan="2" align="center"> <asp:Label ID="lblmessage" runat="server"></asp:Label> </td> </tr> </table>

And Last button click then write code for compare capatcha image and textbox write value and Show error label

string randomStr = GeneratePassword(); int result = string.Compare(randomStr, txtcaptcha.Text); if (result == 0) { lblmessage.Text = "Captcha Value Match Successfully."; } else if (result == -1) { lblmessage.Text = "Test String1 is less than Test String2"; } else if (result == 1) { lblmessage.Text = "Test String1 is greater than Test String2"; }