How To create Captacha Image in C#

Create Captacha image and Match Textvalue is compared or not

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"; }