How to create Captcha in ASP.NET

1. Create one page with name "Captcha.aspx"

2. No any control require in this page
3. Go to Captcha.aspx.vb write the below code

Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        'create object of Bitmap Class and set its width and height.
        Dim objBMP As Bitmap = New Bitmap(180, 51)
        'Create Graphics object and assign bitmap object to graphics' object.
        Dim objGraphics As Graphics = Graphics.FromImage(objBMP)
        objGraphics.Clear(Color.White)
        objGraphics.TextRenderingHint = TextRenderingHint.AntiAlias
        Dim objFont As Font = New Font("arial", 30, FontStyle.Bold)
        'genetating random 6 digit random number
        Dim randomStr As String = GeneratePassword()
        'set this random number in session
        Session.Add("randomStr", randomStr)
        Session.Add("randomStrCountry", randomStr)
        objGraphics.DrawString(randomStr, objFont, Brushes.Black, 2, 2)
        Response.ContentType = "image/GIF"
        objBMP.Save(Response.OutputStream, ImageFormat.Gif)
        objFont.Dispose()
        objGraphics.Dispose()
        objBMP.Dispose()
    End Sub
    Public Function GeneratePassword() As String
        ' Below code describes how to create random numbers.some of the digits and letters
        ' are ommited because they look same like "i","o","1","0","I","O".
        Dim allowedChars As String = "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"
        Dim sep() As Char = {","c}
        Dim arr() As String = allowedChars.Split(sep)
        Dim passwordString As String = ""
        Dim temp As String
        Dim rand As Random = New Random()
        Dim i As Integer
        For i = 0 To 5 - 1 Step i + 1
            temp = arr(rand.Next(0, arr.Length))
            passwordString += temp
        Next
        Return passwordString
    End Function


4. Use this page in you aspx page like this

img alt="" border="0" src="Captcha.aspx" style="cursor: move; height: 60px; width: 200px;"

//
 

                               
your textbox to insert code by user.