we can also use characters or numbers or mixed characters otrnumbers by modifying the code with the single function which is shown below

Imports System.Web.SessionState
Imports System.Drawing
Imports System.Drawing.Imaging

Public Class Captcha

Protected RandomValue As New Random

Protected _captchaImage As Bitmap
Protected _captchaHeight As Integer
Protected _captchaWidth As Integer
Protected _captchaFont As Font
Protected _captchaCode As String

”’
”’ Useful for subclassing
”’
”’
Public Sub New()

End Sub

”’
”’ Loads properties and generates an initial captcha value and graphic
”’
”’ The Width of the Captcha Graphic
”’ The Height of the Captcha Graphic
”’ The Font of the Captcha Graphic
”’
Public Sub New(ByVal imageWidth As Integer, ByVal imageHeight As Integer, ByVal imageFont As Font)
_captchaHeight = imageHeight
_captchaWidth = imageWidth
_captchaFont = imageFont
GenerateNewCaptcha()
End Sub

”’
”’ Generates a new captcha value and graphic for the class
”’
”’
Public Sub GenerateNewCaptcha()
If _captchaFont Is Nothing OrElse _captchaHeight = 0 OrElse _captchaWidth = 0 Then
Exit Sub
End If
_captchaCode = getrandomstring()
_captchaImage = New Bitmap(_captchaWidth, _captchaHeight)
Using CaptchaGraphics As Graphics = Graphics.FromImage(CaptchaImage)
Using BackgroundBrush As New Drawing2D.HatchBrush(Drawing2D.HatchStyle.SmallGrid, Color.GhostWhite, Color.WhiteSmoke)
CaptchaGraphics.FillRectangle(BackgroundBrush, 0, 0, _captchaWidth, _captchaHeight)
End Using
Dim CharacterSpacing As Integer = (_captchaWidth \ 5) - 1
Dim HorizontalPosition As Integer
Dim MaxVerticalPosition As Integer
For Each CharValue As Char In _captchaCode.ToCharArray
MaxVerticalPosition = _captchaHeight - Convert.ToInt32(CaptchaGraphics.MeasureString(CharValue, _captchaFont).Height)
CaptchaGraphics.DrawString(CharValue, _captchaFont, Brushes.DimGray, HorizontalPosition, RandomValue.Next(0, MaxVerticalPosition))
HorizontalPosition += CharacterSpacing + RandomValue.Next(-1, 1)
Next
For Counter As Integer = 0 To 24
CaptchaGraphics.FillEllipse(Brushes.DimGray, RandomValue.Next(1, _captchaWidth), RandomValue.Next(1, _captchaHeight), RandomValue.Next(1, 4), RandomValue.Next(1, 4))
Next
For Counter As Integer = 0 To 24
CaptchaGraphics.FillEllipse(Brushes.WhiteSmoke, RandomValue.Next(1, _captchaWidth), RandomValue.Next(1, _captchaHeight), RandomValue.Next(1, 4), RandomValue.Next(1, 4))
Next
End Using
End Sub

”’
”’ The captcha bitmap image
”’
”’
”’
”’
Private Function getrandomstring() As String
Dim arrstr() As String
arrstr = “A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,1,2,3,4,5,6,7,8,9,0″.Split(”,”.ToCharArray())
Dim strdraw As String
Dim r As Random
r = New Random()
strdraw = String.Empty
Dim i As Integer
For i = 0 To 4
strdraw += arrstr(r.Next(0, arrstr.Length - 1))
Next
Return strdraw
End Function
Public Property CaptchaImage() As Bitmap
Get
Return _captchaImage
End Get
Set(ByVal value As Bitmap)
_captchaImage = value
End Set
End Property

”’
”’ The value of the captcha
”’
”’
”’
”’
Public Property CaptchaCode() As String
Get
Return _captchaCode
End Get
Set(ByVal value As String)
_captchaCode = value
End Set
End Property

”’
”’ The Height of the Captcha Graphic
”’
”’
”’
”’
Public Property CaptchaHeight() As Integer
Get
Return _captchaHeight
End Get
Set(ByVal value As Integer)
_captchaHeight = value
End Set
End Property

”’
”’ The Width of the Captcha Graphic
”’
”’
”’
”’
Public Property CaptchaWidth() As Integer
Get
Return _captchaWidth
End Get
Set(ByVal value As Integer)
_captchaWidth = value
End Set
End Property

”’
”’ The Font of the Captcha Graphic
”’
”’
”’
”’
Public Property CaptchaFont() As Font
Get
Return _captchaFont
End Get
Set(ByVal value As Font)
_captchaFont = value
End Set
End Property

End Class

As you said you have to store the image in your project folder..

If it happen like this.. if the number of user visiting that page in the

sense - so there are no.of images stored in the My Images folder

how to avoid this….

how to use Character Instead of numbers… to display in the image..

Dim myCaptcha As New Captcha
myCaptcha.CaptchaHeight = 50
myCaptcha.CaptchaWidth = 150
myCaptcha.CaptchaFont = New Font(”Verdana”, 20, FontStyle.Regular)
myCaptcha.GenerateNewCaptcha()

myCaptcha.CaptchaImage.Save(Request.PhysicalApplicationPath & “images\captcha\” & myCaptcha.CaptchaCode & “.jpg”)
ltrCaptcha.Text = myCaptcha.CaptchaCode

imgCaptcha.ImageUrl = “../images/captcha/” & myCaptcha.CaptchaCode & “.jpg”

Then on user input just check wheter invisble ltrCaptcha text is same as user’s input!

I would like to know the design part of the code.. i.e.captcha

That is true. However, there are some blackhat scripts around that I’ve seen that will do some equation solving for captchas, particularly if it’s worded like you have in your comment.

It’s a never ending battle against spam. While I don’t update this site often due to other commitments, I do keep an eye on it. In the past 3 months there have been over 10000 spam comment attempts. Akismet, while not perfect, does a good job of keeping them at bay.

Something as simple as “What is the sum of 10 and 5?”

As far as I know, there are no bots out there yet smart enough to decipher such simple questions. A more sophisticated system could randomize a set of simple questions, to make it even more secure.

What color is the sun? Yellow.
What is the name of the planet on which we live? Earth

and so on.

Another advantage is this is more accessible to the vision-impaired.