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Problem with Image not refreshing

Posted on 2008-10-03
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Last Modified: 2012-08-13
Hi all

I am using image verification (CAPTCHA) on my contact form and for some reason the image is not being updated when the page refreshes. The image tag...

<asp:Image ID="verimg" runat="server" Width="100" Height="40" />

..has its imageurl property set programmatically when the new image is generated and saved (see code below) and I was expecting the image to be refreshed whenever the following two lines were executed by the code..

Dim code As String = GenerateVCodeImage()
Session("VCode") = code

..but this isn't happening.

I have also included the following three lines of code in my page_load routine to stop cacheing, but that hasn't helped..

Response.Expires = 0
Response.Cache.SetNoStore()
Response.AppendHeader("Pragma", "no-cache")

Can anyone suggest why my image won't refresh, or provide a better solution for displaying my generated bitmap on the contact form.

Thanks in advance

Jerute
Private Function GenerateVCodeImage() As String
 
	Dim bmfile As String = Server.MapPath("~") & "\capimage.jpg"
	Dim txfile As String = Server.MapPath("~") & "\captext.txt"
	Dim oBitmap As Drawing.Bitmap = New Drawing.Bitmap(90, 35)
	Dim oGraphic As Drawing.Graphics = Drawing.Graphics.FromImage(oBitmap)
	Dim foreColor As System.Drawing.Color
	Dim backColor As System.Drawing.Color
 
	Dim sText As String = generateVCode(5)
	Dim sFont As String = "Comic Sans MS"
 
	foreColor = System.Drawing.Color.FromArgb(220, 220, 220)
	backColor = System.Drawing.Color.FromArgb(190, 190, 190)
 
	Dim oBrush As New Drawing.Drawing2D.HatchBrush(CType(generateHatchStyle(), Drawing.Drawing2D.HatchStyle), foreColor, backColor)
	Dim oBrushWrite As New Drawing.SolidBrush(Drawing.Color.Red)
 
	oGraphic.FillRectangle(oBrush, 0, 0, 100, 50)
	oGraphic.TextRenderingHint = Drawing.Text.TextRenderingHint.AntiAlias
 
	Dim oFont As New Drawing.Font(sFont, 14)
	Dim oPoint As New Drawing.PointF(5.0F, 4.0F)
 
	oGraphic.DrawString(sText, oFont, oBrushWrite, oPoint)
 
	Response.ContentType = "image/jpeg"
	oBitmap.Save(bmfile, Drawing.Imaging.ImageFormat.Jpeg)
	oBitmap.Dispose()
 
	verimg.ImageUrl = "capimage.jpg"
	File.WriteAllText(txfile, sText)
	Return sText
 
End Function
 
 
Private Function generateVCode(ByVal CodeLength As Integer) As String
	Dim VCode As String = String.Empty
	Dim randObj As New Random()
	Dim c As Integer = 63
	For i As Byte = 1 To CodeLength
		c = randObj.Next(35)
		If c >= 10 Then
			c += 7
		End If
		c += 48
		VCode += Chr(c)
	Next
	Return VCode
End Function
 
Private Function generateHatchStyle() As HatchStyle
	Dim slist As New ArrayList
	For Each style As HatchStyle In System.Enum.GetValues(GetType(HatchStyle))
		slist.Add(style)
	Next
 
	Dim randObj As New Random()
	Dim index As Integer = randObj.Next(slist.Count - 1)
 
	Return CType(slist(index), HatchStyle)
End Function

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Question by:jerute
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6 Comments
 
LVL 3

Expert Comment

by:Genome_A_W
ID: 22631867
back when i used to make websites i used to get a problem with the images not updating. Id check everything and by all means they should have refreshed. I would give up and then the next day (or sometimes a few hours later) they would have changed. Something to do with the cache server side maybe?
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LVL 10

Accepted Solution

by:
ericwong27 earned 500 total points
ID: 22633292
Try to generate a unique ID string and append it to the image Like:

verimg.ImageUrl = "capimage.jpg?uid=" + System.Guid.NewGuid().ToString()

0
 

Author Comment

by:jerute
ID: 22633393
Thanks for your reply Eric but I'll have to stand up to read this as it just went over my head. I've never dealt with unique IDs. Can you explain the how's and why's?

Thanks.
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LVL 10

Expert Comment

by:ericwong27
ID: 22634168
This will trick the browser into reloading the image from the server since the image name is difference. if your image name is not change, the browser probably get it from cache.
0
 

Author Comment

by:jerute
ID: 22635620
You mistunderstand me Eric. How do I implement this GUID?

How do I assign one, and how do I use it? Can you give me an example as I have never come across the use of these before.

Thanks again.
0
 

Author Comment

by:jerute
ID: 22639350
A-HA!! I get it. Yes, that seems to do the trick. Thank you EW.
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