Avoiding Image Caching

[Nathan Sokalski]

I have been trying to find a way to prevent the images on my site from being
cached on the user's machine. I want to avoid this because the images used
in an Image control often change, yet have the same name. When they are
cached, the cached image is displayed which is not what I want. I thought
that using
Response.Cache.SetCacheability(HttpCacheability.NoCache)

would have achieved this, but it seems to only prevent the .aspx file from
being cached. Is there any way to prevent images from being cached as well?

 

[Cor Ligthert [MVP]]

Nathan,

I am not sure if this will do it (I did not try it). In this sample I make
the image in the procedure, however you can do it of course using a read.
Now you can get the image by its page url

\\\
Private Sub Page_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
Response.Cache.SetExpires(DateTime.Now.AddTicks(500))
Dim myForeBrush As Brush = Brushes.Black
Dim myFont As New Font("Times New Roman", 8, FontStyle.Regular)
Dim textHeight As Single
Dim bm As New Bitmap(120, 20)
Dim g As Graphics = Graphics.FromImage(bm)
g.Clear(Color.White)
Dim textSize As SizeF = g.MeasureString(now.tostring, myFont)
g.DrawString(Now.ToString, myFont, myForeBrush, _
New RectangleF(0, 0, 120, 20))
Dim ms As New IO.MemoryStream
Dim arrImage() As Byte
bm.Save(ms, Imaging.ImageFormat.Bmp)
arrImage = ms.GetBuffer
Response.BinaryWrite(arrImage)
g.dispose
End Sub
///

I hope this helps,

Cor

 

[Chris Botha]

I don't think it is caching. Check that the new image on the server has a
later create date than the older, if not the older will be shown.
Test this by having 2 images, aaa.jpg, put the one with the older creation
date on the server and browse to it, now put the newer one on the server,
browse to it and the 2nd one will show, all OK.
Now rename them to bbb.jpg, first put the one with the newer creation date
on the server, browse, now the older creation date, browse, the 1st image
still shows up.

 

[W.G. Ryan MVP]

I agree with Chris - but if you want to make sure this doesn't happen, use a
HTTPHandler to draw your own images - then they can't steal your bandwidth
either ;-) http://www.uberasp.net/GetArticle.aspx?id=13

 

[laimis]

Well what you can also do is when generating the URL for the image,
append a random string to it like this:

image.ImageUrl = "someimage.gif?randomstring"

and then the browser will get the fresh image as long as the
randomstring is different. For random string you can use TickCount or
anything else you want. Of course there are some drawbacks to this
technique (such as brwoser getting image even though it hasn't changed).

L

 

[Joerg Jooss]

I don't think it is caching.

Think again. If an image is being cached on the client-side or some
downstream proxy, you can change it on the server-side all day long
without affecting the client.

You *must* specify the appropriate Cache-Control header
("must-revalidate" in theory, thanks to IE "no-cache" in practice) to
force clients and proxies to send conditional GETs and pick up
server-side updates.

Cheers,

 

[Ben Strackany]

You can configure this in IIS MMC by navigating to the folder containing the
images and adjusting the HTTP Headers to either "expire immediately" or
"never expire".

 

[Cor Ligthert [MVP]]

Bill,

I agree with Chris - but if you want to make sure this doesn't happen, use
a HTTPHandler to draw your own images - then they can't steal your
bandwidth either ;-)

I had the idea that that is what I am doing in my sample



The bitmap is embedded in the url by the and sent as bitmap by the redirect.

It is not a reference to an image url.

However if you disagree that with me, please tell where I make a mistake in
that idea?

Cor