Persisting View State to the File System: ASP Alliance

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Persisting View State to the File System

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by Justin Lovell
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Why Does Scott’s Code Fail?

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The ironic thing is: Scott already discussed the reason why that exception will be thrown… indirectly. The LosFormatter is clever enough to know when the view state was tampered with. You might ask - but we never tampered with the view state!? Indirectly, yes we did. If you had to step through the logic of the saving and loading of the view state from the medium (file system), you will quickly find that both of the browser windows that were opened shares the same session ID and page path; therefore, sharing the same view state file (where all the view state is saved to).

Essentially, with Scott's code, the last to write to the view state file is the one which that the view state file will be guaranteed to work in future post backs. With that said, the connection of the tampering has to do with the logic that the last post back may make the legitimism of the pages rendered earlier corrupt (from the latest page request in timeline).

A sharp reader out there might say - but you used the query string for different 'pages' (presentation of different user controls). Surely you can just vary each view state file by query string as well? To be honest, that was only an example that I presented… there are many other pages that do not vary their query string at all but the same page may want to persist two completely different view state values.

It is time for me to crack my knuckles for my solution that follows on the next page.

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User Comments

Title: Will fail for ASP.NET 2.0
Name: TomP
Date: 2007-12-11 9:13:31 AM
Comment:
We used this for ASP.NET 2.0 and found that you will lose your control state. A good article exists here: http://blog.arctus.co.uk/articles/2007/04/23/advanced-asp-net-storing-viewstate-in-a-database

Title: Problem with Microsoft Ajax
Name: AnupT
Date: 2007-06-13 12:17:38 AM
Comment:
Excellent article and i was able to successfully use this in a project.

But I ran in to problem in another project that uses Microsoft Ajax 1.0.

I have a page which uses updatepanel control. In this updatepanel, i have 2 dropdownlist with autopostback enabled. For some weird reason the second dropdownlist is not maintaing the selected value. As soon as i change the 2nd dropdownlist, its value changes back to first item in the list.

This problem is happening for all the page that has 2 or more dropdownlist.

Title: Some problems occurs during long sessions
Name: Raul
Date: 2006-01-16 12:22:06 PM
Comment:
\

Title: Httpmodule problem
Name: Ian
Date: 2005-05-25 12:47:33 PM
Comment:
Great article.
I did run into one problem with the HttpModule. I kept getting the error "Server operation is not available in this context". I think it was because the the httpmodule is created in the Application_OnStart and at that point the context does not contain a value for Server.MapPath. Anyway, I got around this by using System.AppDomain.CurrentDomain.BaseDirectory instead.

Good stuff though

Title: Cache
Name: Wesley
Date: 2004-08-07 5:58:02 AM
Comment:
Justin,

I thought it was all about minimizing site traffic due to the sometimes very large viewstate. Just a little misunderstanding. But...

Your mathwork showed me somthing else as wel. 50 x 8 x 15 = 6000KB an hour! If we're talking about shared hosting most hosts give you a harddisk space of 50MB to 100MB which means that in the best case(not counting object allready there) we got 16.6 hours of viewstate left. So it's not very usefull for the average.

I would like to thank you both(scott and you) an awfull lot for explaning viewstate in such an extended way. I managed to solve my viewstate problems on the composite control mentioned in the url.

Cheers,

Wes

Title: Postback problem still exists
Name: Justin Lovell
Date: 2004-08-07 5:57:14 AM
Comment:
Richard,

As I have already said to, the problem that you are experiencing is due to the fact that my HTML code that I posted is directing you to Scott's example page.

I have upload the new source code with the fixed HTML

Title: re: Cache
Name: Justin Lovell
Date: 2004-08-06 5:43:53 PM
Comment:
Hi Wesley,

You can use the cache object to store the view state. However, it totally depends on what type of site you are running. The main issue that was addressed by persisting the view state to the file system is that an user can fill a form and then leave it to stand over night before he submits his changes.

If you are using the cache object, you will then have to counter that time-restriction by placing the cache time to a couple of hours and sometimes as long as a day. If your site gets an average of 50 users who all visit eight pages on average, and assuming that the average view state size is 15KB, then do the maths: 50 x 8 x 15 = 6000KB of memory (~6MB). That is a low traffic site... now let's multiply the amount of sites doing exactly that (shared hosting for low traffic sites) to about 40: that is ~240MG just for just holding view state into memory.

That is a bit inefficient. However, it becomes even more dramatized if you have to run off medium trafic site. And to make it worst: you can't scale your application to a web farm.

It is all about keeping your options open... and your memory for better use :-)

Title: Cache
Name: Wesley
Date: 2004-08-06 2:57:28 PM
Comment:
As I am quit new on programming in .Net I tend to read a lot just to learn. So don't blame me if this is a stupid idea

One thing I truly don't understand: Why not cache the viewstate by the name of the SessionID+Page and the time a session exists? Something like: cache.insert(SessionId+Page, Viewstate,,DateTime.Now.AddMinutes(SessionTTL))

In that case your sure to get the viewstate corresponding to the current session, it auto cleans up the mess and it's a lot quicker I guess... You just have to make sure the cache isn't cleared.

Cheers,
Wes

Title: Postback problem still exists
Name: Richard Lemmon
Date: 2004-08-06 2:41:38 PM
Comment:
I followed your article and was able to duplicate the same error that was being caused by Scott's code.

Try the following:

1.) http://localhost/ViewStateToFS/MySolution.aspx
2.) Click on the "Show User Control A"
3.) Select Hello world
4.) right-click on "Show user Control B" and select Open in new window
5.) In the new window click Postback
6.) Return to the initial window & click Postback

same error.

Regards,

Rich Lemmon
rich@lindenstreet.com

Title: A couple of very small points on an otherwise excellent article
Name: Scott Galloway
Date: 2004-08-06 11:37:13 AM
Comment:
Great article, I do have a couple of small points though - firstly, as written your code writes the filepath into the HTML source of the generated page; this is not really a great idea as path disclosure is generally to be avoided in web apps. I made a simple modification to allow just the GUID portion to be stored in the hidden field - which is also smaller :-). Second point is to do with the HttpModule you use for cleanup, I tend to use a different approach here, using a Timer object stored as a static object - it has the nice side effect of avoiding using a separate module but it does have the disadvantage of requiring a code change.

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