Persisting View State to the File System
page 1 of 7
Published: 14 Jun 2004
Unedited - Community Contributed
Abstract
This is a run up article to Scott Mitchell's article at MSDN. This article will provide a working example of persisting view state onto the file system with the usage of global unique identifiers; whilst giving a reason why Scott's solution is likely to fail and why this one is a fail-safe solution.
by Justin Lovell
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Introduction

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How would one start an article based on top of an excellent article from a worthy author? The point of that question is that Scott Mitchell is a fantastic author who recently got an excellent article published at MSDN about the ins and outs of view state.

It is quite a lengthy article - I think that it is at least 10 000 words. I was lucky enough to review that article (as mentioned in the credits of Scott's article) a month ago and I added a few suggestions in.

One of my suggestions that I put forward was regarding the part of where Scott demonstrates that the view state may be saved to the server's disk; the intention being to reduce the download bandwidth that the web browser will have to do and possibly the additional upload of the same information to the server when a post back occurs.

The thought was excellent, I must admit. However, there is a major downfall to his code which could result in ASP.NET throwing an exception stating that view state is corrupted and/or invalid. The reason for the error being thrown is to avoid security exploits which Scott also discussed in his article. The exact reason (or should I say reproduction) of the exception being thrown is a bit complex to explain for the time being but I will explain on the next page of this article.

Exception message

Figure 1: A screen shot of the exception that Scott's code could raise.

The suggestion that I put forward was to avoid the above issue. Due to the extremely long length of the article, Scott could not fit my suggestion in his article (although he did give a hint of my solution but I think it was too brief)… and that is what this article will discuss - using hidden fields and global unique identifiers combination.

Note: I will be referencing to Scott's article at regular intervals so I would recommend that you read his article before continuing reading this one.


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User Comments

Title: Will fail for ASP.NET 2.0   
Name: TomP
Date: 12/11/2007 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: 6/13/2007 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: 1/16/2006 12:22:06 PM
Comment:
\
Title: Httpmodule problem   
Name: Ian
Date: 5/25/2005 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: 8/7/2004 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: 8/7/2004 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: 8/6/2004 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: 8/6/2004 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: 8/6/2004 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: 8/6/2004 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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