Persisting View State to the File System
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by Justin Lovell
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Where Scott’s Code Will Fail On Some Occasions

[Download Source Code]

The best way for demonstrating where Scott's code will fail is in a practical example. For the complete source code for the practical example in which I will demonstrate the downfall can be downloaded with the attached source code. The specific file that demonstrates the downfall is conventionally called "ScottsProblem.aspx"

Scott already explained his code in his article in immense detail; hence, I will not regurgitate what he has already written. However, I will explain the code that allows the exception to be thrown. The first code listing to step up to the plate follows (without Scott's code but it is included in the example):

public class ScottsProblem : PersistViewStateToFileSystem {
   protected System.Web.UI.WebControls.PlaceHolder ControlHolder;</font>


<font face="Verdana" size="2">   protected override void OnLoad(EventArgs e) {
      if (Request.QueryString["Control"] != null) {
         ControlHolder.Controls.Add(LoadControl(String.Format(
            "UserControl{0}.ascx", Request.QueryString["Control"])));
      }</font>


<font face="Verdana" size="2">      base.OnLoad(e);
   }
}

That code listing is the code behind for the ScottsProblem page. The ASP.NET page code looks like so (I left out the common code pieces):

<form id="Form1" method="post" runat="server">
   <div>
      <a href="ScottsProblem.aspx?control=A">Show User Control A</a><br/>
      <a href="ScottsProblem.aspx?control=B">Show User Control B</a>
   </div>
   <asp:PlaceHolder id="ControlHolder" runat="server"></asp:PlaceHolder>
</form>

You have guessed it correctly! The above code is a simple demonstration of two user controls being dynamically added to the page based on the query string. That query string variable name is 'Control'. Essentially, the two hyperlinks are there only to facilitate the testing purposes.

There is also one thing that I would like to add: the two user controls are named UserControlA.ascx and UserControlB.ascx respectively. The only thing that they have in common is that their control tree has similar naming. For example, the ID's of the controls inside both user controls looks like the following:

|- Ctl1
|- Ctl2

Because view state saves the data according to the ID of the controls and not by it's control type (ie. DataGrid or TextBox), ASP.NET will throw an exception when it detects that the control structure is completely different between the two post backs.

With that said, time for the exact reproduction of the ASP.NET exception being thrown with Scott's code.

  1. Open a new browser window and browse to (assuming that you downloaded my source code and made a virtual directory named "ViewStateToFS" to the root of the source code):

    http://localhost/ViewStateToFS/ScottsProblem.aspx
  2. From there, instruct the browser to open UserControlB in a new window. Please note that this is an essential step because:

    a) We are replicating a casual user's browsing habits.
    b) Opening a new instance of a browser to show UserControlB will result in the two "browser windows" taking on two different session ID's.
  3. On the first browser window, make the browser browse to the page that will output UserControlA.
  4. And the part that I enjoy (because I have a thing with atomic weapons going off), click the post back button on second windows (the one displaying UserControlB). And then you will find that you are going face to face to with the exception which I said ASP.NET will raise.

    Exception message

Stay tuned because on the next page, I will explain why the exception is being thrown.


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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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