Juri Strumpflohner
Juri Strumpflohner Juri is a full stack developer and tech lead with a special passion for the web and frontend development. He creates online videos for Egghead.io, writes articles on his blog and for tech magazines, speaks at conferences and holds training workshops. Juri is also a recognized Google Developer Expert in Web Technologies

The ViewState hell: controls loosing their values

3 min read

Recently at work I encountered severe problems with the ASP.net viewstate. We have build a very nice framework which we use on our web projects for dynamically adding validation controls to our text-fields or dropdown lists on the UI according to the validity of rules defined in the business logic. Moreover we make use of the .NET's new AjaxControlToolkit's UpdatePanel (I'll come to that back in a later post since that's another story).
The problem: after a postback suddently all of my dropdown lists were empty. My first assumption was immediately that there must be something wrong with the viewstate especially because often it happened after the first and often only after the second postback. I'm not yet an expert in ASP.net since I'm now working for approximately 2 months with it but as far as I know, the viewstate is stored in some obscure way according to an index which corresponds to the position of the control on the UI. So that position could have been changed due to the adding of new controls dynamically after the postback. Actually that was the case. The strange thing is just that all of the newly added controls had their "EnableViewstate" property set to false. Shouldn't it then be ignored by the viewstate?? Apparently it isn't. Anyway, trying to this I set the [ViewStateModeById] attribute on the UserControl's code:
partial class MyUserControl : UserControl
By setting this property, the control's direct children are managed in the viewstate by using their ID instead of the index. This attribute can be applied to basically all containers. After that change, 3 out of the 4 dropdown lists worked and 1 didn't...damn :( .
So the final approach was to change our framework such that ALL of our validator controls are added immediately, handling their IsValid property depending on the outcome of the rules. In that way no controls were added dynamically...and look at that, everything worked out fine, no more problems of empty dropdown lists or broken viewstate.
The résumé: if you're experiencing similar problems, check whether you add new controls dynamically. Your problem might be there.
What one would have to do is to further research how the viewstate is managed by the ASP.net framework in order to understand these things in deep. My assumption is that there is some kind of tree-like structure (i.e. a binary tree or so ??) which is mapped to the controls present on the page after the postback. If there are a lot of changes, whole subtrees may be discarded wherefore 1 of my dropdowns wasn't loaded correctly also after I changed to the ViewStateModeById. If you have any further informations you're invited to leave a comment :)
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