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:
[ViewStateModeById]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 :( .
partial class MyUserControl : UserControl
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 :)