Viewstate and controls in ASP.NET

Question

I posted a question a couple of days ago about viewstate and after running some tests I have come to some conclusions/results. Based on these results I have a few questions as to how someone would do certain things.

Here are the results of my tests that I ran:

1. If usercontrolA is loaded from OnInit of a Page, then his viewstate will be available in OnLoad. All other controls that usercontrolA loads from it's OnInit, will have their viewstate ready in their OnLoad.
2. If usercontrolA is loaded from OnLoad of a Page, then his viewstate will be available in OnPreRender. All other controls that usercontrolA loads from it's OnLoad, will have their viewstate available in their OnPreRender.
3. If usercontrolA is loaded from an event (Example: button click. Events fire after OnLoad and before OnPreRender) of a Page, then his viewstate will not be available. All other controls that usercontrolA loades will not have their viewstate available.

So in a perfect world you would always load all controls using situation #1, so that their viewstate is available on their OnLoad. Unfortunately when you need to load a control from a button click or from a OnLoad, is there no way for control to get its viewstate before OnPreRender stage?

I have read a bunch of articles on viewstate and thought I understood it, but working on my current application which loads usercontrols which load other usercontrols, I am having a real hard time with being able to get viewstate on my leaf (last in the chain) usercontrol.

Any suggestions and/or links are appreciated.

Answer 1

I don't think I can add anything that this article doesn't cover.

Look specifically at the Life Cycle Events section.

http://msdn.microsoft.com/en-us/library/ie/ms178472.aspx

Answer 2

It is accepted practice to load dynamic controls in OnInit, so that they get the full control lifecycle. I'm not sure I particularly understand your situation though - if you're loading a control based on a button click, why would it have viewstate at that point? On the next OnInit, you should load the control again (I usually use a page level Viewstate item to track that a particular control needs to be loaded) so that it can restore from Viewstate. Something like:

class Default : Page {
   enum LoadedControl { Textbox, Label, GridView }

   override OnInit() {
      if (IsPostback) {
        var c = Viewstate["LoadedControl"] as LoadedControl;
        if (c != null) LoadDynamicControl(c);
      }
   }

   void Button_Click() {
     var c = (LoadedControl)Enum.Parse(typeof(LoadedControl), ddl.SelectedValue);
     LoadDynamicControl(c);
   }

   void LoadDynamicControl(LoadedControl c) {
     switch (c) {
        case LoadedControl.Textbox:
           this.ph.Controls.Add(new Textbox());
           break;
        ...
     }

     ViewState["LoadedControl"] = c;
   }
}

The slightly more interesting bit, though, is that according to catch-up events - it really shouldn't matter. The callstack for dynamically loading a control looks something like:

Control.Controls.Add(Control)
   Control.AddedControl(Control)
      Control.LoadViewStateRecursive(object)
          Control.LoadViewState(object)

Taking Label as an example, it overrides LoadViewState and pulls it's Text property directly from ViewState. TextBox is similar. So, by my reading, it should be OK to add at any point, and then access ViewState. That doesn't seem to be jive with my experience, though, so further investigation seems warranted.