asp.netcustom-server-controlspage-lifecycle

Viewstate vs Postback

I sort of answered my own question I think but I want to make sure I am understanding correctly. I initially thought that when a user provided values in a form, that on postback the values were submitted as part of the Viewstate, because TextBox.Text is part of the viewstate. Now I have found that user supplied values actually aren't applied to the controls until after the OnLoad event. This confused me because I thought that viewstate was loaded into the controls before OnLoad(or when calling Controls.Add()). I have gone over the documentation on page and control lifecycles a few times and I am just now realizing that there was a different step for handling postback data(this step didn't appear in a lot of documentation :(

1) So postback data, the values user's type into the fields, is applied after the OnLoad event, and Viewstate data is applied just before the OnLoad event?

2) So essentially all this means is that on postback the server gets two values for a TextBox.Text property, the one in Viewstate, which is like the "old" value from the previous request, and the new value supplied by the user in the form?

3) Does the .net framework apply postback data the same was as Viewstate, in that it finds the appropriate control via it's ID property? This is important because I am creating controls dynamically and I may even have forms that change structure overtime and need to think about how I handle ID's. So far I haven't been setting the ID property and everything works fine but things may be more complicated later on.

4) Does viewstate data ever get modified at all on client side? Or is the viewstate identical to what was sent by the server in the previous request(assuming no tampering)? My impression used to be that the server encoded all the control properties into the viewstate, and on the client side when the user submitted the form, the viewstate field was decoded, modified, encoded, and submitted to the server with modifications. I assumed there was a bunch of javascript doing all this for me. Now I think I had it all wrong. Instead it seems that the Viewstate never changes on client side, and all the client changes are in the postback data such that the next request the server loads viewstate, loads postback, and provides a new updated viewstate in the next response?

Solution

  • 1) Both are loaded before Load
    2) Basically, yes
    3) ViewState is applied first, then Post Data

    To quote Scott Mitchell(see below)

    dynamically added controls must be programmatically added to the Web page on each and every page visit. The best time to add these controls is during the initialization stage of the page life cycle, which occurs before the load view state stage. That is, we want to have the control hierarchy complete before the load view state stage arrives. For this reason, it is best to create an event handler for the Page class's Init event in your code-behind class, and add your dynamic controls there.

    4) Unless you're doing something way outside of the box, ViewState is never modified client-side. "ViewState" is an HTML form field and is processed on the server side.

    Here's a few images from Understanding ASP.NET View State by Scott Mitchell that may help you.

    alt text
    (source: microsoft.com)

    alt text
    (source: microsoft.com)

    Bonus Reading Material: http://weblogs.asp.net/infinitiesloop/archive/2006/08/03/Truly-Understanding-Viewstate.aspx