Page Life cycle 2

Let’s build a simple Web application to examine how ViewState works.

Create a blank Web project and paste the code given below in the page:

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<script runat="server">

    Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) 

                                                                                            Handles btnSubmit.Click

        lblMessage.Text = "Goodbye everyone"

        lblMessage1.Text = "Goodbye everyone"

        txtMessage.Text = "Goodbye everyone"

        txtMessage1.Text = "Goodbye everyone"

    End Sub

</script>

<form id="form1" runat="server">

   <asp:Label runat="server" ID="lblMessage" EnableViewState =true  

   Text="Hello World"></asp:Label>

   <asp:Label runat="server" ID="lblMessage1" EnableViewState =false  

   Text="Hello World"></asp:Label>

  <asp:Textbox runat="server" ID="txtMessage" EnableViewState =true  

   Text="Hello World"></asp:Textbox>

   <asp:Textbox runat="server" ID="txtMessage1" EnableViewState =false  

   Text="Hello World"></asp:Textbox>

<br />

<asp:Button runat="server" 

  Text="Change Message" ID="btnSubmit"></asp:Button>

<br />

<asp:Button ID="btnEmptyPostBack" runat="server" Text="Empty Postback"></asp:Button>

</form>

The page rendered will have four controls (two text boxes and two labels) initialized with Hello World and two buttons.

Click on the Change Message button, the value in controls will be changed to Goodbye Everyone.

Now click on the Empty Postback button.

The expected result is, after postback the Textbox (txtMessage) and label (lblMessage) with EnableViewState = false should not retain the value and hence the value should be Hello world, while the controls with ViewStateenabled (txtMessage1 and lblMessage1) should retain the value and hence value should be Goodbye world.

But this does not happen. Both the Textbox will maintain the value irrespective of whether ViewState is enabled or disabled, but in the case of label control if ViewState is disabled, the value we changed programmatically is not retained.

Let's examine why this happens?

Page LifeCycle and ViewState

In page life cycle, two events are associated with ViewState:

·         Load View State: This stage follows the initialization stage of page lifecycle. During this stage, ViewStateinformation saved in the previous postback is loaded into controls. As there is no need to check and load previous data, when the page is loaded for the first time this stage will not happen. On subsequent postback of the page as there may be previous data for the controls, the page will go through this stage.

·         Save View State: This stage precedes the render stage of the page. During this stage, current state (value) of controls is serialized into 64 bit encoded string and persisted in the hidden control (__ViewState) in the page.

·         Load Postback Data stage: Though this stage has nothing to do with ViewState, it causes most of the misconception among developers. This stage only happens when the page has been posted back. ASP.NET controls which implement IPostBackEventHandler will update its value (state) from the appropriate postback data. The important things to note about this stage are as follows:

1.     State (value) of controls are NOT retrieved from ViewState but from posted back form.

2.     Page class will hand over the posted back data to only those controls which implementIPostBackEventHandler.

3.     This stage follows the Load View State stage, in other words state of controls set during the Load View State stage will be overwritten in this stage.

Hey sivaraj,

Thank you for all the analysis. The post in indeed really helpful but I see one flaw in it. The interface that you've talked about is IPostBackEventHandler. It should actually be IPostBackDataHandler. IPostbackDataHandler is the interface implementing which helps controls like text box restore their text data on page post back even if their view state is disabled. TextBox control doesn't implement IPostbackEventHandler interface at all. You can check what all controls implement IPostbackEventHandler interface at following link :

http://msdn.microsoft.com/en-us/library/system.web.ui.ipostbackeventhandler(v=VS.71).aspx[^]

You can check what all controls implement IPostbackDataHandler interface at following link :

http://msdn.microsoft.com/en-us/library/system.web.ui.ipostbackdatahandler(VS.71).aspx[^]

Answers

Now with the above information, let us try to answer the question:

Why some controls retain values even after disabling the ViewState while others do not?

The answer is Controls which implements IPostBackEventHandler like Textbox, Checkbox, etc. will retain the state even after disabling the viewstate. The reason is during the Load Postback Data stage, these controls will get state information from Posted back form.

But controls like label which do not implement IPostBackEventHandler will not get any state information from posted back data and hence depend entirely on viewstate to maintain the state.

Following are the changes in textbox during the page life cycle.

Textbox with ViewState Enabled

Page Events	Page is visited for first time	On click of “Change Message” button	On click of “Empty Post back” button
Init	Textbox is set value Hello World	Textbox is set value Hello World	Textbox is set value Hello World
Load View State		In section "Textbox with ViewState Enabled" the cell (Load View State, On click of “Change Message” button) should read as "Texbox is set with value Hello World from ViewState" "Texbox is set with value Hello World from ViewState"	Textbox is set with value Good Bye Everyone from ViewState
Load Post back data stage		Postback data is Hello World so Textbox is set with Hello World	Postback data is Goodbye Everyone so Textbox is set withGoodbye Everyone
Controls Postback event (button click )		Textbox is set with Goodbye everyone
Save view state	Hello World is saved toViewState	Goodbye Everyone is saved to ViewState	Goodbye Everyone is saved toViewState
Render	Textbox is rendered with textHello world	Textbox is rendered with text Goodbye Everyone	Textbox is rendered with textGoodbye Everyone

Textbox with ViewState Disabled

Page Events	Page is visited for first time	On click of “Change Message” button	On click of “Empty Post back” button
Init	Textbox is set value Hello World	Textbox is set value Hello World	Textbox is set value Hello World
Load View State			In the section "Textbox with ViewState Disabled" the cell (Load View State, On click of “Empty Post back” button) should be empty because as the section heading explains it has no view state from previous page. (Textbox is set with value Good Bye Everyone from ViewState)
Load Post back data stage		Postback data is Hello World so Textbox is set with Hello World	Postback data is Goodbye Everyone so Textbox is set withGoodbye Everyone
Controls Postback event (button click )		Textbox is set with Goodbye everyone
Save view state
Render	Textbox is rendered with textHello world	Textbox is rendered with text Goodbye Everyone	Textbox is rendered with textGoodbye Everyone

Label with ViewState Enabled

Page Events	Page is visited for first time	On click of “Change Message” button	On click of “Empty Post back” button
Init	Label is set valueHello World	Label is set value Hello World	Label is set value Hello World
Load View State		In section "Label with ViewState Enabled" the cell (Load View State, On click of “Change Message” button) should read as "Label is set with value Hello World from ViewState"	Label is set with value Good Bye Everyone from ViewState
Load Post back data stage
Controls Postback event (button click)		Label is set withGoodbye everyone
Save view state	Hello World is saved to label	Goodbye Everyone is saved to ViewState	Goodbye Everyone is saved toViewState
Render	Label is rendered with text Hello world	Label is rendered with text Goodbye Everyone	Label is rendered with textGoodbye Everyone

Label with ViewState Disabled

Page Events	Page is visited for first time	On click of “Change Message” button	On click of “Empty Post back” button
Init	Label is set valueHello World	Label is set value Hello World	Label is set valueHello World
Load View State
Load Post back data stage
Controls Postback event (button click )		Label is set with Goodbye everyone
Save view state
Render	Label is rendered with text Hello world	Label is rendered with textGoodbye Everyone	Label is rendered with text Hello World

In page life cycle, two events are associated with ViewState:

·         Load View State: This stage follows the initialization stage of page lifecycle. During this stage, ViewState information saved in the previous postback is loaded into controls. As there is no need to check and load previous data, when the page is loaded for the first time this stage will not happen. On subsequent postback of the page as there may be previous data for the controls, the page will go through this stage.

·         Save View State: This stage precedes the render stage of the page. During this stage, current state (value) of controls is serialized into 64 bit encoded string and persisted in the hidden control (__ViewState) in the page.

·         Load Postback Data stage: Though this stage has nothing to do with ViewState, it causes most of the misconception among developers. This stage only happens when the page has been posted back. ASP.NET controls which implement IPostBackDataHandler will update its value (state) from the appropriate postback data. The important things to note about this stage are as follows:

1.     State (value) of controls are NOT retrieved from ViewState but from posted back form.

2.     Page class will hand over the posted back data to only those controls which implement IPostBackDataHandler.

3.     This stage follows the Load View State stage, in other words state of controls set during the Load View State stage will be overwritten in this stage.

There is one correction

IPostBackDataHandler 

IPostBackEventHandler

 IPostBackEventHandler Interface

Button,Linkbutton etc control implements this interface

******************

Where you can add dynamic cotrol  to the Page At page _init.

http://aspnet.4guysfromrolla.com/articles/092904-1.aspx

Adding Controls at the Right Time

We already know that when adding controls dynamically through the page's code portion the controls must be added on every postback. But when in the page lifecycle should the controls be added? At first guess, we might decide to put such code in the Page_Load event handler, causing the controls to be added during the Load stage of the page's lifecycle. This would work fine if we don't need to worry about saving the controls' view state across postbacks, but if we do need to persist the view state of the dynamically added controls the Load stage is not where we should be adding these controls.

If we need our dynamically added controls to maintain their view state it is paramount that these controls be added before the Load View State stage. That is, these controls must exist within the page's control hierarchy before the view state is loaded. There's only one stage before Load View State - Initialization. That means, if we want our dynamic controls to persist view state we must add them to the control hierarchy in the page's Init event.

If you are using Visual Studio .NET, the code-behind class already contains an event handler for the page's Init event. This event handler (named Page_Init in VB.NET code-behind classes and OnInit in C# code-behind classes) is tucked away in the hidden "Web Form Designer Generated Code" region. What I typically do is create a separate method in the code-behind class and simply call this method from the Init event handler. Once you have configured everything so that the dynamic controls are added during the Initialization stage, you can then read/write the controls' properties in the Load stage. You'll find that the users' interactions with the dynamic controls remains "remembered" across postbacks if you follow this pattern.