what is the difference between postback data and view state data

ViewState data is data that ASP.NET encoded end sent to the client in the _ViewState hidden field. It's basically the page as it was when it was sent to the client.

PostBack data is data that the user submits.

For example suppose you have a have a textbox on a page defined like so:

<asp:TextBox id="TextBox1" runat="server" text="Some Text" />

You type in My user input into the textbox and submit the form. Some Text would be ViewState data and My user input would be the PostBack data.

 If you set view state of the page to false than check b/w postback view state value( in the View source of page)  will always be same no matter how many time you post back same page.
Each server control manages to its own information after the post back since it manages in View state so next in 

Load	During load, if the current request is a postback, control properties are loaded with information recovered from view state and control state.

Reference  ViewState and Postback

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:

 Collapse | Copy Code

<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.

Post back data is what user enter in the Text box.

<asp:Label runat="server" ID="lblMessage" EnableViewState =true  
   Text="Hello World"></asp:Label>

 

 <asp:Textbox runat="server" ID="txtMessage" EnableViewState =true  
   Text="Hello World"></asp:Textbox>

We set some default text to both text box and Label, When get request goes to the server for this page PLC occurs and on SaveViewState event View state is saved in the form of hidden filed, If you view source of page for the first time when request the page than you can see values in the hidden field. For the first time Load View State stage and Load Postback Data stage will not happen, only Save View state happens

Now when user click on button(If link button then request will be get for the new page) or submit the page than IsPostBack becomes true again page life cycle occurres again all the event take place. Now

Load View State stage and Load Postback Data stage ,Save View state happens.

 <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>

 protected void btnSubmit_Click(object sender, EventArgs e)
    {
                       
        lblMessage.Text = "Goodbye everyone";//state false
        lblMessage1.Text = "Goodbye everyone";//True
        txtMessage.Text = "Goodbye everyone";//
        txtMessage1.Text = "Goodbye everyone";
    }

If submit is clicked than event occurs and set to the new values. but when click on emptypostback than Label with VS true will get its previews value and other lose its values. but for read full article.

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			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			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			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

An interesting behavior is if we make a control which implements IPostBackEventHandler interface disabled then the ASP.NET will not process the control during postback. So in the above sample, if we make the Textbox (one withEnableViewState = false) disabled then it will not retain the changed value and behave like a label control.

In this article, we examined how the ViewState of control is persisted during the life cycle of page and why some controls maintain the state even if the ViewState is disabled. Hope the information provided here is useful.