Introduction
The aim of this article is to clarify the question that many new Web developers might have about ViewState
.
Why do some Web controls like Textbox
retain values even after disabling the ViewState
while others do not?
Background
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:
<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 ViewState
enabled (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,
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 IPostBackEventHandler
will update its value (state
) from the appropriate postback data. The important things to note about this stage are as follows:
- State (
value
) of controls are NOT retrieved from ViewState
but from posted back form.
- Page class will hand over the posted back data to only those controls which implement
IPostBackEventHandler
.
- 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.
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 with Goodbye Everyone |
Controls Postback event (button click ) |
|
Textbox is set with Goodbye everyone
|
|
Save view state |
Hello World is saved to ViewState |
Goodbye Everyone is saved to ViewState |
Goodbye Everyone is saved to ViewState |
Render |
Textbox is rendered with text Hello world |
Textbox is rendered with text Goodbye Everyone |
Textbox is rendered with text Goodbye 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 with Goodbye Everyone |
Controls Postback event (button click ) |
|
Textbox is set with Goodbye everyone
|
|
Save view state |
|
|
|
Render |
Textbox is rendered with text Hello world |
Textbox is rendered with text Goodbye Everyone |
Textbox is rendered with text Goodbye 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 value Hello 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 with Goodbye everyone
|
|
Save view state |
Hello World is saved to label |
Goodbye Everyone is saved to ViewState |
Goodbye Everyone is saved to ViewState |
Render |
Label is rendered with text Hello world |
Label is rendered with text Goodbye Everyone |
Label is rendered with text Goodbye 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 value Hello World
|
Label is set value Hello World
|
Label is set value Hello 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 text Goodbye Everyone |
Label is rendered with text Hello World |
Point of Interest
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 with EnableViewState = false
) disabled then it will not retain the changed value and behave like a label
control.
Conclusion
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.
History
- 12th December, 2008: Initial post