problems understanding ViewState

[Chris Peeters]

Hi,

I have a Listbox with 3 strings added at design time : "Red" "Green"
"Blue".
I add a fourth string at run-time in Page_Load:

protected void Page_Load(objec t sender, EventArgs e)
{
if (!IsPostBack)
lstColors.Items .Add("Yellow");
}

Enabling the EnableViewState-property of the Listbox always shows the 4
strings.
Disabling the EnableViewState-property never shows "Yellow".

I find it very confusing: all 4 string are part of Listbox's ViewState
no ?
I'd say, or you see all 4 of them (when EnableViewState =true) or you
don't see anything listed (when EnableViewState =false).

But it doesn't work that way apparently. What is the logic behind
EnableViewState ?

It is confusing as well when using a Label: with EnableViewState set or
not, the Text property is ALWAYS shown, even when that text was set at
runtime.

So, what does ViewState really mean for a Control (ListBox, Label, ...)
?

Thank you
Chris


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[=?Utf-8?B?TGFkaXNsYXYgTXJua2E=?=]

Hi Chris,

if you turn off view state for your list box you should see the yellow item
only in first request to your page because you fill it in only when page is
not posted back. View state is actually a hidden field where all registered
controls store their state from previous request (encoded and usually
encrypted). This state is used when next request is precessed so if you turn
on view state all four items are stored to view state. When next request
arrives (post back) asp.net engine knows that list box contains four items
but if you turn off the view state asp.net engine does not have this
information and it has to fill list box again. But at this time only three
items with declarative definition are filled because yellow item is added
only if page was not posted back.

View state is usually turned off for controls binded to large dataset to
reduce size of http response. But there you have to rebind them in each time.

I hope this explanation helps you to understand the view state.

Regards,
Ladislav

"Chris Peeters" wrote:

Hi,

I have a Listbox with 3 strings added at design time : "Red" "Green"
"Blue".
I add a fourth string at run-time in Page_Load:

protected void Page_Load(objec t sender, EventArgs e)
{
if (!IsPostBack)
lstColors.Items .Add("Yellow");
}

Enabling the EnableViewState-property of the Listbox always shows the 4
strings.
Disabling the EnableViewState-property never shows "Yellow".

I find it very confusing: all 4 string are part of Listbox's ViewState
no ?
I'd say, or you see all 4 of them (when EnableViewState =true) or you
don't see anything listed (when EnableViewState =false).

But it doesn't work that way apparently. What is the logic behind
EnableViewState ?

It is confusing as well when using a Label: with EnableViewState set or
not, the Text property is ALWAYS shown, even when that text was set at
runtime.

So, what does ViewState really mean for a Control (ListBox, Label, ...)
?

Thank you
Chris


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[Chris Peeters]

Hi Ladislav,

thank you for your explanation !

I understand the Postback mechanism but what is still unclear to me is
why are the 3 other strings (red, green, blue) shown even when
EnableViewState is turned off ?

thank you
Chris

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[=?Utf-8?B?TGFkaXNsYXYgTXJua2E=?=]

Hi Chris,

Label is processed in same way as any other web control. If you set Text
attribute in asp:Label tag (means you set the Text in design time) it will
always be used as initialization. ViewState stores only values assigned at
runtime. Also if you use something like this:
protected void Page_Load(objec t sender, EventArgs e)
{
MyLabel.Text = "some text";
}
you will override the value (at runtime) in each request = ViewState can be
turned off and behavior will be the same as when you turn the ViewState on
and use:
protected void Page_Load(objec t sender, EventArgs e)
{
if (!IsPostBack)
{
MyLabel.Text = "some text";
}
}

Regards,
Ladislav

"Chris Peeters" wrote:

Hello Ladislav,

it's clearer now for the Listbox !

Maybe a final question: what about when using a Label: with
EnableViewState set or not for the label-control, the Text property is
ALWAYS shown, even when that text was set at runtime ???

thank you
Chris

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