In an email follow up, Neven asked me if I could be more clear since he was
still seeing the TextChanged event fire despite turning off viewstate. Here
was my response, in case it helps others:
The TextChanged event is broken under special conditions, I wasn't
very clear about this.
Create a simple page:
<form id="form" method="post" runat="server">
<asp:Textbox ID="name" Runat="server" EnableViewState="False" />
<asp:Button ID="x" Runat="server" />
</form>
with the following codebehind:
protected TextBox name;
protected Button x;
private void Page_Load(object sender, EventArgs e)
{
name.TextChanged += new EventHandler(this.name_TextChanged);
x.Click += new EventHandler(this.x_Click);
}
private void name_TextChanged(object sender, EventArgs e)
{
Trace.Write("TextChanged");
}
private void x_Click(object sender, EventArgs e)
{
Trace.Write("ButtonClicked");
}
Enter text and click on the button, "TextChanged" WILL fire. However, now
CLEAR the text from the textbox and hit the button, "TextChange" WILL NOT
fire. Because the old value isn't preserved in viewstate, ASP.Net assumes
that the "old" value is blank. That means that textChanged won't fire if
you go from a value to a blank one (even though the value has changed).
Also, if you enter text, say "aa" and hit the button, the hit the button
again, you'll see that TextChanged fires. That's because it's comparing the
2nd "aa" to blank, instead of the old value which is actually also "aa".
Hope that makes some sense :)
The problem with the radio button list is for code like this:
private void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostback)
{
rad.DataSource = "XX";
rad.DataBind();
}
x.Click += new EventHandler(this.x_Click);
}
private void x_Click(object sender, EventArgs e)
{
Trace.Write(rad.Items[0].Value); //won't work unless 0 was selected
if viewstate is off for radiobutton list
}
The problem is that values are only added when it isn't postback
(!Page.IsPostBack). With viewstate on, no problem because the control is
re-created from viewstate, with it off however, it isn't recreated, and only
the selected value can be retrieved via Request.Form("rad"). One solution
is to re-bind the control, on postback as well (remove the check for
!PAge.IsPostback). you have to decide what you prefer, fetching the data
source again (might involve a database hit) or loading the data into
viewstate? I often prefer to reload the data, especially if it's cached!
Karl
--
MY ASP.Net tutorials
http://www.openmymind.net/ http://openmymind.net/redirector.aspx?documentId=51 - Learn about AJAX!
"Neven Klofutar" <ne***********************@vip.hr> wrote in message
news:eC**************@tk2msftngp13.phx.gbl...
I see what you mean Karl, thanx !
Neven
"Karl Seguin" <karl REMOVE @ REMOVE openmymind REMOVEMETOO . ANDME net>
wrote in message news:et**************@TK2MSFTNGP12.phx.gbl... Any control who's value can be deduced from Request.Form.
However, note that even in the most obvious case, such as a textbox, the
viewstate still preserves the OLD value, which is the ONLY way for
TextChanged event to fire server-side.
Also, only the _selected_ value is preserved without viewstate. This
means if you have a radiobutton list with out viewstate, not only will
your Changed event not fire, but you won't have access to the
non-selected radio buttons.
In other words, all form elements can make due with only
REquest.Form...if you've programmed in classic ASP/PHP, the meaning of
this should be intuitive.
Karl
--
MY ASP.Net tutorials
http://www.openmymind.net/
"Neven Klofutar" <neven.klofutar@**re...m.o..v...e**vip.hr> wrote in
message news:em**************@TK2MSFTNGP10.phx.gbl... Hi,
I recently read an article on MSDN about viewstate and postback.
http://msdn.microsoft.com/library/de.../viewstate.asp
The author states the following:
"It is a common misconception among developers that view state is
somehow responsible for having TextBoxes, CheckBoxes, DropDownLists, and
other Web controls remember their values across postback. This is not
the case, as the values are identified via posted back form field
values, and assigned in the LoadPostData() method for those controls
that implement IPostBackDataHandler."
I would like to know exactly what ASP.NET controls don't need viewstate
to remeber value across postbacks.
thanx, Neven
