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DropDownList without Autopostback

I need a DropDownlist which can cause SelectedIndexChanged event but do not do post back(do not need page_load event when ddl index changed).
After looking into solutions in Stack here is my code:
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:DropDownList ID="a" runat="server" OnSelectedIndexChanged="a_SelectedIndexChanged">
<asp:ListItem Text="a"></asp:ListItem>
<asp:ListItem Text="b"></asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="a" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
and C# code:
public partial class TestApp : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
txtDate.Text = DateTime.Now.ToString("yyyy-MM-dd");
}
protected void a_SelectedIndexChanged(object sender, EventArgs e)
{
int a = 1;
}
}
But the break point in put in a_SelectedIndexChanged never gets fired. What could be the issue here?
As you see the above is an accpeted answer on stack, then how its not workining:
How DropDownList's SelectedIndexChanged() works without PostBack?
Edit: While the below answer correctly explains that page_load should fire all the times. I still want to know why SelectedIndexChanged never gets fired. So what EventName="SelectedIndexChanged" in the Trigger does?

While a update panel (UP for this post) will "ajax" up, and ONLY post-back that part of the page. (and code behind for that UP can only update controls in that UP?).
The page load even will ALWAYS fire each time.
In fact, page load fires for any button click, post-back, changed event or anything at all.
Thus, as a STANDARD design pattern?
If your page load event loads up a drop down list, listbox, gridview etc.?
Then you ONLY want that code to run on the "real" first page load.
As a result, the last 200+ web pages I have built?
They have this:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
LoadData();
}
void LoadData()
{
Repeater1.DataSource = General.MyRst("SELECT * from tblHotelsA");
Repeater1.DataBind();
}
So, in above example, I am loading up a "repeater" with some data.
However, REGARDLESS if you using a UP or not?
Your page load code, first time MUST quite much do the above.
If you don't do the above, then simple code that say loads up a dropdown list (combo box) will fire each and every time.
If then if you select a combo box value, then say click a button on the page?
the page load fires first, AND THEN your button code. But, if page load re-loads the combo box, then you lose your selection.
So, update panel or not?
Your page load event fires each and every time - including when you use that UP.
So, even without a UP, it stands to reason you can't really even build a working web page if you page load event EVERY time loads up data and controls, since then any changes to such controls will be "blowen" out, and over written each time.
So, while the UP results in what "appears" to not be a post-back?
it is in fact what we call a partial post-back. And the standard page life cycle does occur, but ONLY the bits and parts in the UP are posted back to the server, and in that case, ,then the code behind of course can only change/modify the controls inside of that UP (quite much the whole idea).
So, while the UP will "eliminate" the browser spinner, and while it KEEP your current scroll position on the page?
Make no mistake here - that partial post-back is in fact a page post-back, but just a smaller one, and one that looks to the user that no post-back occurred.
so, it is fine and dandy to have auto-post back for that dropdown list. (and you do NOT even require a trigger, and I suggest you remove the trigger, and put back in the auto post-back.
So, the UP does not "eliminate" the post-back, but hides this from the end user, and for the most part it can be rather nice, quick, and of course will not refresh the whole page. But, page load still fires each and every time.
So, for your code to work correctly, and only fire on the VERY first time, and what amounts to the "real" first page load?
Then your code has to be this:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
txtDate.Text = DateTime.Now.ToString("yyyy-MM-dd");
}
}
Now, if you really don't want any kind of post back?
Then you have to start writing some JavaScript, and start making ajax calls to some web methods.
Now, a ajax call certainly is "less" of a post-back then a UP, which in turn is less of a full page post-back.
If you absolute have a "poor" page design in which a UP can't be used, then yes, you have to start writing and adding client side js code, and thus you can setup a ajax call, and no page post-back will occur. it is quite rare that a UP can't be used, but ajax calls are better, but they also take significantly more time to setup, write and adopt.
However, keep in mind that a ajax call to server code does not have any use of controls on the page. So, you have to return values, and then have your js code update the controls on the page.
So, if a UP can suffice, then that's the least effort. The fantastic part is you don't have to write up and wire up a whole bunch of JavaScript code in place of using that UP.
For sure, a ajax call is best, but that takes more code, more time, and has more moving parts (and requires JavaScript skills on your part). Often this extra effort and time (cost) can't be justified to adopt ajax calls for the page, so a UP is a good middle ground. it is somewhat of a compromise compared to ajax calls, but it boatloads of less work, and thus often a great choice cost and time wise. In other words, in most cases a UP is "good enough".

Postback causes whitescreen when posting data to url containing parameters

I have encountered an unexpected behaviour and/or bug in the .net postback system.
I have a page that uses a master page to provide common elements, with form inputs split between the child and master pages. The form submit button is located on the master page, and I am attempting to process postback on the masterpage.
Any time I attempt to submit data where the form contains any non empty values and the url contains parameters, the page fails to process correctly. This does not occur if the page is submitted under either condition by itself.
The form postback method is post.
The page fails to load and in firefox returns the no element found error.
I have checked for correct class names ect and I do have empty attributes in non form elements, but as the page loads correctly at first I don't think that is relevant. I have also checked for infinately looping code.
This is the current postback handling code:
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack)
{
save_page();
}
page_render();
}
//save
private void save_page()
{
dev_text.Text = "save in progress";
}

Setting text in an HTML element on the server will only be seen on the browser when the HTML is sent to the browser. Normally this happens once the entire processing of the page has completed... so normally quite some time after the user initiated the post-back.
Instead of setting the text on the server, consider setting the text directly on the browser at the moment of submission. Something like...
function setSavingText(){
// Vanilla javascript...
document.getElementById("<%=dev_text.ClientId%>").innerHTML = "save in progress";
// JQuery...
$("#<%=dev_text.ClientId%>").text("save in progress");
}
<asp:Button runat="server" ... OnClientClick="setSavingText();" />
The above function contains both a line for vanilla (normal) javascript, and one for the jQuery library. You only need one of them.

What is the effect of IsPostBack Condition?

I have a aspx Page where I am using AJAX. like
<asp:UpdatePanel runat="server" ID="upPanelDDLProgram">
<ContentTemplate>
<asp:DropDownList ID="DDLProgram" runat="server" Width="194px" Height="18px" OnSelectedIndexChanged="OnDDLProgramChanged" AutoPostBack="true">
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
and my code behind is like
protected void Page_Load(object sender, EventArgs e)
{
//if (!IsPostBack)
//{
// BindProgramDDL();
//}
BindProgramDDL();
}
protected void BindProgramDDL()
{
List<CcProgramEntity> programEntities = FormSaleSubmit_BAO.GetAllPrograms();
DDLProgram.DataSource = programEntities;
DDLProgram.DataTextField = "Shortname";
DDLProgram.DataValueField = "Id";
DDLProgram.DataBind();
string programCode = programEntities[DDLProgram.SelectedIndex].Code;
}
protected void OnDDLProgramChanged(object sender, EventArgs e)
{
List<CcProgramEntity> programEntities = FormSaleSubmit_BAO.GetAllPrograms();
string programCode = programEntities[DDLProgram.SelectedIndex].Code;
}
the If condition is the page load event, is commented out. If I toggle the comment part of the page load event, it works perfect in both cases. My question is why is this heppening?

IsPostBack tells you if it is a second request to the page. The benefit here is if you need to do anything costly, such as a database call to fill a dropdownlist or similar, you can do it when !IsPostback, then use ViewState to retain the values.
To put it specific to your situation
Using:
if (!IsPostBack)
{
BindProgramDDL();
}
Will result in BindProgramDDL being called ONLY on the first time the page is loaded, all AJAX or other user interaction with the page will NOT call BindProgramDDL;
Without that, in place EVERY page load would call the method, un-necessarily hitting the database for the records.

If I am getting you correct .......
DropDown list has data even you are not binding it second time after post back..........its becasuse its server side control and each serverside control has its view state with it thats y its not removing data.
IsPostBack - it true when do the post back by using serverside control like dropdown, checkbox , textbox............When you load page first time this property is false but in subsequent request to same page value of this property is true. you can check msdn document for more detail about it.

It's basically saying are you visiting the page for the first time (not a post back), or has the user clicked on a control (a post back).
Useful for when you only want to run methods once when the page is initially loaded
You're code should probably look like this to achieve best results
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
BindProgramDDL();
}
}

I suspect that the DropDownList saves the items in the ViewState and then work with them during all subsequesnt requests. That is why your code works even if the editor's DataSource is set only when IsPostBack returns false.

PostBack event appears on every action (ajax too), except of first page load.

Page.IsPostBack
indicates whether the page is being rendered for the first time or is being loaded in response to a postback.
see http://msdn.microsoft.com/en-us/library/system.web.ui.page.ispostback.aspx
Since you've bound your datasource the first time the page was loaded, the data are still in the viewstate and you don't need to update the control (unless the datasource has changed).
Take also into account that, since you're using ajax, you may also want to intercept if there was an 'asynchronous postback'.
See http://encosia.com/are-you-making-these-3-common-aspnet-ajax-mistakes/

How do I ensure javascript is fired after an update Panel has, well, updated?

Morning all.
How are we? Excellent I hope.
I have the following little bits of code that I am using to check/uncheck a couple of radio buttons on my page:
<script type="text/javascript">
function SetOption1RadioButton() {
document.getElementById('<%=radOption1.ClientID%>').checked = true;
document.getElementById('<%=radOption2.ClientID%>').checked = false;
</script>
I then assign this to a few controls in the code behind and add this to the page load:
protected void Page_Load(object sender, EventArgs e)
{
SetJavascriptAttributes();
}
public void SetJavascriptAttributes()
{
ddl1.Attributes.Add("onFocus", "SetOption1RadioButton()");
ddl1.Attributes.Add("onClick", "SetOption1RadioButton()");
}
Now this works great on the initial load - lovely stuff. When I click on ddl1, the radio button is set accordingly. However, when the update panel posts back, the javascript no longer works.
I'm thinking this is because the html is sent back and the javascript is no longer there.
How do I go about ensuring that, despite an update panel post back occurring, the javascript stays around a little longer and performs as it should?
Any help or suggestions gratefully received.

Since you're using updatepanel's I assume you already have an istance of the ScriptManager on your page. I suggest you use the ScriptManager to register any client scripts, this will insure they get called on partial postbacks:
ScriptManager.RegisterStartupScript();
ScriptManager.RegisterExpandoAttribute();

UpdatePanel reloads the whole page

I'm building an asp.net cutom control inside which I have two dropdownlists: companyIdSelection and productFamilySelection.I populate the companyIdSelection at Page_Load and in order to populate the productFamilySelection depending on the selected item in companyIdSelection.I'm using UpdatePanels to achieve this, but for some reason every time I update companyIdSelection Page_Load is being called ( which as far as I know should happen only when the entire page is reloaded ), the list is being reloaded again and the item the user selected is lost( the selected item is always the top one ).Here's the code
<asp:UpdatePanel ID="updateFamilies"
runat="server"
UpdateMode="Always">
<ContentTemplate>
Company ID:<br>
<br></br>
<asp:DropDownList ID="companyIdSelection"
runat="server"
AutoPostBack="True"
OnSelectedIndexChanged="companyIdSelection_SelectedIndexChanged">
</asp:DropDownList>
<br></br>
Product Family:
<br></br>
<asp:DropDownList ID="productFamilySelection" runat="server"
AutoPostBack="True"
onselectedindexchanged="productFamilySelection_SelectedIndexChanged">
</asp:DropDownList>
<br>
</ContentTemplate>
</asp:UpdatePanel>
protected void Page_Load(object sender, EventArgs e)
{
this.companyIdSelection.DataSource = companyIds(); //companyIds returns the object containing the initial data items
this.companyIdSelection.DataBind();
}
protected void companyIdSelection_SelectedIndexChanged(object sender, EventArgs e)
{
// Page_Load is called again for some reason before this method is called, so it
// resets the companyIdSelection
EngDbService s = new EngDbService();
productFamilySelection.DataSource = s.getProductFamilies(companyIdSelection.Text);
productFamilySelection.DataBind();
}
Also, I tried setting the UpdateMode of the UpdatePanel to "Conditional" and adding an asyncpostback trigger
but the result was the same.
What am I doing wrong?
PS:
I fixed the updating problem, by using Page.IsPostBack in the Page_Load method, but I would still want to avoid a full postback if possible

I think you misunderstand how UpdatePanels work. They DO actually do a full page postback, it's just that during the rendering stage they capture only a portion of the output and send it back in the AJAX response so the page can be updated. More info here.
So you will still need to check whether it is a postback in your page_load event and only perform your data load if it isn't one.

The update panel call back will go through the page load on every call back. In face, the pull page lifecycle (minus render and prerender) will occur. Update panels give the appearance of ajax, but your client side code still posts back to the same page - which is the problem you are describing. If you can avoid using Update Panels, I suggest you do so. Use something like jQuery instead. If not, then use this in your Page_Load
if (Page.IsCallback) { } //Do callback specific code here
else { } //Do Postback specific code here
Hope this helps. Good Luck.