Develop Reference

__doPostBack only works if there is a LinkButton, Calendar or WizardStep control on the page

I have identified a problem with __doPostBack and found a work around. I am seeking an explanation for the cause and/or a better solution than my work around.
Scenario:
I have a dropdown populated with the values; "-Select-", "One" & "Two". If the user selects "One" than client side script is executed. If the user selects "Two" than server side script is executed.
Problem:
The client script initiates the postback by calling __doPostBack. However, no post back actually occurs unless there is also a LinkButton, Calendar or WizardStep control on the page. I actually went through all of the standard tools in the Visual Studio Toolbox and tested them all. It has to be one of those three.
Work Around:
Add a link button surrounded by a span with display set to none.
<span style="display:none;">
<asp:LinkButton ID="LinkButton1" runat="server">LinkButton</asp:LinkButton>
</span>
Question: Can somebody provide an explanation for this behavior or provide a better fix than my "Work Around"?
Source - Javascript (I placed it between the head tags)
<script language="javascript" type="text/javascript">
function DropDownList1_change(elementRef) {
var selectedIndex = elementRef.selectedIndex;
if (selectedIndex > 0) {
var selectedValue = elementRef.options[selectedIndex].value;
if (selectedValue == "One") {
alert("Because you selected 'One', special javascript code will be executed");
// Special javascript code goes here
return;
}
else if (selectedValue == "Two") {
// Special server code gets executed on server DropDownList1_SelectedIndexChanged
__doPostBack('DropDownList1', '');
}
}
}
</script>
Source - ASPX Controls
<asp:DropDownList ID="DropDownList1" runat="server" onchange="DropDownList1_change(this)" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
<asp:ListItem>-Select-</asp:ListItem>
<asp:ListItem>One</asp:ListItem>
<asp:ListItem>Two</asp:ListItem>
</asp:DropDownList>
<br />
<!-- For some unknown reason __doPostBack only works if there is a LinkButton, Calendar or WizardStep control on the page -->
<span style="display:none;">
<asp:LinkButton ID="LinkButton1" runat="server">LinkButton</asp:LinkButton>
</span>
Time of last Post Back: <asp:Label ID="Label1" runat="server"></asp:Label><br />
Time of OnSelectedIndexChanged: <asp:Label ID="Label2" runat="server"></asp:Label>
Source - Code Behind
protected void Page_Load(object sender, EventArgs e)
{
Label1.Text = DateTime.Now.ToLongTimeString();
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
Label2.Text = DateTime.Now.ToLongTimeString();
}
Additional Resource - I found the following article after posting this question. Its a very old Microsoft article and the only Microsoft article I found that mentions the specific limitation of DropDowns, return values & postbacks. I haven't digged deep into their solution and not sure time will allow me to. Mainly posting it in case my solution fails down the road or doesn't work for someone else.
Intuitively you might think adding a confirm dialog box for a
DropDownList is identical to adding such a dialog box for a Button Web
control. That is, simply set the DropDownList's client-side onchange
attribute to something like: return confirm(...);. using:
DropDownListID.Attributes("onchange") = "return confirm(...);"
Unfortunately, this won't work as desired because an AutoPostBack
DropDownList's onchange attribute will be set to a bit of JavaScript
that causes a postback, namely a call to the client-side __doPostBack
function. When setting the onchange attribute programmatically
yourself, the end result is that the rendered client-side onchange
event handler has both your code and the call to __doPostBack:
The article is long so search for "Confirmation with AutoPostBack DropDownLists"
https://msdn.microsoft.com/en-us/library/aa479302.aspx

There are 2 solutions.
Solution 1:
A better work around than adding a link button surrounded by hidden span tags is to add the following to the page load event. This ensures that the function __doPostBack is available.
protected void Page_Load(object sender, EventArgs e)
{
Page.ClientScript.GetPostBackEventReference(this, string.Empty);
}
The function __doPostBack is generated only if a control in the form needs it to perform postbacks. This includes controls like the LinkButton and other controls for which AutoPostBack is set to true. In fact, only the Button and ImageButton controls can perform postbacks without __doPostBack (see this article). For example, we can see in the HTML output that a LinkButton is rendered this way:
<a id="lnk" href="javascript:__doPostBack('lnk','')">My link</a>
Solution 2: The following approach achieves the same thing without using __doPostBack.
In the present case, you could set AutoPostBack="true" for the DropDownList:
<asp:DropDownList AutoPostBack="true" onchange="if (!confirmPostBack(this)) return false;" ... >
The onchange event handler would return false when you want to prevent the postback. The Javascript function could be something like this:
function confirmPostBack(ddl)
{
if (condition) {
...
return true;
}
else {
...
return false;
}
}
Important: The onchange event handler should not return anything to allow the postback to occur. You can use this syntax:
onchange="if (!confirmPostBack(this)) return false;"
For reasons probably explained in the article mentioned in the question, the following syntax does not work. Returning true still prevents the postback.
onchange="return confirmPostBack(this);" // Does not work!

Save data before leave the page in ASP.NET

I have asp.net page.
I don't have button "Save" on it (not my requirement).
I need to save page data before I leave this page.
I've tried to do it with events like Page_Unload but it doen't work...
Cause Save method uses data from elements on page like txtBox1.Text I can't use .ashx handler to make some ajax save (Or I don't know how to use page elements in handler as they are protected).
So is there any solution except serealizing form and pass it to handler?

You can use the onbeforeunload event.
It should fire before unload.
It allows you to ask back if the user really wants to leave. Check simillar example here.
<script language="JavaScript">
window.onbeforeunload = confirmExit;
function confirmExit()
{
return "You have attempted to leave this page. If you have made any changes to the fields without clicking the Save button, your changes will be lost. Are you sure you want to exit this page?";
}
</script>
In return you can save you required values.
Alternatively, you can send out an Ajax request when user leaves.

I know its late to answer , but its not late at all for people who is still working on same problem :
you just have to surround your execution function area with update panel like below
<asp:UpdatePanel runat="server" ID="blahblahId">
<ContentTemplate>
<asp:Button CssClass="hidden" runat="server" ID="btnAddInfo" OnClick="execBforeUnload"/>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnAddInfo"/>
</Triggers>
</asp:UpdatePanel>
and your javascript code will be
var unloadEvent = function (e)
{
alert("hi");
__doPostBack("<%=btnAddInfo.ClientID%>", "");
};
window.addEventListener("beforeunload", unloadEvent);
Finally your C# method will be as follows:
public void execBforeUnload(object sender ,EventArgs e)
{
Session[lblNumber.Text] = null;
Application[lblNumber.Text] = null;
}

Editing a gridview cell using Modal which is shown using rel attribute

I open a (css made)modal dialog using the rel="#showEditModal" attribute of a button.
I used it to add values to database then update gridview and got it working at last.
Problem is now i need to edit this values when clicked upon in the gridview.
It is something like this:
<asp:TemplateField>
<ItemTemplate>
<asp:Button ID="BtnEditDep" rel="#showEditModal" CssClass="btn showModal"
runat="server" CausesValidation="false" Text="Test" OnClientClick="window.scrollTo =
function(x,y) { return true; };" />
</ItemTemplate>
</asp:TemplateField>
This pretty much screws me because i need to send a variable(ID) to that modal
1) If i put it inside the Grid_RowCommand it won't go there until after the modal shows up
tried putting the value inside a viewstate => can't seem to be able to extract the value from the gridview
2) Tried to open the modal dynamically by adding attribute ("rel","#showEditModal") dynamically INSIDE the Grid_RowCommand but apparently I will need to click it again to show the modal (looked into programmatically clicking the button but failed)
3) Also tried it like this(didn't work - couldn't send parameter)
OnClientClick="SaveValue('<%= (BtnEditDep.ClientID).Text %>);
window.scrollTo = function(x,y) { return true; };"
....
function SaveValue(name)
{
$("hiddenelement").value = "<%= (BtnEditDep.ClientID).Text %>";
}
How can i accomplish this because I am currently stumped.
Thanks.

I think you should try to pack your function of saving data and refreshing gridview in a service and use ajax to fire it on client side, that's the most nature solution for your problem.

OnClick not working when i use both Onclick & OnClientClick

With this code, i try to Close a Window (the way i'm doing it works) but i have also an Onclick event which is ignored!
<script type="text/javascript">
function GetRadWindow() {
var oWindow = null;
if (window.radWindow) oWindow = window.radWindow;
else if (window.frameElement.radWindow) oWindow = window.frameElement.radWindow;
return oWindow;
}
function CloseDialog() {
GetRadWindow().close();
}
ASPX page:
<asp:Button ID="Button1" runat="server" Text="Soumettre ce ticket"
onclick="Button1_Click" OnClientClick="CloseDialog();return false;"/>
My application never enters Button1_click event, can anyone help me to find out why ?
thanks in advance
EDIT: HTML GENERATED FOR THE BUTTON
<input type="submit" id="Button1" onclick="CloseDialog();" value="Soumettre ce ticket" name="Button1"/>

This article kind of explains the problem. You need to return true in your JS if you want the server event to trigger. Otherwise, you have to return false.
And, it also looks like you will have to add the UseSubmitBehavior = false based on: OnclientClick and OnClick is not working at the same time?
This is especially evident after seeing that your generated HTML only has the CloseDialog() and not the call to Button1_Click. This change will concatenate to the end of your onclick.
<asp:Button ID="Button1" runat="server" Text="Soumettre ce ticket"
onclick="Button1_Click" OnClientClick="CloseDialog();"
UseSubmitBehavior="false"/>

I ran into this problem and using UseSubmitBehavior="false" nearly did the trick. Be sure to have your OnClientClick call set up correctly:
My code was using OnClientClick="return ValidateSearch();" which is incorrect. It should just be
<asp:Button ID="keywordSearch" runat="server" Text="Search" TabIndex="1"
OnClick="keywordSearch_Click" OnClientClick="if (!ValidateSearch()) { return false;};" />
See the Source here (and tell them thanks!)
If it is set up incorrectly, the OnClick function will not fire.

You are returning false in the onclientclick, so the event is returned before the postback. As a result, onclick never fires.

I just came across the same issue, and I think we're getting confused between server side enable="false" and client-side "disable".
The serverside property to enable a control using control.Enabled = "false"; is not the same as applying an Attribute, like control.Attribute.Add("disabled","disabled");
If you apply the Enabled = false; from the server side, you're actually turning off the control entirely, even if it's shown on the screen! Go on, try and right click the control and select Inspector or FireFly to see it. Nothing shows up. Nothing happens, because the control does not "exist".
Yet if you apply the Attribute property the control is visible to the server and the client, and you're able to inspect it.
What I did is set up the default environment on the ASP.net side by saying the control (asp:Button in my case), has Enabled="true" (or not saying anything, as that's the default anyway).
On the server side, upon PageLoad(), I make my button Visible (in my case I also had it defaulted to visible="false"), and add the appropriate Attribute values.
btnSEND.Visible = true;
btnSEND.Attributes.Add("disabled", "disabled");
That way the button is not enabled, but it's not entirely "invisible" to the client, and by using JavaScript in another area of my program, I control when to enable it or not.
This method also avoids having to use the UseSubmitBehavior="false".

in page_load, you pass the java script function using Add.Atribute method with return true. first it will load the javascript function. then it will load the button click event

Conditional confirm prompt in asp.net code behind

I have looked around for a way of implementing this. Here is a pseudocode representation of what I have:
bool hasData = ItemHasData(itemid);
Confirm = "false"; // hidden variable
if (hasData)
{
//Code to call confirm(message) returns "true" or "false"
if (Confirm == "true")
{
//Delete item
}
else if (Confirm == "false")
{
return;
}
}
The code to call confirm uses a asp:Literal control and sets it equal to the confirm. I can get the popup but only after the function exits. And it does nothing with the conditions after that.
The general consensus seems to be that calling the javascript at that specific line is impossible (makes sense due to the server side/client side gap), but how can I achieve this? I tried using the ConfirmButtonExtender from the ASP.NET AJAX Toolkit but I couldn't interact with the confirmbuttonextender object from the code behind when the object is set to runat="server".
edit:
Sorry, I did miss those tidbits. Thanks Icarus.
The control itself is the GridView (the pseudo version is actually from the gvData_RowCommand function)'s rowcommand. The first check looks to see if the CommandName is DeleteItem and if so goes into this.
The gvData's columns are set based off a list of headers (and the dataset) passed as the table it is working against is for multiple items with different required information. The gvData's data is there, I just need to get a Yes/No (or in reality it'll end up being Ok/Cancel) dialog to verify they want to delete the item when there is data.

One method I end up using in some situations is to have a Panel that displays the Confirm / Cancel buttons. This avoids the need to handle JavaScript events and uses ASP.NET entirely.
<asp:Panel ID="pDeleteConfirm" runat="server"
CssClass="AlertDialog"
Visible="False">
<p>Do you wish to delete the selected record?<br />
<asp:Button ID="btDeleteYes" runat="server" OnClick="btDelete_Click" Text="Delete" />
<asp:Button ID="btDeleteNo" runat="server" OnClick="btDelete_Click" Text="Cancel" />
</p>
</asp:Panel>
<asp:GridView ID="gvData" runat="server"
AutoGenerateColumns="False"
CssClass="GridView"
DataKeyNames="ID"
DataSourceID="sqlData"
EmptyDataText="There is no data entered in the system."
OnRowDeleting="gvData_RowDeleting">
......
</asp:GridView>
I use the OnRowDeleting event to show the Panel
protected void gvData_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
// Show confirmation dialog
pDeleteConfirm.Visible = true;
// Select the row to delete
gvData.SelectedIndex = e.RowIndex;
// Cancel the delete so the user can use the confirm box
e.Cancel = true;
}
Handle the button Click events
protected void btDelete_Click(object sender, EventArgs e)
{
Button bt = (Button)sender;
switch (bt.ID)
{
case "btDeleteYes": // they confirmed a delete
sqlData.Delete();
break;
case "btDeleteNo": // they clicked cancel
// Do nothing
break;
default:
throw new Exception("Unknow button click in btDelete_Click");
}
// clear selection and hide the confirm box
gvData.SelectedIndex = -1;
pDeleteConfirm.Visible = false;
}
This isn't JavaScript but you can add in some UpdatePanels to do AJAX work on it.
Just one method to do it through ASP.NET rather than JavaScript handling.

The job of your code-behind is to render HTML out to the browser. It does this, and then the socket is closed and your server side code is no longer executing.
You'll have to implement this logic using a Javascript function on the client side.
Are you attempting to warn the user if there's data loaded before they take a certain action? Or perhaps before they try to leave the page? You'll have to popup the alert using script when that action happens on the page.