Page with AJAX controls posts back twice to the server

I have a form containing three AJAX UpdatePanels, two of which contain a pair of listboxes and a pair of buttons, with the third one containing a pair of radio buttons in a radiobutton list and either three CascadingDropDown lists or two CascadingDropDown lists and a textbox depending on which radio button is selected. The only other control on my form is a button to submit the information on the form to the server.

One thing I've noticed is the information gets posted back to the server twice when the button is clicked. This causes a duplicate record to be written to two tables and a SQL exception being thrown when it tries to write a duplicate record to a third table since it violates that table's primary key constraint. At first, I thought the cause might've been due to the submit button being contained in the third UpdatePanel I mentioned, so I removed it from there and placed it by itself ... but the problem still occurs. One other thing I've noticed is that when data for one of the UpdatePanels is to be updated via a postback to the server, the other two UpdatePanels act as if they're also being posted back to the server; in other words, all three panels noticeably flicker simultaneously. I have a sneaking suspicion this may somehow tie into my entire page being posted back twice when the submit button is clicked, but I'm not at all sure.

What can I do to prevent my page from being posted back twice?

Hi,

any sync postback at the server side is almost identical to regual postback including:

- all control values are posted to the server
- Viewstate is transferred to the server and back

The most important difference is in the rendering phass: only update panels are rendered and their content transferred to the client

So, you must design your application in such a way, that removing all update panels will not break server logic. UpdatePanels reduce flickering, but they don't change how server code process data.

-yuriy

I think there a couple things that can cause this... I think ImageButtons can get rendered as "Submit" buttons, and have a __doPostBack call attached to their OnClick, which seems to make a double postback. Also, I think a button has a UseSubmitBehavior property... try setting that to false.

pageLoad and pageUnload called when using PageMethods ?

I have pageLoad and pageUnload functions in my page where I do my initialization and cleanup. I also use UpdatePanels and PageMethods. I am noticing that pageLoad and pageUnload methods are getting called on every PageMethod (ajax) call !!.

Doesn't pageLoad and pageUnload correspond to window.load and window.unload events respectively ?

Basically, I want methods which should be called when _entire_ page is loaded or unloaded and should not be called while using PageMethods or postbacks from within UpdatePanel

Please guide.

Regards & thanks

Kapil

Hi ksachdeva17,

I hope i understand your situation correctly. You don't want that the page life cycle will be invoked when there is an event triggered by an action from a control inside your updatepanel (or trigger). Well the situation on the server will be the same when you're using an updatepanel. The async postback will take the same cycle as an synchronous postback. The thing that is different is the rendering of the page. The controls in your updatepanel will only be rendered and not the whole page. I hope this will make things clear for you.

Regards,

Thanks Dennis,

You are saying that window.onload and window.unload (DOM) events will occur if we make XMLHttprequest from the page, I would guess they should not as XMLHttpRequest is independent of browser post !!. I understand that on server side async postback would take the same cycle as synchronous postback but on the page (in client browser) it should not.

Regards

Kapil

Hi Kapil,

I was saying that the page life cycle on the server will stay the same no matter if it's a synchronous of asynchronous postback. I found this in the documentation of Ajax.

Client Page Life-cycle Events

During ordinary page processing in the browser, thewindow.onload DOM event is raised when the page first loads. Similarly, thewindow.onunload DOM event is raised when the page is refreshed or when the user moves away from the page.

However, these events are not raised during asynchronous postbacks. To help you manage these types of events for asynchronous postbacks, the PageRequestManager class exposes a set of events. These resemblewindow.load and other DOM events, but they also occur during asynchronous postbacks. For each asynchronous postback, all page events in the PageRequestManager class are raised and any attached event handlers are called.

Hope this helps!

Regards,

Thanks Dennis,

So my inference is that pageLoad is not same as window.onload. If you could please verify my understanding. I think I should be able to add the event handler using $addHandler with 'load' as an event name to execute things when page is loaded for the first time (no async) . Correct ?

Regards

Kapil

HI Kapil,

PageLoad and window.load are not the same indeed. you can handle also the logic you want in you code behind using thefollowing funciton:

if

(!ScriptManager.GetCurrent(this).IsInAsyncPostBack)

{

myTextBox.Text =

"Test";

}

Probably you already see what it's doing. It will check if it's in a asynchronous postback, when you want to handle some functionality when the page is in a synchronous postback you can you the statement above.

It is also possible to add a handeler to the load event of your window! like you suggested

Good luck

Regards,