Wednesday, March 28, 2012

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?


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.


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.

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