Thursday, April 02, 2009

Invalid postback or callback argument

I've been meaning to start blogging on various things, but hadn't found sufficient motivation or time to do so, until now. I've been struggling with the invalid postback or callback argument problem for a day and a half now, and have finally found the solution to the problem.

ASP.NET 2.0 introduced the EventValidation framework to ensure that form values posted to an aspx page are valid and that they originated from the appropriate controls. This is a security feature to ensure that ASP.NET pages are not the target of cross-site scripting and other attacks. The EventValidation framework is the cause of this error, so one obvious and very simple solution to the problem is to turn EventValidation off. This is not recommended for obvious reasons!

There is a solution to this problem, I hope this post will help you to find the cause of the problem. After much research on forums around the web, there seem to be several causes of this error:

1) Form values generated dynamically on the client browser.
2) Form values generated in an AJAX call without an appropriate call to UpdatePanel.Update().
3) More than one form element on an aspx page.
4) More than one control with the same ID.
5) Page is not given sufficient time to load before a postback or callback occurs.

I will discuss each cause in more detail and try to give some potential solutions below.

1) Form values generated dynamically on the client browser.

For controls that have a predefined set of values that can be posted back, ASP.NET will determine that set of values on the server side and will throw this exception if a value is posted that is not in this set. This set obviously cannot include values that are added on the client browser. In this case, you have two solutions:

- Use an Html control instead of an ASP.NET server control (eg HtmlSelect control instead of a DropDownList).
- Use the ClientScriptManager.RegisterForEventValidation method in the Page.Render method to register all possible values that can be submitted by the control.


2) Form values generated in an AJAX call without an appropriate call to UpdatePanel.Update().

This is similar to the above situation in that ASP.NET has determined the valid set of values that a control can post back to the server, but the value submitted is not contained in that set. There seem to be a fairly limited set of circumstances in which this will occur:

- The offending control must be in an UpdatePanel with UpdateMode set to "Conditional".
- The AJAX call must not originate from the same UpdatePanel.
- The AJAX call must change values in the offending control without calling Update() on the UpdatePanel containing the control.

The solution is simple - call UpdatePanel.Update().

This is an unlikely situation, but I'm sure there are other similar situations where the same problem will occur. This is the situation that was causing me problems as I was using an AJAX modal dialog containing a drop down list and was updating the drop down list from an event fired from a control elsewhere on the page.


3) More than one form
element on an aspx page.

I have not been able to reproduce this error, but from what I have read, having more than one form element on a page can cause the "Invalid postback or callback argument" error. The forms can be in the same aspx page or in separate master/content pages. The solution is to remove the extra form tag.


4) More than one control with the same ID.

In this case, the common culprit is dynamically generated controls that are incorrectly assigned an ID, either by the ASP.NET framework, or by the developer. If more than one control on the page is assigned the same ID, ASP.NET is not able to determine which control submitted the value and will throw this exception. Again, I've not been able to reproduce this error, but check what IDs are being assigned to dynamically generated controls and ensure they are unique.


5) Page is not given sufficient time to load before a postback or callback occurs.

EventValidation information is posted back to the server in a hidden form field named "__EVENTVALIDATION". This field does not appear at the top of the page and may sometimes not be fully rendered before a postback or callback occurs. In this situation, the only option would appear to be to prevent postbacks or callbacks from occuring until the page is fully rendered by enabling controls only when the page is fully rendered.


There are other situations that can cause this error, you may find the the post at http://forums.asp.net/t/922994.aspx helpful in troubleshooting. I hope this post has helped you!

0 Comments:

Post a Comment

<< Home