How Does ASP.NET Knows Which Button Caused a PostBack?

Question

I'm using ASP.NET 4 to create a page with elements that are used in a jQuery UI Sortable plugin.

All of these elements contain a button (implemented as a submit button) with the name SubmitButton. All but one of these buttons are hidden in the page's Load event. But if I drag the element to another position, and then submit the page using that button, ASP.NET gets confused.

ASP.NET thinks I've clicked a different button, one that wasn't even visible on the page, but is associated with content at the position where the clicked button was before the move. (If I don't move the element, it works fine.)

I can't seem to determine how this is happening. As I understand it, ASP.NET knows which submit button caused a postback because the button's name and value is included in the postback data. (__EVENTTARGET plays no role here and is empty.) So how can it think a different button submitted the page?

How could ASP.NET get confused about which button submitted the page?

(Sorry, it's not possible to put this page on a public website.)

I'm assuming your dragging the button via javascript of some sort. After you drag the button does the name and id of the control stay the same as when it was rendered? — Michael Grassman
The jQuery UI Sortable plugin takes care of repositioning the elements. It moves elements, not rename them. — Jonathan Wood

Jonathan Wood Jonathan Wood · Accepted Answer · 2011-11-12T15:29:40

I worked through this issue and here's what I found.

First of all, buttons by default are rendered as <input> tags with type = submit. The postback mechanism in this case has nothing to do with ASP.NET. The postback data includes, among other things, the name of the submit button that caused the postback along with the button's text (in the form name=text).

But ASP.NET button names, by default, include the names of all parent controls. When those controls are unnamed, they are given an ID like ctl00, ctl01, ctl02, etc. The result is that all my buttons are guaranteed to have a unique name.

The problem is when these buttons and parent controls are all created dynamically. On the postback, these controls are reconstructed in the load event but in the new order. Because the order affects the name (ctl01 vs ctl02), this means my submit button has a different name than it did when the page was originally rendered.

The result is that ASP.NET sees the wrong name associated with the submit button used to trigger the postback.

How Does ASP.NET Knows Which Button Caused a PostBack?

1 Answers