Asp – Nested UpdatePanel Behavior

Is there a question here?

One thing you can do is add a "busy" display while waiting for the first Ajax call and disable the controls that would cause another postback. Remember what the first A stand for in AJAX: asynchronous.

First off, keep in mind: UpdatePanels do not alter the lifecycle of a page.

All of the Control Tree (including the UpdatePanels) must be reconstructed just as with a Normal Postback Life-cycle.^{1 2} The UpdatePanels ensure that only a portion of the rendered (HTML) view is returned. Removing all UpdatePanels should result in the same behavior, except with a full postback. For instance, only the HTML representing a nested UpdatePanel (presumably because data changed) might be sent back in the XHR response.

To get "true" AJAX consider Page Methods. Alternatively, DevExpress (and perhaps Telerik and others?) offer their own form of "Callback Panels", which are similar to UpdatePanels, but can bypass parts of the life-cycle (and, as a result often do not support the ViewState model entirely or may introduce their own quirks).

While not understanding the above is the most likely reason for the controls "disappearing", here is my rule: Do Not Let [Nested] UpdatePanels Work "Automatically".

Edge cases with dynamic controls and nested UpdatePanels will be encountered. There might be a nice way to handle this, but I have failed on multiple different attempts.

Instead, for every update panel, run with:

UpdateMode="Conditional"
ChildrenAsTriggers="False"

With the "Conditional" UpdateMode, make sure to manually specify the Trigger control or call panel.Update() (although this hard-wires the Control) as required. Depending on needs ChildrenAsTriggers="True" might work as well. The big thing is that UpdateMode is not "Always" which is the default.

After switching to this approach, I have no problem -- well, almost none -- with nested UpdatePanels.

Happy coding!

¹ If the page doesn't render correctly where Partial Rendering (in the ScriptManager) is disabled (e.g. all requests are full postbacks) then there is no reason to expect/believe it will work correctly with UpdatePanels.

² There are times when it's warranted to "cheat" expensive recomputations in the control tree for controls that will not be re-rendered. However, I would consider these advanced cases that should only be done when performance analysis indicates there is a specific need.