I have developed a custom control that extends ListBox. The idea is that the control 'remembers' modifications to its elements which occurred client-side, e.g. as a result of an AJAX request.
The way it works is that the control also renders a hidden input, and the result of the AJAX request is stored in the hidden input. This is posted back, and the control's LoadPostData() method looks for the hidden input, and if the hidden input has data, creates the ListItem collection from it.
This works perfectly so long as the user has made a selection from the list box. If they have not, the LoadPostData() method doesn't get called, and consequently the new ListItem collection is not created. (I've established this using the debugger.)
I assume that the LoadPostData method is only called if the POST data collection includes data corresponding to the control's UniqueID (i.e. 'name' attribute in HTML). If the user hasn't made a selection from the list box, nothing is included in the post data for the list box's UniqueID and LoadPostData() isn't called. Is that correct?
Can anyone suggest how I can ensure that my custom ListBox's LoadPostData() method is called every postback regardless of whether the user has made a selection?
Thanks in advance - I'm really stuck with this one.
David
I've established that the LoadPostData() method is not called unless the post data contains an item with the same name as the control's UniqueID. [Edit: calling Page.RegisterRequiresPostback during Init() overcomes this.] I can see why, but it is quite limiting.
I have overcome the problem by not handling it during the LoadPostData() method at all. Instead, I have handled it in a method which I call in OnLoad() instead.
Two things need to be borne in mind when using this approach:
1) You no longer have access to the postCollection NameValueCollection object which is passed in to the LoadPostData() method as an argument. This means you have to extract the post data from the Request.Form collection, which is slightly harder work. 2) Since OnLoad() occurs after the ViewState processing code, you will need to manually set the SelectedValue after you create the ListItems. If you don't, if the listbox is populated via AJAX and the user makes a selection, the selection will be lost.
I hope this helps someone in the future.