postback issue with drop downlists

emachines · January 6th, 2005, 01:42 PM

I have a problem with which I am struggling for many days.
I have a page which has six drop down list (which are webform elements and not html elements). Auto postback property is enabled for all the elements. The data in the dropdown lists are related in a hierarical fashion. Now to give you the general idea of the working of the page, suppose, When the client changes the selection in any of lists in the page (say list number 2), it updates the items of the subsequent lists (that is lists 3, 4, 5, 6). The process is database intensive, so it takes quite a while before any results comes up.
What I want is to prevent the client from changing the selection before their previous request is serviced, by disabling the dropdown lists in the page.
I am trying to use Javascript to disable the lists by adding a "OnChange" attribute to the dropdown list, in the Page_Load event of the page.
            ddllist1.Attributes.Add("OnChange", "disable();")
            ddllist2.Attributes.Add("OnChange", "disable();")
        ...........
            ddllist6.Attributes.Add("OnChange", "disable();")
The disable function is defined on the <Head> of the aspx page. Now the trouble is, this "OnChange" event is suppressing the "OnSelectedIndexChanged" event of the dropdown lists!! So though the lists are getting disabled, when the client is changing the selection, the code in the "OnSelectedIndexChanged" is not executing and the lists are not getting updated.
Please suggest.

planoie · January 6th, 2005, 11:26 PM

I have seen similar problems to this. It has to do with the way the form is processed. When you disable the dropdownlist, the value for it never get's posted to with the form data. So what you need to do is disable it with a time delay. Modify your disable() JS function to do something like this:

setTimeout("<put current disable code in here>", 100);

Or you could modify the server side code like so:

ddllist1.Attributes.Add("OnChange", "setTimeout('disable()', 100);")

Either way, your onChange event handler will return before the DDL is disabled and the onchange autopostback call will happen with useful data, then after the 100 millisecond delay, the DDLs will be disabled. Usually this is quite suitable to let the post fire off before the use has a chance to try clicking again.

emachines · January 7th, 2005, 01:23 PM

hello planoie,
You tip saved my day. You are great.

thanks

pradeepac · March 26th, 2005, 01:36 PM

Hello emachines,

There is a way of making this faster without having to go to the database everytime on each selectedIndexchanged event

Get the contents of all the ddls as separate tables in a dataset.

In the dataset, create a relation between a column of one table to the same column of another table.

This way you do not have to go to the database, because it is very resource-intensive.

Thanks