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Answer 1

+1 A:

The AutoPostBack property means that selection of the control will cause a post. If it is not set to true, then you will need to do something else to submit the form (i.e. a button click)

The possiblity on OnSelectedIndexChanged event, is check to make sure that you don't have it adding the event handler twice. If you have it defined in the ASPX, then do not have it being added on the codebehind and vice versa.

Stephen Wrighton 2010-03-09 15:08:38

yes iam sure that I added is only in my aspx file

codymanix 2010-03-09 15:14:15

I've had this happen to me a few times before. Be sure not to set in ASPX and Codebehind otherwise it can possibly screw up the page... and try to call it twice. You really only need one or the other (I prefer the code behind).

Rob 2010-03-09 15:14:42

I edited my question. I found that some kind of timercallback is causing this, but it isn't from me.

codymanix 2010-03-09 15:25:17

Answer 2

A:

If I recall correctly the events that's named OnSomething are more for internal use (like if you create a child-class you'd override the OnSelectedIndexChanged). So I think it's good practice to use the SelectedIndexChange.

As for the AutoPostBack, this is to make shure that the page posts back to the server when an item is changed. If this is not set to true then you (the server) will never now that it changed before the user clicks some submit-button or does something else that causes and autopostback (on another controll).

EDIT:

One more thing, if I don't recall wrongly a OnSomething method normally is the one that calls the Something event (Edit2, this seems not to be the case with the markup-language in Asp.NET (the html). OnSomething="" in html maps to the Something event it seems, my bad). So if you'd write a class "Person" with a name that could be changed you'd do it like this:

public class Person
{
    private string name;
    public string Name
    {
        get { return name; }
        set { name = value; OnNameChanged(new EventArgs()); }
    }

    protected OnNameChanged(EventArgs args)
    {
        if(NameChanged != null)
            NameChanged(this, args);
    }

    public event EventHandler NameChanged;
}

or something like that.

Microsofts example's probably better than mine though :-P: http://msdn.microsoft.com/en-us/library/5z57dxz2.aspx

Alxandr 2010-03-09 15:10:08

I tried it without "On", just with SelectedIndexChanged but then the handler isn't called at all. Also the page you are giving is about .net events in general and not how they work with asp.net

codymanix 2010-03-09 15:34:14

You're quite right actually. http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.listcontrol.selectedindexchanged.aspx(what one can do with a little googleing). I forgot that in the markup-language for Asp.net (the html in other words) they've added "On" in front of everything. So it's also called OnClick... My mistake. Though it still sounds strange that it's fired twice.. Sorry, can't help you there.

Alxandr 2010-03-09 15:56:51

Answer 3

+3 A:

what is the difference between [asp:DropDownList 1] and [asp:DropDownList 2]?

SelectedIndexChanged is not a valid attribute for an asp:DropDownList tag, so ASP.NET won't do anything with it.

I am asking because it seems my OnSelectedIndexChanged-event seem always to be called twice ... Why could this be?

Your stack trace suggests that the second event invocation is caused by a re-entrant thread. Are you running any asynchronous delegates from your page, using client callbacks, or using a third-party control?

Jeff Sternal 2010-03-09 15:26:38

Iam using the microsoft ajax library. I don't know how I could possible accidently have included asynchronous delegates or client callbacks without knowing it.

codymanix 2010-03-09 15:39:08

Thank, now I found it because you brought me on the right track. Iam using a AJAX ListSearchExtender which is causing the problem. The Question now is how to fix that..

codymanix 2010-03-09 15:51:15

Hooray! Are you sure there's anything you need to fix? The `ListSearchExtender` changes the drop-down selection: isn't it appropriate to call `OnSelectedIndexChanged` when it does that?

Jeff Sternal 2010-03-09 15:56:35

But I don't need the event handler called twice, this is just causing unnecessary traffic.

codymanix 2010-03-09 16:38:23

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