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ModalpopupExtender button click event is changing the alignment of the aspx page

I have an aspx page with multiple controls.
When an image button is clicked, a modalpopup will open. In the modal popup, I have label, textbox and few buttons - Search, Add, Cancel.
When I click search button a grid will open with results matching the text box text. When ever this grid loads the alignment(All the labels and listboxes below this image control) of the page behind(aspx - mainpage) gets messed up.
There is no problem with other button click events in the modal popup.
How can I retain my table structure?
Please help. Sorry cannot post code.

AutoPostBack ONCE on pageload

I have a listview with a dropdown, with autopostback enabled.
The user can select yes - which postsback and brings up loads more dropdowns, no - which brings up a textbox, and blank - which does nothing.
As the original dropdownbox is databound it displays one of those values but at the moment always acts as if it was blank ie no other controls showing..
Can I make the page do ONE autopostback as soon as it's loaded without any user input to display the correct controls if it's a yes or no as opposed to blank?

try calling dropdownbox_selectedindexchanged(null,null) after binding in page load. it is just calling selectedindexchanged event programmatically. as far as i know you want to load controls based on dropdown selection when page first load and user haven't changed dropdown value. so this code will call it from code behind.

gridview click causing postback in usercontrol

I have three user controls in one aspx page, where each user control is loading in each separate tabpanel.
Each usercontrol has gridivew and objectdatasource(which has selectmethods).
The gridview is binding data from the objectdatasource, on every postback.
I have some dropdowns, the selected values are passed to the objectdatasource on submit button click, which then populates the results in the gridview.
Now, if I click on any linkbutton on the gridview, the postback is happening and gridview is rebinding with null values, so no click event of linkbutton is happening.
Anybody can suggest me how to stop the postbacks for gridview. I tried placing the updatepanels but didnt help

You can control the flow of the program by checking IsPostback in Page_Load() and filling the grid only when it's supposed to be filled. You can also specify your UI elements to not postback as well, if certain ones are posting back when you don't want them to. Look into the AutoPostBack property.
If (IsPostBack == true) {
// do something, load that
}
Else {
// do something else, don't load that
}

modal popup after post back c#

I have one requirement like,
One master page with three image buttons, once clicking on any button it redirect's to appropriate page, in that page(it's requester form) i have placed all controls in one panel control(id="MyPanel"), one link button(MyLinkButton) out side to that panel and one modal popup control(id=MyModalpopup) as target control id=MyLinkButton,popupcontrolid=MyPanel. On this page load i did code (MyModalpopup.show()) to show all controls in modal popup.
every thing is working fine but, i have controls like dropdown,checkboxlist with autopostback=true and these are in updatepanel and for these controls i am binding data from code behind. Now the problem here is the data i am unable to bind and when these controls are postback the modal popup is not visible.
i am using C# and 3.5 framework.

What I would do, is have a hidden field (with runat=server) that has a true/false value that is set when the modal is activated/hidden.
Then during the client-side page-load of the postback, read that value and make the modal hidden/shown appropriately.
The other option would be to figure out how to make those controls work without the auto-postback (i.e. populating the controls via ajax).

choose a dynamic item in a dropdown without losing choice upon submit

Every time I click an ASP button on my page to select an item in my dropdown list, it is only choosing the first item in the list. It is as if every time I click a button the page refreshes and all variables reset. How do you choose an item from a dropdown when using dynamic data from a database?

This sounds indicative of populating the DropDownList in Page_Load. Is that the case? (I'm assuming you're using WebForms.) If you're populating the control in Page_Load then you'll want to wrap it in a conditional:
if (!IsPostBack)
{
// populate your controls from data
}
Otherwise, they'll get re-populated with each postback. When you click a button or perform some other action on the page which initiates a postback, Page_Load is called before the event handler. So in effect, this is happening:
User navigates to the page
Page_Load clears and re-populates the DropDownList
User chooses an item in the DropDownList
User clicks a button
Page_Load clears and re-populates the DropDownList
Button handler gets the current selection from the DropDownList, which is the default.

What do you mean by "click ASP button"?
If you are talking about the downarrow to open the dropdown list, then you need to set AutoPostback options to false if you don't want that opening the dropdown list triggers postback (that is on the other side required if you have any events related to the opening of your list).