I have the control dropdownlist which has been loaded inside the template column of RadGrid.
While loading I have set AutoPostBack='True' for dropdownlist and also created the event SelectedIndexChanged.
DropDownList ddlConditions = new DropDownList();
ddlConditions.ID = "ddl" + name;
ddlConditions.AutoPostBack = true;
ddlConditions.SelectedIndexChanged += new EventHandler(ddlConditions_SelectedIndexChanged);
My question is while i change the selected index of dropdownlist the event SelectedIndexChanged is not getting triggered.
Can anyone help me to solve this problem?
Thanks in advance.
Usually this caused by a page life cycle problem. When your index changed event of Dropdownlist fires the control doesn't exist to bind it on the postback.
Example:
-> MyEvent fires.
-> Drop-down created.
-> Event Handler specified.
-> Index Changed event triggered. Page reloads. Drop-down not found, cannot fire.
So you have to ensure that the drop-down is created before .NET attempts to handle the event.
Please refer this answer for more information regarding this type of issue and life cycle.
I can suggest you to check the place where you have created DropDownList. Dynamic controls should be added on OnInit or at least on OnLoad. After OnLoad finishes executing ASP.NET starts processing control's events and values.
My question is while i change the selected index of dropdownlist the
event SelectedIndexChanged is not getting triggered.
Answer: because you have created DropDownList after the events have been processed.
Is the page posting back? If so, you'll need to make sure that the control is recreated on the page on every postback.
If it's inside the usual if(!IsPostBack) block, then put it outside - Usually, it's prudent to create controls in page_init as well, but that can depend on your specific setup.
Related
I am trying to take advantage of the page lifecycle in Asp.NET so that I don't bind to my datasources more than I actually need to.
This has led to the following problem. I have an ObjectDataSource, GridView, and a button on my page. The button adds a record to the database, which should be picked up by the data source and presented on the Grid. The problem is that the item is not showing up on the gridview until I refresh the page.
I can solve the problem by calling GridView.DataBind() in my button's event handler, but this goes against what I understand about the .NET Page Lifecycle.
Based on this article, the lifecycle should be as follows:
In addition the article states that the Databinding event is "Raised after the control's PreRender event, which occurs after the page's PreRender event."
So, my button click event should fire first, during the Event Handling phase. It should add the record the database.
Next PreRender should be called on the controls.
Finally DataBind should be called on the controls, and the Grid should update to capture the new record.
Yet this doesn't seem to happen. What am I not understanding?
I think the issue is that your viewstate is not enabled on your GridView. This is what I experienced, but then I also had to call DataBind() on the GridView from the page PreRender event if the request was a postback to get the data updated in the GridView on postback.
Edit:
It would help to understand the issue and context better if you could post the source code of your page (aspx + codebehind). How and where do you connect your GridView to your datasource? Statically in markup or dynamically? Do you make any calls to page.DataBind()? ... These things may influence the behaviour of the GridView.
Assume I have an asp:GridView with some customer information. On that page I also have a button which allows me to add a new user to the GridView. When I add the click the user, I databind the gridview again in the button click. Correct me if I am wrong, but if the asp.net lifecycle or at least part of it is in the following order:
page_load
Hookup event handlers such as my button click
page_preRender
Does this mean that if I put the databinding for the gridview in preRender, that is the only place I need to worry about calling it. I can remove it from the button click. This is what I think, but I am not sure if my thinking is correct, so I would like some more insight as to the benefit of putting code in PreRender as opposed to PageLoad
By PreRender, you assume that most logic in the page that would affect the binding result has been completed (usually in Page_Load, but anywhere earlier really).
Controls in the .Net framework by default do their binding in the PreRender event (makes sense, bind to the datasource at the last possible moment...could be that you changed the data 400 times earlier in the lifecycle). Here's the full layout of the 2.0 lifecycle for reference.
PreRender - Before this event occurs:
The Page object calls EnsureChildControls for each control and for the page.
Each data bound control whose DataSourceID property is set calls its DataBind method. For more information, see Data Binding Events for Data-Bound Controls later in this topic.
The PreRender event occurs for each control on the page. Use the event to make final changes to the contents of the page or its controls.
I forgot to mention this asp.net 2.0.
The user control has a unique id and it is loaded in the PageLoad event.
The user control is loaded into a panel and the panel is inside of a webpart.
The dropdown has autopostback set to true.
EnableViewState = true on the dropdown.
The ListItems are created in the dropdowns pre-render event method.
This is why I don't understand why it is not firing, the dropdown is the only thing that causes postback on this user control.
The event methods for the dropdown should occur since the user control is loaded in the page load method on postback again right?
Make sure there is no OnLoad or PageLoad event that is rebinding the datasource of the dropdown list. Rebinding the data with a new set of data may cause the clickhandler to not ever get executed.
make sure you have if (!Page.IsPostBack) around dropdownlist.datasource = and dropdownlist.databind()
I am not sure if this is your problem, but it is the most common.
Try with EnableViewState set to
true for the DropDownList
If the ViewState is set to false, on post back the selected Index gets back to default which is normally the first Item. First item, if selected, does not cause SelectedIndexChange event to fire
My understanding of the order of page events is this:
Page : Load
Control : DataBind (for a GridView or whatever)
Control : Load
Control : Clicked (for a Button)
Page: PreRender
Control : PreRender
(There are lots of others - but these are the ones I'm interested in)
The important thing to notice here is that the button's click event comes after the gridview's bind event. If the button causes a change to the data, the GridView displays the old data. I could rebind the control in the PreRender event, but that seems totally ugly.
This must be a very common pattern (a button that updates data). How do I put this together so that the GridView binds to the data after the Button click changes it?
The answer was in the Button Click event, after the data has been changed, call DataBind() on the page to cause the GridView (and anything else that needs it) to rebind. I didn't realise you could do that.
Thank you Ocdecio & Mufasa - I would mark your answers as helpful, but I got no rep yet.
ASP.NET does, by default, lots of binding and rebinding. Rebinding after a click event is normal.
The only reason the button click event comes after GridView bind is because you programmed your page to do that . I don't see any problem in binding the control in the PreRender event, in fact that is the only way to take action after a control event (such as Button onclick).
I'm writing an ASP.Net webform with some DropDownList controls on it. Then user changes selected item in one of dropdowns, ASP.Net doesn't seem to handle SelectedIndexChanged event until form is submitted with a 'Submit' button click.
How do I make my dropdowns handle SelectedIndexChanged instantly?
P.S. It's a classic question I have answered too many times, but it seems no one asked it before on stackoverflow.
Setting the AutoPostback property to true will cause it to postback when the selection is changed. Please note that this requires javascript to be enabled.
You need to set the AutoPostBack property of the list to true.
Also, if you're populating the contents of the drop down list from the code behind (getting the contents of the list from a database, for example) - make sure you're not re-binding the data in every postback.
Sometimes people are caught out by binding the drop-down in the page load event without putting it in an If Not IsPostBack. This will cause the event not to fire.
The same is also true of repeaters and ItemCommand events.
if you are populating the dropdown list during page load then each time the page postback it will reload the list thus negating your postback method.
you need to be sure to load the dropdownlist only if (!ispostback)
Set the AutoPostBack property of DropDownList to true.