I face a weird problem. I have a simple aspx page with a dropdownlist. The dropdown gets filled through a function which is called from Page_Load() event. The dropdown item selection triggers event OnSelectedIndexChanged. Now the event triggers rightly
but what happens that upon post back the dropdownlist gets initialized, that is, it shows empty. Never faced this type of issue before so i wonder what's happening wrong.
The piece of code follow:
protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostBack)
PopulateCompanyList(GetCompanies(serverUNCPath));
return;
}
Is ViewState disabled on your dropdownlist, or perhaps the whole application?
The very first ideas:
Check EnableViewState property of
your dropdown and all the parent
controls up to the root (should not be false)
You perform rebinding on postbacks without storing
SelectedValue property
If one of your parent controls is
custom or dynamic it may incorrectly
persist ViewState info (including children).
Related
So I have an ASP.NET page with two controls:
a GridView which displays rows from a SqlDataSource, and in which the user can select a row;
a DetailsView in which the user can see and edit the values of the selected row.
The DetailsView can update the underlying data, but even though the page reloads, the GridView still displays the old data until I manually reload the page in the browser.
How do I ensure that the GridView displays the correct data after the update in the DetailsView?
P.S. It may be important to note that due to reasons outlined in this other question, I had to use a custom OnItemUpdating event with the DetailsView. This event performs the SQL update command (successfully), sets the DetailsView back to ReadOnly mode (out of Edit mode) and then cancels the event (e.Cancel = true). If this event canceling also somehow cancels the GridView updating, how do I manually tell it to update itself?
P.P.S.: I discovered this similar question, but the answer doesn’t work: it resets the entire page back to pristine state, which means the GridView loses its selected row and the DetailsView disappears. I don’t want that.
on page load:
YourGridViewID.DataBind()
If your OnItemUpdating event generates postback
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack)
{
GridView.DataBind();//rebinding it with considering changes DetailView
}
}
If it doesn't work let me know.
I have been reading about the Page LifeCycle. I understand the LifeCycle, however, it's not clear on what to do, and when to do it. The problem is I use Page_Load to get database values, and set form fields. I use a button's onClick method to update the database. However, the form fields text properties were set during Page_Load, so it's really only updating the database with the OLD values.
Page_Load: I gather data, and set control text properties to reflect data.
Button_onClick: I update the database from the form
Problem: It's updating values gathered from Page_Load and not the actual form.
Certainly, I am not supposed to perform everything in the Page_Load. So where am I going wrong during this process?
Page_Load
If you are loading your database data in the Page_Load event, the very first thing to do is to wrap it within a if (!IsPostBack) statement.
IsPostBack
Gets a value that indicates whether the page is being rendered for the
first time or is being loaded in response to a postback.
http://msdn.microsoft.com/en-us/library/system.web.ui.page.ispostback.aspx
So IsPostBack = true when the page cycle is the result of postback.
In your Page_Load, you should only gather your data when IsPostBack = false, not on every page load.
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
// gather your data here
}
}
Setting fields
I personnaly prefer to set the fields content on the PreRender event handler (but honnestly i don't know it should/must be done there, it just seems more logic to me).
PreRender is executed after your postback events (click on a button, drop-down selection change...) so it ensures that your updates and more generally data modifications are done before rendering the page.
In an effort to speed up my site, I am trying to disable the viewstate as I don't think I am using it everywhere. I have a master page setup with user controls loaded (using LoadControl) in default.aspx. My typical page setup would be:
Main.master -> Default.aspx -> ControlWrapper.ascx -> MyControl.ascx
I have put EnableViewState="false" in my Default.aspx page. Now when I try and read a value from a DropDownList in MyControl.ascx it comes back blank when the form is posted. First all, why is this? I thought I should still be able to read the value from the drop down list?
I then tried enabling the ViewState on that control and it didn't work.
I also tried enabling the viewstate on the Page_Init event of MyControl.ascx using Page.EnableViewState = True; but that didn't help either.
I guess I am misunderstanding the viewstate somewhat, can someone point me in the right direction please?
p.s I don't know if this information is relevant but I am adding the contents of the DropDownList dynamically in the Page_Load event. (Thinking about it, could this be the issues - Will test this now).
Thanks.
With viewstate turned off, the values you are loading in Page_Load are no longer in the list when you post back (until you reload them obviously). If you want to work without viewstate, you will need to set the selected item from the value in Request.Form.
protected void Page_Load(object sender, System.EventArgs e)
{
ddlItems.Items.Add(new ListItem("test1", "test1"));
ddlItems.Items.Add(new ListItem("test2", "test2"));
ddlItems.Items.Add(new ListItem("test3", "test3"));
if (Page.IsPostBack)
ddlItems.SelectedValue = Request.Form["ddlItems"];
}
When you've set ViewState to false the dropdown needs to get populated before page load - which means you probably should do it at page init. Something like this:
protected void Page_Init(object sender, System.EventArgs e)
{
ddlItems.Items.Add(new ListItem("test1", "test1"));
ddlItems.Items.Add(new ListItem("test2", "test2"));
ddlItems.Items.Add(new ListItem("test3", "test3"));
}
Then you should be able to read the value at load:
protected void Page_Load(object sender, System.EventArgs e)
{
someTextBox = ddlItems.SelectedValue;
}
A bit of background:
On this page: Microsofts page cycle
At the image with the page cycle there is the methods "ProcessPostData" and "LoadPostData" firing in between Init and Load. The post data for the drop down contains the selected value - but not the possible values, so when it loads the post data it is essential that the possible values are already there (or it won't be able to set the selected value). Also before the post data has been loaded the selected value has not been set.
If viewstate is enabled it saves and retrieves the possible values in between postbacks.
I will assume you're using .NET 4. View State is the method that the ASP.NET page framework uses to preserve page and control values between round trips.
The reason it didn't work for you when View State was turned off is because that control was rendered again when you performed a PostBack to the server, meaning you lost your selected value.
The reason it didn't work for you when View State was off for the page, but on for the control is because in order for that to work, the following conditions must be met:
The EnableViewState property for the page is set to true.
The EnableViewState property for the control is set to true.
The ViewStateMode property for the control is set to Enabled or inherits the Enabled setting.
ASP .NET View State Overview
When you did EnableViewState = false; on a page then you should not expect DropdownList.SelectedValue after postback.
It will be good if you Enable/Disable ViewState on particular controls rather than disabling whole view state by specifying it on page directive.
I am relatively new to this, but here is my problem.
In asp.net, I have a parent and a child control. Within the child control I have a dropdown list. Based on dropdown list's selected value I would like to toggle Visibility of Panel in parent control. For instance if I select Show in child control dropdown list, I need to pass true to parent control to make Panel visible and vice versa. How should I do that. I have read that can be done via the event handling and have seen certain scenarios but I am not clear on that. Please help!
Thanks.
Raise an event that your parent control listens for.
In the code behind for your parent control, create an object of the type of your child control. Something like:
private MyWebControl childControl;
Then in the child control, define an event
public event System.EventHandler SelectionChanged;
In the OnIndexChanged event of your DropDownList, after you do your processing, raise your event:
if(this.SelectionChanged!= null)
{
this.SelectionChanged(this, new EventArgs());
// You can send the index of the DDL in the event args
}
In your parent control, wire up the event. Page_Init is good
this.childControl.SelectionChanged+=new EventHandler(childControl_SelectionChanged);
Still in the parent control, define your method
private void childControl_SelectionChanged(object sender, EventArgs e)
{
/// Do your processing here.
/// Grab the DDL's index from the EventArgs and do your processing
}
Should be all you need to get it working!
One way to do it is to expose the dropdown list (public) and in your parent control check the child controls dropdown to see if it should show or hide the panel on page load. If this works or not depends a little on the page lifecycle.
Another way to do it is to store the drop-down value in ViewState on the change event. That way the ViewState parameter can be read by the parent control.
If possible you should definitely go for the first option.
Basically, you just need to subscribe to the SelectedIndexChanged event and handle it. The event is fired when the selected item was changed. Note that you should allow auto-postback on the drop down control to make sure the event is fired just after the user changed the drop down's value.
In the ASPX file:
<asp:DropDownList … OnSelectedIndexChanged="OnDropDownChanged">…</asp:dropDownList>
In case you are creating the control in the code-behind, subscribe after creating the control like this:
dropDown.SelectedIndexChanged += OnDropDownChanged;
And then handle it:
public void OnDropDownChanged(object sender, EventArgs e)
{
// alter the panel's visibility here; the drop down's value contains
// the selected item; note that you shoud use "(DropDownList)sender"
// to access it
}
EDIT: Also, have a look on a more elaborate example on MSDN. Note that the event is declared in DropDownList's ancestor 'ListControl'.
I Have a UserControl called TenantList.ascx which contains a slightly modified GridView from DevExpress (Web.ASPxGridView). This control makes Callbacks without causing a postback which is exactly what I need.
The specific event I need to react on is CustomButtonClicked. I have made my on OnCustomButtonClicked event on the usercontrol TenantList.ascx that fires when the the GridView CustomButtonClicked event fires.
I have an eventhandler on the page where I use the UC. When I debug using VS I can see that I get into the eventhandler as I am suppose to.
My Eventhandler looks like this:
protected void uc_TenantList_CustomButtonCallback(object sender, ASPxGridViewCustomButtonCallbackEventArgs e)
{
Tenant tenant = (Tenant)uc_TenantList.GetGridView().GetRow(e.VisibleIndex);
switch (e.ButtonID)
{
case "btn_show":
ShowRow(tenant);
break;
case "btn_edit":
EditRow(tenant);
break;
case "btn_delete":
DeleteRow(tenant.Id);
break;
default:
break;
}
}
private void EditRow(Tenant tenant)
{
uc_TenantDetails.SetTenantData(cBLL.GetTenant(tenant.Id));
UpdatePanel1.Update();
}
The EditRow function get's called and the UserControl TenantDetails.ascx gets filled with data correctly. However the UpdatePanel1.Update(); is not updating the panel where my TenantDetails UserControl is in.
However if i call UpdatePanel1.Update(); from a normal control registered to the ScriptManager it updates just fine.
protected void Button1_Click(object sender, EventArgs e)
{
uc_TenantDetails.SetTenantData(cBLL.GetTenant(17));
UpdatePanel1.Update();
}
That works without a problem... I am 100% stuck and without any idea of what might be the problem here.
Any suggestion is welcome!!
Cheers
The Real Napster - In real trouble :)
Okay solved this issue
What I needed to do was to enable postback on the gridview control inside my usercontrol.
Then place the Gridview usercontrol in a updatepanel and still keep the details usercontrol in another updatepanel.
That way it all worked out. Not impressed by the solution though. Looks a bit ugly.
Make sure that your update panel is set to always update (not conditionally). You may also find the information in this articles useful:
http://www.asp.net/AJAX/Documentation/Live/overview/PartialPageRenderingOverview.aspx
http://msdn.microsoft.com/en-us/library/system.web.ui.updatepanel.update.aspx
The first link will give you some history regarding Partial Page Rendering, the second gives you some more information about the Update method.
From the documentation calling UpdatePanel.Update will cause an update panel to be re-rendered on the client after calling, but only if the UpdatePanel is set to Conditionally update. If it is set to always update this should throw an error.
If your update panel is set to always update, could it be that it is nested in another UpdatePanel which has its update mode set to conditional?