The more I use ASP.NET, the more if (!IsPostBack) {} seems pointless...
First example:
For example, I just Googled an issue, they said use this as part of the solution:
if (!Page.IsPostBack)
{
Page.LoadComplete += new EventHandler(Page_LoadComplete);
}
Which does exactly as coded, LoadComplete will only fire on the first load. After clicking a button, or anything that triggers a postback, the LoadComplete event is left unhooked, thus skipping the event handler. Therefore, their "fix" only works upon the first load = worthless. I promptly commented out the if (!Page.IsPostBack) {} and now the event always triggers as desired.
Second example:
I am attempting to hook events to a dynamically created button (which by the way, I can't get to work [GRR!]). I see examples showing this:
myEditToggleButton = new Button();
myEditToggleButton.ID = "editToggleButton";
//^GOTTA HAVE THIS FOR EVENTS TO WORK! (supposedly, I haven't seen it work...)
if (!IsPostBack)
{
myEditToggleButton.Click += new EventHandler(myEditToggleButton_Click);
}
Controls.Add(myEditToggleButton);
Like the first example, my understanding is that the event wouldn't be hooked after the first page load, thus the button is "inert" after one click (because clicking triggered a postback).
Question:
When should you use if (!IsPostBack) {}? I am guessing it has to do with mark-up created controls only.
In short, you use it everytime you need to execute something ONLY on first load.
The classic usage of Page.IsPostBack is data binding / control initialization.
if(!Page.IsPostBack)
{
//Control Initialization
//Databinding
}
Things that are persisted on ViewState and ControlState don't need to be recreated on every postback so you check for this condition in order to avoid executing unnecessary code.
Another classic usage is getting and processing Querystring parameters. You don't need to do that on postback.
When there is no need to repeat the operation other than the first time.
use it with expensive operations (such as getting data from a database or populating ListItems) that must be performed only the first time the page or control is loaded. If the page is posted to the server and then reloaded, there is no need to repeat the operation. By testing the value of IsPostBack, you can skip the expensive operation,
It's for processing form data.
If you want to handle POSTed data, you only want to do so if the page actually posted data, not on first load. Hence, the IsPostBack flag.
What can happen if you cause a postback is you can change the state of your controls, without meaning to. For example in using a gridview, if you postback during edit mode, you will no longer have access to your edit-ed fields.
Often you need to preserve the information from disapearing on a page when you hit the server, this is the point of
if(!Page.IsPostBack)
Your event handlers should be wired up whenever the event can be fired (irrespective of PostBack status)
Also, when adding controls dynamically, be sure to observe the asp page lifecycle
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack) {
SqlConnection conn = new SqlConnection("Data Source=-----; Database=-----; Integrated Security=True");
SqlDataAdapter da = new SqlDataAdapter();
conn.Open();
da.SelectCommand = new SqlCommand("Select Command",conn);
conn.Close();
DataTable dt = new DataTable();
da.Fill(dt);
ddlSearch.DataSource = dt;
ddlSearch.DataTextField = "---";
ddlSearch.DataValueField = "---";
ddlSearch.DataBind();
}
}
Also, you must use IsPostBack if you want to initialize controls, otherwise they will be reverted to the default on every load. This will confuse the user as when they try to use the form their entered values will get reset to your defaults.
First you need to understand what is postback,when you start your project in Visual Studio,
if you have a if statement which checks whether isPostBack is true or false in your Page_Load method, at this point, isPostBack is false, means it is not a postback, then what is postback,
now click a button (if you don't have a button,please add one and the button click method as well), at this point, you send a request back to the server, the server then response, this process is the so called postback, which is triggered by clicking the button,
one thing you really need to notice, the Page_Load method will be executed again, not only the Button_click method will be executed, so now, the isPostBack is true, means it is postback, yes, it really is a postback, because you clicked the button.
Related
I know there are similar questions about this issue (for example, here and here) but no one results helpful for my problem.
I have a ListView control showing all the users registered in the database and there is a CheckBox for each user shown if the user is approved or not, and I wanna save the changes directly when the CheckBox's Checked property changes.
I know it's not correct to add the event handler on the ListView_ItemDataBound, because after the CheckBox's AutoPostback there is not a new binding, thus the event handler get lost. On the other hand, I can't append the method directly on the ASPX file because this way I can't know which user is affected by the change (at least, I think I can't).
Any sugestion?
Thanks a lot
You have a couple of options.
It's possible that the ListView.ItemCommand event might fire. I'm not sure, though, because the documentation just specifies buttons. You might want to experiment.
The other option would be to harness the ListView.ItemCreated command. I believe that this always runs, regardless of whether the ListView is bound or not, because the item always has to be created, even if it's from ViewState. What you would do in the event handler for that event would be to attach an event handler to the CheckBox Click or CheckChanged event (I forget what the server-side name is for a CheckBox state change event).
So i ran into the same problem. i'm curious, on your page load are you checking if its a postback?
if(!Post.IsPostBack){
//normal page load
}
If you don't have that check, it will call your page load logic, in my case it was resetting the checkbox everytime with my data object.
On the aspx on the checkbox OnCheckedChanged="ckbNameOfCheckbox_CheckedChanged" AutoPostBack="true"
In the code behind
Protected Sub ckbNameOfCheckbox_CheckedChanged(sender As Object, e As EventArgs)
Dim chkBox As CheckBox = CType(sender, CheckBox)
' Gets the item that contains the CheckBox object.
Dim item As ListViewDataItem = CType(chkBox.Parent, ListViewDataItem)
NameOfTheListView.UpdateItem(item.DisplayIndex, sender.Checked)
End Sub
I have a set of programmatically added user controls on the page (they are created and added during the Init event of the main page). Each of these control programmatically add buttons during his own Init event.
(So it's "two levels" of dynamically created controls. Don't know if it matters.)
But I need to access the viewstate while creating those buttons (I use the viewstate to remember user data).
protected void Page_Init(object sender, EventArgs e)
{
// I need my ViewState right now !
Button myButton = new Button();
myButton.Text = "Click me";
myButton.Command += new CommandEventHandler(myCommandHandler);
myButton.CommandName = "Delete";
myButton.ID = "myButtonID";
myPlaceholder.Controls.Add(myButton);
}
I understood that the viewstate isn't available during the Init event. But buttons don't fire their events (OnClick, etc.) if they are created after the Init.
How to create dynamic controls and access the viewstate at the same time ?
I don't think you can. Not ideal but I would put the user data in the Session in this instance. Dynamic controls are difficult to work with and I think this is one of the trade offs. ViewState loads after Page_Init - this forum post confirms
Another option might be to store the information in your own hidden fields. These will become available in the Request.Form collection during postback and will be available. Of course if you make them runat="server" (which you will probably have to) the framework will append all the naming container ids to the name so you will have to be a bit clever when digging them out. It's going to get a bit hacky though so I would be tempted to stick to the Session
Someone else might know different of course.
I am having a problem with an C# ASP.NET form. The purpose of the form is to load some key/value pairs from a database to allow a user to view, edit or delete them.
The Key/Value pairs are dynamically generated in the Page_Load event, and everything works well when the page is first openend. Here is some example code (trimmed down from the original)
TableRow row = new TableRow();
TableCell cell = new TableCell();
cell.Text = keyName;
row.Cells.Add(cell);
cell = new TableCell();
cell.Text = keyValue;
row.Cells.Add(cell);
KeyValueTable.Rows.Add(row);
I just loop through all of the Key/Value pairs and each are added dynamically to a table. At the point of page loading, everything is running smoothly, and all Key/Value pairs show as expected.
The problem occurs when a user clicks a button, which since is set to runat="server", generates a post back event. What this does is it appears to do is clear the dynamic content generated above, and then recreates it (since it is created in the Page_Load event).
What I would prefer to do is choose when that dynamic content is cleared, since I don't want it refreshing unnecessarily, since each refresh is actually occurring from a database.
What I have attempted to do is perform the generation of the dynamic content if it isn't a postback. I.E:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
GenerateDynamicContent();
}
}
However what that now does is generate the dynamic content when the page first loads, however as soon as some postback event occurs (such as a user clicking a button), the dynamic content is wiped, but not created.
Basically what I am hoping for is to discover some way where I can create dynamic content, as shown above, that isn't wiped on any postback event. I want to be able to control when it is refreshed, rather than it occurring on any postback event. Is this possible?
Many thanks
I've found that putting the dynamic code into Page_Init instead of Page_Load prevents the content from being wiped during a postback
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.
On Page_Init I create a table of dynamically created controls based on a couple of database tables. One of the controls is an ImageButton for moving an list item up the list. What this event handler does is to update the SortOrder column in the database for the affected items.
Now the problem is that since the controls are created in the Page_Init event and the SortOrder is updated later on when the ImageButton command event is fired. What's the best procedure for updating the table with the correct SortOrder. If I recreate the table after the event has fired the ImageButton command event does not work any more.
Should I implement a method for updating the data in the table without recreating it?
Should I reload the page in code after the event has fired?
What's your preferred way for solving this problem?
Page events such as Init and Load will always fire before the event handler that raised the postback. This is the basis of the Page lifecycle (For a visual representation by Peter Bromberg, see here). Most developers new to ASP.NET have a major problem understanding and appropriately handling this "quandary".
The ideal way to do this is:
a. Your Page_Init should call a procedure (let's call it BindData() for illustration) that handles the creation of the table based on database data. This method would be similar to a binding method that binds to the database data and renders UI elements on the basis of that binding. IOW, you should remove the table creation code from the Page_Init method and put it in a separate method so that it can be called when needed.
Important note: This BindData() method also handles the attaching of the eventhandler for the dynamically created ImageButton control to the control. We'll call this ImageButton_Click. This is crucial for the control to the event to fire on subsequent postback.
b. When your ImageButton_Click method executes, it calls the BindData() method to recreate the table and it's bindings but with new sort order rules.
So, the order of execution on first load is:
Page_Init
BindData()
The order of execution on subsequent loads (on postback) is:
Page_Init
BindData() - Eventhandler for ImageButton attached.
ImageButton_Click
BindData()
You'll need something like this...
OnInit (IsPostBack = false)
Dynamically create ImageButton
Wireup ImageButton Event Handler
Load Table - Check for a sort-order in Session/Variable. If none; use the default
Click the button
OnInit (IsPostBack = true / 1st Postback)
Dynamically re-create ImageButton
Wireup ImageButton Event Handler
Load Table - with default sort order
ImageButton_OnClick (Still the same 1st postback)
Reload Table - with specific sort order
Save this sort-order variable in Viewstate/Session variable
Cause some other Postback
OnInit (IsPostBack = true / 2nd & Subsequent Postbacks)
Dynamically create ImageButton
Wireup ImageButton Event Handler
Load Table - Check for a sort-order in Session/Variable. If FOUND, use that.
Firstly, you seem to be binding your data manually to UI controls. In Asp.Net there and many ways to avoid this using built-in data binding techniques. Many controls like the GridView allow automatic creation of Html tables from a given data source. There are many other options including Repeaters.
However you do choose to bind your data, the technique is to rebind at some point every time through the page lifecycle.
You need to...
Bind you data on first page load with the default sort order
Rebind the data in the image button’s event handler after the sort order has been changed.
The code would look something like this...
private void Page_Load (...)
{
if (!IsPostBack)
//On First Load
BindData(defaultSoortOrder);
else
BindData(currentSortOrder);
}
private void ImageButton_Click (...)
{
currentSortOrder = newSortOrder;
BindData(currentSortOrder);
}
If the Image button is clicked, you will end up calling BindData twice. But this is necessary since a page postback could be initiated from any control, you need to always ensure you bind the data when the page loads.