I encountered this comment recently:
protected void Page_PreRender(object sender, EventArgs e) {
// doing this at PreRender so we don't have to worry about when/if
// we should bind based on if it's a postback or callback and what not.
OrderList.DataSource = OrderItems;
OrderList.DataBind(); }
I was under the impression that PreRender fired every time Load fires, as part of normal page lifecycle, so what's the advantage of doing databinding here?
if you use PreRender, you can manage any control at this point, just before page goes for display. PreRender occurs in the very end, after all events have occured
This article explains all from start to finish, in
Control Execution Life Cycle
hope this helps
I see no advantage of doing that in response to that event. As far as the specific PostBack comment, he's referring to code like this:
if(!IsPostback)
{
...
}
Wild guess: Lots of code in Page_Load is dependent on whether or not the processing is being done as a result of a postback, and there are some things in that code that must be decided before the binding is made. The commenter therefore believes that maintenance will be easier if the .databind() call is made in some later part of the lifecycle, which makes more clear the fact that the binding is independent of the codepaths in Page_Load.
Related
Here's some background to help better understand the problem:
The company I work for makes x-ray scanners for security purposes. We have a page in our web app for viewing current scan activity. Currently, it's set to refresh every minute. I've been tasked with reworking the page so that it's event driven, refreshing every time a scan is saved. The information is displayed in a GridView, which is inside an update panel. The first column contains a thumbnail image of the scan, and an Ajax HoverMenuExtender for the purpose of enlarging the thumbnail when the cursor hovers over it.
When the page loads, everything works fine. I assume this is because all the extenders are created before the OnPreRender event is handled. Upon my scan event being fired, however, it fails when the GridView.DataBind() method is called. I assume this is because the OnPreRender event already occurred when the page first loaded and, as the exception says, extender controls may not be registered after prerender.
I've done a lot of digging on this exception, but have yet to find a solution that works. I read that moving the ToolkitScriptManager to the master page can fix it, but it's already on the master page. I also tried overloading the OnInit and OnPreRender handlers as follows:
protected override void OnInit(EventArgs e)
{
base.OnInit(e);
if (this.DesignerMode)
{
this.EnsureChildControls();
}
this.Page.RegisterRequiresControlState(this);
}
protected override void OnPreRender(EventArgs e)
{
base.OnPreRender(e);
}
I tried putting that on the current activity page, the master page, and the base page, all to no avail.
I've come across slews of answers simply saying to register the controls before the prerender, which of course makes sense, but since new controls are being created dynamically upon event trigger after the initial page load, how can I accomplish that? Unfortunately, I just don't understand this well enough to know what else to try.
In my Asp.Net Web Form,
Assuming that, I have Main_page and Save_button that it is on Main_page.
When I click Save_button, firstly Page_Load eventexecute and after this btnSave_Click button execute.
I thought that when I click button firstly and only button might execute.
Is it correct or my program doesn't work correctly ?
It is normal that Page_Load is executed first and event handlers afterwards. So your program behaves as designed.
Excerpt from MSDN on the page lifecycle:
Load
During load, if the current request is a postback, control properties are loaded with information recovered from view state and control state.
Postback event handling
If the request is a postback, control event handlers are called. After that, the Validate method of all validator controls is called, which sets the IsValid property of individual validator controls and of the page. (There is an exception to this sequence: the handler for the event that caused validation is called after validation.)
If you are interested in details on the lifecycle of an ASP.NET Page, have a look at this link.
Resolution
If you need to perform certain steps in Page_Load (or any other method on your page) only when the page is requested first, you can check the IsPostBack property and thus make your program behave as you describe in your question:
public void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
// Steps are only run on initial GET, not when request is a PostBack.
}
}
It is correct that load event fires first then button event handler. If you want to execute code only at first, but not when any postback check IsPostBack property.
I am working in a asp.net project(ASP.NET 4.0,c#,WebForms).
I need to add a function to the Page Load event based on some conditions in Page Prerender event.How can i achieve this.
I tried the following code,
Page_Load += new EventHandler(delegate(object sender, EventArgs e) { ShowForm(); });
But it doesn't work.Can someone provide some insight on this?
You can't, in the pre-render event you're too late, the Load event has already happened!
See http://msdn.microsoft.com/en-us/library/ms178472(v=vs.100).aspx
Not possible - Page_Load event happens before Page_Prerender event.
I also suspect that perhaps you are mixing between server side events and client side events - you may want to perform some logic when page gets loaded at the client side (which is different than page_load event - which happens on the server side).
Perhaps you can try to explain what exactly you want to achieve by the code that you have sighted and you may get better answers that would solve your actual problem.
I have a button that calls function A()
When I click on it I want the calls to be made in that order:
A()
Page_Load()
Right now it's doing:
Page_Load()
A()
Is there a way around that or is it just by design and there's nothing I can do about it?
The easiest way to do this would be to use a HTML Submit button and check to see if it is in the Form on every postback in Page_Init
public void Page_Init(object o, EventArgs e)
{
if(!string.IsNullOrEmpty(Request.Form["MyButtonName"]))
{
A();
}
}
And in your ASP.NET code:
<Button Type="Submit" Name="MyButtonName" Value="Press Here To Do Stuff Early!" />
I think that will work.
Control events (such as the click events of buttons) are called after page_load. The controls are not guarenteed to be fully initialized prior to page_load. If you really need to call a function before page_load has been called based on whether a button has been pressed you'll have to examine the request to check if the button has been pressed (basically old school ASP)
You need to call your function in the Page_Init. Page_Init will happen before Page_Load.
Here's an Overview of the ASP.NET Page Lifecycle.
Not exactly: ASP.NET will always call Page_Load before handling postback events like Button_Click.
However, you can accomplish what you want by redirecting to your page after handling the postback event. (Using the Post-Redirect-Get pattern.)
Inside your Page_Load method, you can avoid running any relevant code twice by checking to see if it's a postback first:
if (!this.IsPostBack) {
// Do something resource-intensive that you only want to do on GETs
}
As Jeff Sternal answered, The Post-Redirect-Get pattern is a good way of solving a problem like this.
In my circumstances i had a calendar and if you clicked a date it would add that to a scheduler. The scheduler would have buttons on each new date that needed to have onclick functions tied to them.
Because the new row was being added with a linkbutton(on the calendar), in the code the new scheduler date was being added at the Postback event handling meaning that the new set of buttons wouldn't have a command tied to them.
The page life Cycle
Post Get Redirect
I don't think it's possible, at least, not in the way described by your question. When you click a button it will send a request to the server which in turn will start processing it, and follow the ASP.NET Page Lifecycle as posted by Joseph.
Alternatively you could try making an AJAX call to a page without reloading the current one you're on and do whatever processing you require.
This is what you want to do for Page Init is called before Page Load.
Take a look at the ASP.net Page Life Cycle
public void Page_Init(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
//CALL YOU FUNCTION A()
}
}
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
}
}
If your actual goal here is to have your "page loading code" happen after your event handler runs -- for example, if clicking your button changes something in your database, and you want the updated data to be reflected on the page when it loads -- then you could have your "page loading code" get called from a method that gets called later in the ASP.NET page life cycle than your event handler, such as Page_PreRender, instead of calling it from Page_Load.
For example, here's a simplified excerpt from an .aspx.cs page class that has a button event handler that runs before the page population logic, and a confirmation message that is visible on the page only after the button was clicked:
// Runs *before* the button event handler
protected void Page_Load() {
_myConfirmationMessage.Visible = false;
}
// Runs *after* the button event handler
protected void Page_PreRender() {
// (...Code to populate page controls with database data goes here...)
}
// Event handler for an asp:Button on the page
protected void myButton_Click(object sender, EventArgs e) {
// (...Code to update database data goes here...)
_myConfirmationMessage.Visible = true;
}
I have tried, 'PreviousPage', 'PreviousPage.IsCrossPagePostBack' 'Page.previousPage', page.title
It causes the client to stop rendering the page after this line.
simple example
protected void Page_Load(object sender, EventArgs e)
{
response.write("I can see this");
string test = PreviousPage.IsCrossPagePostBack.toString(); //Any page call Causes client rendering to freeze
response.write("But i cant see this");
System.Windows.Forms.MessageBox.Show("However i can see this,proving that the server is still running the code");
}
Anybody Please, any ideas?
ANSWER
Well it ended up it was something stupid. code smell over.
The button i was using to fire the PostBack had a handler that fired to redirect, i just deleted the handler, keeping the PostBackUrl setting and magic.
Have you checked PreviousPage for null?
From msdn:
The PreviousPage property is a null
reference (Nothing in Visual Basic)
when the user requests that page
directly from the server.
Also - MessageBox in a web form, not a great idea... perhaps use the inbuilt trace.axd