This document discusses events, postbacks, and auto postbacks in ASP.NET. It explains that ASP.NET is event-driven and events can trigger code execution. When a control like a button is clicked, it raises a click event. Event handlers can be created to handle these events. Postbacks occur when a page is submitted and processed on the server. The AutoPostBack property can be used to automatically post back the page on certain events like a dropdown selection change without requiring a submit button.

1. Events,Postback,AutoPostback
Session 6

2. Objectives
• Events
• Postback
• Auto postback property in Asp.net
application

3. Events
• ASP.NET is an event driven environment, which means that code
can execute based on certain events that occur in your code.
Events are raised by certain objects in the application and then
handled by others. There are many objects in the .NET Framework
capable of raising an event, and you can even add your own events
to classes that you write.
• To be able to handle an event raised by an object, you need to write
an event handler, which is basically a normal method with a special
signature. You can wire up this event handler to the event using
event wiring syntax. When an object, such as a control in a web
page, raises an event, it may have the need to pass additional
information to the event handler, to inform it about relevant data that
caused or influenced the event. You can send out this information
using an event arguments class, which is the class
System.EventArgs or any class that inherits from it.

4. • To see how all these terms fit together, consider what happens
when you click a button in a web page. When you click it, the
client-side button in the browser causes a postback. At the
server, the Button control sees it was clicked in the browser
and then raises its Click event.
• Usually, the code that is interested in the button’s Click event
is your own page that needs to have an event handler to handle
the click. You can create an event handler for the Button by
double-clicking it in the designer.
• Alternatively, you can double-click the relevant event on the
Properties Grid of the control with the Events category listed
in the following diagram which you can open by pressing the
button with the lightning bolt on the toolbar.

5. • If you double-click the control in Design View or the event
name in the Properties Grid, Visual Web Developer writes the
code for the event handler for you. The following snippet
shows the handler in C#.

6. • The C# version doesn’t have this Handles keyword.
Instead, with C# you’ll find that VWD has added the
following bold code to the Button control in the markup
of the page:

7. • With this piece of markup, the .NET runtime will generate the
necessary code to link up the Button1_Click method to the
Click event of the button. At runtime you’ll see the exact same
behavior: when you click the button, the code in
Button1_Click is executed.

8. Post Back
• Each of the asp .net pages will be a separate entity with
ability to process its own posted data. That is, the values
of the Form are posted to the same page and the very
same page can process the data. This model is called
post back.
• Each Asp .net page when loaded goes through a regular
creation and destruction cycle like Initialization, Page
load (already discussed in Page Life cycle) etc., in the
beginning and unload while closing it.
• This Postback is false when the first time the page is
loaded and is true when the page is submitted and
processed. This enables users to write the code
depending on if the PostBack is true or false (with the
use of the function Page.IsPostBack()).

9. AutoPostBack
• Autopostback is the mechanism by which the page will
be posted back to the server automatically based on
some events in the web controls.
• The AutoPostBack property is used to set or return
whether or not an automatic post back occurs when the
user presses "ENTER" or "TAB" in the TextBox control.
• If this property is set to TRUE the automatic post back is
enabled, otherwise FALSE. Default is FALSE.
• If we set autopostback property to true of any control
then after processing on any control a request
(postback) is send to the server.

10. Example
• <form id="form1" runat="server">
• <div>
• Select transportation type:<br />
• <asp:DropDownList ID="DropDownList1"
runat="server"
• OnSelectedIndexChanged="DropDownList1_Selecte
dIndexChanged"
• AutoPostBack="true">
• <asp:ListItem>Select an Item</asp:ListItem>
• <asp:ListItem>Car</asp:ListItem>

11. • <asp:ListItem>Airplane</asp:ListItem>
• <asp:ListItem>Train</asp:ListItem>
• </asp:DropDownList>&nbsp;
• <asp:DropDownList ID="DropDownList2"
runat="server" Visible="false">
• </asp:DropDownList>
• <asp:Button ID="Button1" runat="server"
Text="Select Options"
• OnClick="Button1_Click" />
• </div>
• </form>

12. Summery
• PostBack Concept
• AutoPostBack property