if the AutoEventWireup="true" propety in dropdown when 
index changed automatically post back to the server.

if the AutoEventWireup="true" propety in dropdown when 
index changed will not  post back to the server.

Sorry in the above example is small mistake 


if the AutoEventWireup="true" propety in dropdown when 
index changed automatically post back to the server.

if the AutoEventWireup="false" propety in dropdown when 
index changed will not  post back to the server.

I guess what you mean is AutoPostBack="true" in which the 
index change of a drop down box will be posted back to 
server.
Please correct me if I am wrong.

To know the difference between autoEventWireup="true" and 
autoEventWireup="false" please refer the below link:

http://support.microsoft.com/kb/324151

Regards,
Meganadha Reddy K.

When you set the value of the AutoEventWireup attribute to 
false, you must manually hook up events to event handlers. 

When you set the value of the AutoEventWireup attribute to 
true, the ASP.NET page framework can automatically raise 
events.

When you set the value of the AutoEventWireup attribute to 
false, you must manually hook up events to event handlers. 
such as Page_Load() etc.

When you set the value of the AutoEventWireup attribute to 
true, the ASP.NET page framework can automatically raise 
events.
such as Page_Load() etc.

The default value for AutoEventWireup is true for a C# web 
form, and false for a VB.NET web form. The IDE adds the 
default values to the @ Page directive for a new web form. 
The difference in defaults is partly because VB.NET has a 
mechanism for defining an event handler and subscribing to 
an event in one graceful motion (the Handles keyword). 


Protected Sub Page_Load(ByVal sender As Object, _
                        ByVal e As System.EventArgs) _
                        Handles Me.Load
    ' ...

End Sub


An easy way to generate the above code is to use the drop 
down controls that sit just above the editor. Note: C# 
doesn t make the dropdown list of events available when 
editing a code-beside file, but the dropdown is available 
when writing in-line code. 

There is no equivalent to the Handles keyword in C# 
(anonymous event handlers are arguably close, but just not 
the same). When AutoEventWireup is true, all we need to do 
is follow the method naming convention of 
Page_EventToHandle. The ASP.NET runtime will automatically 
find and fire the method for the appropriate event. 


protected void Page_Load(object sender, EventArgs e)
{

}


Once Again, In Reverse
If we switch to AutoEventWireup= true  for a VB.NET web 
form, we can use the magic Page_EventName approach. The 
only change to the earlier VB.NET code would be to drop the 
Handles clause, and the events fire correctly. 

If we switch to AutoEventWireup= false  for a C# web form, 
there is a little extra work to do. Somewhere we need to 
explicitly wire up events. Here is one approach. 


public partial class _Default : Page
{
   public _Default() // ctor
   {
      Load += new EventHandler(Page_Load);
      PreInit += new EventHandler(Page_PreInit);         
   }

    protected void Page_Load(object sender, EventArgs e)
    {
       // ...
    }

   protected void Page_PreInit(object sender, EventArgs e)
   {
      // ...
   }
}

First of all the above answer that default value for auto
eventwireup=true is false bcoz the default value of
autoeventwireup=false in c#

A false value indicates that you must explicitly write a
code to bind events related to a page ,such as the load
event of a page

and true value indicates that events will be generated
automatically

here is the proper valid answer
AutoEventWireup attribute
AutoEventWireup is a Boolean attribute that indicates
whether events of a Web Forms page are autowired.The default
value for AutoEventWireup is TRUE.

In Visual Basic .NET or in Visual Basic 2005, the designer
performs this binding using the Handles keyword in the
declaration of the event-handler method.
Private Sub Page_Load(ByVal sender As System.Object, ByVal e
As System.EventArgs) Handles MyBase.Load

The ASP.NET page framework supports an alternative mechanism
that uses the AutoEventWireup attribute of a Web Forms page
to automatically associate page events and event-handler
methods. If the AutoEventWireup attribute of the @ Page
directive is set to TRUE (or if it is not specified because
its default value is TRUE), the ASP.NET page framework
automatically calls page event-handler methods.

For example, the Page_Init and Page_Load event-handler
methods are explicitly called by the ASP.NET page framework,
without using the Handles keyword or an explicit event delegate.

Conclusion
When you explicitly set AutoEventWireup to TRUE, Visual
Studio .NET or Visual Studio 2005, by default, generates
code to bind events to their event-handler methods. At the
same time, the ASP.NET page framework automatically calls
the event-handler methods based on their predefined names.

Dont read all the those things simply understand

actually AutoEventWireup is written in page directive like 
this 

<%@ Page Language="C#" AutoEventWireup="true" 
CodeFile="Default2.aspx.cs" Inherits="Default2" %>

by default it is "true", that means all the events that are 
automatically rises when page is loading


if AutoeventWireUp=false then we have to manually write 
events and delegates  like this




 public _Default() // ctor
   {
      Load += new EventHandler(Page_Load);
      PreInit += new EventHandler(Page_PreInit);         
   }

    protected void Page_Load(object sender, EventArgs e)
    {
       // ...
    }

   protected void Page_PreInit(object sender, EventArgs e)
   {
      // ...
   }