PreInit()
In this Page level event, all controls created during 
design time are initialized with their default values. For 
e.g., if you have a TextBox control with Text property 
= “Hello”, it would be set by now. We can create dynamic 
controls here. 

This event occurs only for the Page class and 
UserControls/MasterPages do not have this method to 
override. 

Sample code where you can override this method and add your 
custom code:


protected override void OnPreInit(EventArgs e)
        
{
            //custom code            
           base.OnPreInit(e);
 }
Note that PreInit() is the only event where we can set 
themes programmatically. 

Special Case with MasterPages 

It is important to note that Master Page is treated like a 
control in the Content Pages.
So if a Page has a  Master Page associated with it, then 
the controls on the page will not be initialized and would 
be null in this stage. Only after the Init() event starts, 
you can access these controls directly from the page class. 
Why? 

The reason being that all controls placed in the Content 
Page are within a ContentPlaceholder which is a child 
control of a MasterPage. Now Master Page is merged and 
treated like a control in the Content Pages. As I mentioned 
earlier, all events except the Init() and Unload() are 
fired from outermost to the innermost control. So PreInit() 
in the Page is the first event to fire but User Controls or 
MasterPage (which is itself a Usercontrol) do not have any 
PreInit event . Therefore in the Page_PreInit() method, 
neither the MasterPage nor any user control has been 
initialized and only the controls inside the Page class are 
set to their default values. Only after the Page_PreInit() 
event the Init() events of other controls fire up.