To mark something notable happening during the page's execution lifecycle and let you know about it when it happens,ASP.NET page events are used.
A D V E R T I S E M E N T
You can place your own code into the Page_Load event procedure and a prime example of this is Page_Load, this is raised early in the pages lifecycle.
An Event Handler is a subroutine that is responsible for executing the code for a given event.
ASP.NET - Event Handlers
Look at the code which is given below:
<%
lbl1.Text="The date and time is " & now()
%>
<html>
<body>
<form runat="server">
<h3><asp:label id="lbl1" runat="server" /></h3>
</form>
</body>
</html>
The Page_Load Event
The Page_Load event is one of many events that ASP.NET understands and the Page_Load event is triggered when a page loads.
ASP.NET is responsible for automatically calling the subroutine Page_Load, and execute the code inside it:
<script runat="server">
Sub Page_Load
lbl1.Text="The date and time is " & now()
End Sub
</script>
<html>
<body>
<form runat="server">
<h3><asp:label id="lbl1" runat="server" /></h3>
</form>
</body>
</html>
Note: The Page_Load event contains no event arguments or object references!
The Page.IsPostBack Property
EVERY time the page is loaded,the Page_Load subroutine runs.
You can use the Page.IsPostBack property,if you want to execute the code in the Page_Load subroutine only the FIRST time the page is loaded and if the Page.IsPostBack property is false,
the page is loaded for the first time, if it is true, the page is posted back to the server
(i.e. from a button click on a form):
<script runat="server">
Sub Page_Load
if Not Page.IsPostBack then
lbl1.Text="The date and time is " & now()
end if
End Sub
Sub Submit(s As Object, e As EventArgs)
lbl2.Text="Hello World!"
End Sub
</script>
<html>
<body>
<form runat="server">
<h3><asp:label id="lbl1" runat="server" /></h3>
<h3><asp:label id="lbl2" runat="server" /></h3>
<asp:button text="Submit" onclick="submit" runat="server" />
</form>
</body>
</html>
The example above will write the "The date and time is...." message only the first time the page is loaded and when a user clicks on the Submit button, the submit subroutine will write "Hello World!" to the second label,but the date and time in the first label will not change
