ASP.NET - Event

An event handler (event handler) is a subroutine that executes code for a given event.

ASP.NET - Event Handler

Please see the following code:

<%
lbl1.Text="The date and time is " & now()
%>
<html>
<body>
<form runat="server">
<h3><asp:label id="lbl1" runat="server" /></h3>
</form>
</body>
</html>

When will the above code be executed? The answer is: 'I don't know...'

Page_Load event

The Page_Load event is one of the many events that ASP.NET can understand. The Page_Load event is triggered when the page is loaded, and then ASP.NET automatically calls the subroutine Page_Load and executes the code within 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 does not contain object references or event parameters!

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Page.IsPostBack property

The Page_Load subroutine runs every time the page is loaded. If you only want to execute the code in the Page_Load subroutine on the first load of the page, you can use the Page.IsPostBack property. If the Page.IsPostBack property is false, the page is loaded for the first time, and if it is true, the page is returned to the server (for example, by clicking a button on the 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 creates the message "The date and time is...." only when the page is initially loaded. When the user clicks the Submit button, the submit subroutine creates "Hello World!" in the second label, but the date and time in the first label will not change.

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