Web Forms events
ASP.NET Web Forms - Events
An 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 many events that ASP.NET understands. The Page_Load event will be triggered when the page is loaded. ASP.NET will automatically call the Page_Load subroutine and execute the code in it:
Example
<script runat="server"> Sub Page_Load lbl1.Text="The date and time is " & now() End Sub </script> <!DOCTYPE html> <html> <body> <form runat="server"> <h3><asp:label id="lbl1" runat="server" /></h3> </form> </body> </html>
Run Instance»
Click the "Run Instance" button to view the online instance
Note: Page_Load event does not contain object references or event parameters!
Page.IsPostBack property
The Page_Load subroutine runs every time the page loads. If you only want the code in the Page_Load subroutine to execute when the page is first loaded, you can use the Page.IsPostBack property. If the Page.IsPostBack property is set to false, the page is loaded for the first time, if set to true, the page is posted back to the server (for example, by clicking a button on the form):
Example
<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> <!DOCTYPE html> <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>
Run Example»
Click the "Run Example" button to view the online example
The above example is only on the page The "The date and time is...." message appears when loading for the first time. When the user clicks the Submit button, the submit subroutine will write "Hello World!" in the second label, but the date and time in the first label will not change.
