ASP.NET Web Forms - Events
An event handler is a subroutine that executes code for a given event.
When a related event is triggered in ASP.NET, the event's subroutine will be called. Please see below for details.
Please look at the code 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>
When will the above code be executed? The answer is: "I don't know...".
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:
<script runat="server">Sub Page_Loadlbl1.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!
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):
<script runat="server">Sub Page_Loadif Not Page.IsPostBack thenlbl1.Text="The date and time is " & now()end ifEnd SubSub 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 above example only displays the "The date and time is...." message the first time the page loads. 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.
The above is an explanation of the use of ASP.NET event handlers.