ASP .net material Part-3
ASP.NET - Events
An Event Handler is a subroutine that executes code for a given event.
ASP.NET - Event Handlers
Look at 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 code above be executed? The answer is: "You don't know..."
The Page_Load Event
The Page_Load event is one of many events that ASP.NET understands. The Page_Load event is triggered when a page loads, and ASP.NET will automatically call the subroutine Page_Load, and execute the code inside it:
Example
<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>
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 object references or event arguments!
The Page.IsPostBack Property
The Page_Load subroutine runs EVERY time the page is loaded. If you want to execute the code in the Page_Load subroutine only the FIRST time the page is loaded, you can use the Page.IsPostBack property. 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 for
Example
| <script runat="server"> The example above will write the "The date and time is...." message only the first time the page is loaded. 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. 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> ASP.NET Web FormsAll server controls must appear within a <form> tag, and the <form> tag must contain the runat="server" attribute. ASP.NET Web FormsAll server controls must appear within a <form> tag, and the <form> tag must contain the runat="server" attribute. The runat="server" attribute indicates that the form should be processed on the server. It also indicates that the enclosed controls can be accessed by server scripts:
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Note: An .aspx page can only contain ONE <form runat="server"> control!
If you select view source in an .aspx page containing a form with no name, method, action, or id attribute specified, you will see that ASP.NET has added these attributes to the form. It looks something like this:
| <form name="_ctl0" method="post" action="page.aspx" id="_ctl0"> ...some code </form> Submitting a FormA form is most often submitted by clicking on a button. The Button server control in ASP.NET has the following format:
In the following example we declare a Button control in an .aspx file. A button click runs a subroutine which changes the text on the button: ASP .NET Maintaining the ViewStateYou may save a lot of coding by maintaining the ViewState of the objects in your Web Form. Maintaining the ViewStateWhen a form is submitted in classic ASP, all form values are cleared. Suppose you have submitted a form with a lot of information and the server comes back with an error. You will have to go back to the form and correct the information. You click the back button, and what happens.......ALL form values are CLEARED, and you will have to start all over again! The site did not maintain your ViewState.When a form is submitted in ASP .NET, the form reappears in the browser window together with all form values. How come? This is because ASP .NET maintains your ViewState. The ViewState indicates the status of the page when submitted to the server. The status is defined through a hidden field placed on each page with a <form runat="server"> control. The source could look something like this:
Add a ScriptThe contents and settings of a TextBox control may be changed by server scripts when a form is submitted. A form can be submitted by clicking on a button or when a user changes the value in the TextBox control.In the following example we declare one TextBox control, one Button control, and one Label control in an .aspx file. When the submit button is triggered, the submit subroutine is executed. The submit subroutine writes a text to the Label control:
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Example
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ASP.NET - The Button Control
The Button control is used to display a push button.
The Button Control
The Button control is used to display a push button. The push button may be a submit button or a command button. By default, this control is a submit button.
A submit button does not have a command name and it posts the page back to the server when it is clicked. It is possible to write an event handler to control the actions performed when the submit button is clicked.
A command button has a command name and allows you to create multiple Button controls on a page. It is possible to write an event handler to control the actions performed when the command button is clicked.
The Button control's attributes and properties are listed in our web controls reference page.
The example below demonstrates a simple Button control:
| <html> <body> <form runat="server"> <asp:Button id="b1" Text="Submit" runat="server" /> </form> </body> </html> |
Add a Script
A form is most often submitted by clicking on a button.
In the following example we declare one TextBox control, one Button control, and one Label control in an .aspx file. When the submit button is triggered, the submit subroutine is executed. The submit subroutine writes a text to the Label control:
Example
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