k Me button, you should see text appear in the Label control on the page.
What happened? Clicking a CommandButton control always triggers an immediate postback to the server. At that point, the page''''s Load event procedure runs. Then, the btnClickMe_Click event procedure takes its turn, running the code you just added that writes text to the Label control on the page.
The SelectedIndexChanged Event Selecting an item from a DropDownList control triggers that control''''s SelectedIndexChanged event. To test this out, open Events.aspx in Design view and double-click the DropDownList control (ddlProducts) on the page. Modify the event procedure stub so that it looks like this:
Private Sub ddlProducts_SelectedIndexChanged( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles ddlProducts.SelectedIndexChanged lblMessage.Text = "You selected " & _ ddlProducts.SelectedItem.Text End Sub
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The SelectedItem property of the DropDownList control retrieves a ListItem object representing the item you selected from the list. The Text property of this object contains the text of the selected item in the control.
Try running Events.aspx and select an item. Nothing happens. Click the Click Me button, and you''''ll see that the page posts back to the server and that the text is inserted. If you look carefully, you might see the selected item from the DropDownList control before you see the text from the command button.
What''''s up? Selecting an item in a DropDownList control doesn''''t trigger an automatic postback, unlike clicking a command button, which does. When you do click a command button, you trigger a postback, and the page framework runs both event procedures (btnClickMe_Click and ddlProducts_SelectedIndexChanged). The order of event handlers isn''''t clear, but both run.
If you want to trigger an immediate postback when you select an item from the DropDownList control, you''''ll need to set the control''''s AutoPostBack property to be True (it''''s False by default). Try that now. With Events.aspx open in Design view, set the AutoPostBack property of ddlProducts to True.
Try running Events.aspx again, and this time, select an item from the DropDownList control. Now, selecting an item triggers an immediate postback, and the event procedure writes text into the lblMessage control.
The CheckChanged Event A RadioButton control raises its CheckChanged event when you change the "checked" state of the control. Just like the DropDownList control, the RadioButton control doesn''''t trigger an automatic postback unless you set the control''''s AutoPostBack property to True.
In addition, you normally want RadioButton controls to work in a group梩hat is, if you select one control, you''''d like other controls in the same group to be deselected (in other words, the selections in the group are mutually exclusive). In order for this to happen, you must set the GroupName property of associated controls to be the same group name.
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If you simply require a group of radio buttons that work dependently, you might consider using the RadioButtonList control. This control provides a single group of radio buttons, all working together, allowing for a single choice only. Using individual RadioButton controls requires more effort (you must set the GroupName property for each one). What you lose in convenience, you gain in flexibility.
It would be nice if you could share the same CheckChanged event handler for both RadioButton controls, because both controls require similar actions when you select them. In order to make that happen, you''''ll take advantage of the power of the Handles clause. Double-click one of the RadioButton controls, and Visual Studio .NET places you in the CheckChanged event procedure for that control. Modify the procedure so that it looks like Listing 7.1.
Listing 7.1 You Can Use the sender Parameter to Determine Which Control Was Selected Private Sub Sex_CheckedChanged( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles rdoMale.CheckedChanged, rdoFemale.CheckedChanged If sender Is Me.rdoMale Then
lblMessage.Text = _ "You clicked on the Male radio button" Else lblMessage.Text = _ "You clicked on the Female radio button" End If End Sub
Because this procedure lists multiple object.event items in its Handles clause, the page framework will run this event procedure in reaction to the events of either control. Because the code runs for either control, you must use the sender parameter to determine which control raised the event.
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The sample code uses the Is operator to determine which RadioButton control raised the event. This operator compares objects (not values). Because the code is attempting to determine which object raised the event and is comparing one object (sender) to another (the controls), the Is operator is required. (The alternative would be to use the = operator, which compares values, not objects.)
The Page''''s Load Event Each and every time you request a page, that page''''s Load event runs the code you''''ve placed in the Page_Load event procedure. You may have some code that you''''d like to have run only the first time the page is loaded, and other code for subsequent loads. To make that possible, the Page object provides the IsPostBack property梐 Boolean value that indicates whether the page is being loaded because of a postback.
The sample page, Events.aspx, only needs to fill in the drop-down list of products the first time it''''s loaded. To accomplish this goal, choose the View, Code menu item to display the code-behind file for the page and then find the Page_Load procedure, which looks like Listing 7.2.
Listing 7.2 You Can Use the Page_Load Procedure to Preload Values into Controls Private Sub Page_Load( _ ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles MyBase.Load If Not Page.IsPostBack Then With ddlProducts .Items.Add("Apples") .Items.Add("Pears") .Items.Add("Plums") End With End If End Sub
You''''ll see that the code only loads the list (ddlProducts) with values if Page.IsPostBack isn''''t True. You can take advantage of this same mechanism to take different steps, depending on whether your page is posting back to itself.
There are clearly many more controls, many more objects, and many more events to be investigated in ASP.NET. We''''ve selected a small subset to give you the flavor of working with event procedures and controls. When working with any control, investigate its event procedures carefully and take advantage of the event-specific parameters passed to those procedures from the page framework.
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