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Welcome to the fifteenth chapter of the FREE online programming course:
A Beginner's Guide to Asp.Net Programming for Delphi developers.
ASP.NET selection controls allow users to select from a series of predefined values. This chapter explores list-type controls: CheckBox, CheckBoxList, RadioButton, RadioButtonList, DropDownList and ListBox from the perspective of a Delphi ASP.NET web developer.
In the Introduction to Web Server Controls chapter, we've divided ASP.NET Web Server controls into several categories.
We continue our exploration of the System.Web.UI.WebControls namespace by investigating a variety of different controls that allow selection.
Throughout this chapter, you'll learn about the following:
How to enable users to make selections on a Web Form
Using RadioButtons and CheckBox
Using RadioButtonsList and CheckBoxList
Using ListBox and DropDownList
Selections in Web Forms
Much similar to Win32 applications, a Web application must enable a user to select a value from a predefined set. Starting with individual controls, we have the CheckBox (in Win32: TCheckBox), RadioButton (in Win32: TRadioButton), and then we have group versions of the same controls: the CheckBoxList (in Win32: CheckListBox) and RadioButtonList (in Win32 TRadioGroup).
There are also the ListBox (in Win32: TListBox) and DropDownList (in Win32: TComboBox with Style := csDropDownList).
Many of these controls share the same set of features - though each offers a slightly different functionality and look.
CheckBox
Much similar to the (Win32) TCheckBox Delphi VCL component, the CheckBox allows Boolean (true/false) selection by the user - user can check the box to select the option, or uncheck it to deselect the option. The Checked property specifies whether the check box is checked.
The most important event for the CheckBox control is CheckedChanged. Much similar to the TextBox's TextChanged property, after a postback, if the value (Checked) of the control has changed, the CheckedChanged event is raised.
When you drop a CheckBox control on a Web Form, the generated HTML in the "aspx" view will look like:
<asp:checkbox id=CheckBox1
runat="server">
</asp:checkbox>
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The CheckBox control renders as an HTML <input type="checkbox"> element to the browser.
A CheckBox can have a label (Text property determines the text displayed) that renders "near" the CheckBox depending on the TextAlign property.
RadioButton
Again, similar to the TRadioButton, the RadioButton ASP.NET control can operate in groups. OptionButton's GroupName property determines the group name. Several option buttons sharing the same group are mutually exclusive - meaning that only one choice can be selected.
When you drop a RadioButton control on a Web Form, the generated HTML in the "aspx" view will look like:
<asp:radiobutton id=RadioButton1
runat="server">
</asp:radiobutton>
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The RadioButton control renders as an HTML <input type="radio"> element to the browser.
An example
Here's one funny example to understand how CheckBox and RadioButton web control operate.
Drop 3 radio buttons on a web form, and one Check Box. Set the AutoPostback property of the check box to true, and handle the CheckedChanged event:
procedure TWebForm5.CheckBox1_CheckedChanged
(sender: System.Object; e: System.EventArgs);
begin
if CheckBox1.Checked then
begin
RadioButton1.GroupName := 'GroupA';
RadioButton2.GroupName := 'GroupA';
RadioButton3.GroupName := 'GroupA';
end
else
begin
RadioButton1.GroupName := '';
RadioButton2.GroupName := '';
RadioButton3.GroupName := '';
end;
end; //CheckBox1_CheckedChanged
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When the check box is on, only one option button can be selected - all three option buttons share the same Group.
When the Checked property oh the check box is false ("not checked") you can select all three option buttons (since the code clears the GroupName property).
Next page > RadioButtonList, CheckBoxList > Page 1, 2, 3
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