Programming Microsoft ASP.NET 4 - Dino Esposito [223]
TextAlign
Gets or sets the text alignment for the check boxes
The CheckBoxList does not supply any properties that know which items have been selected. But this aspect is vital for any Web application that uses selectable elements. The CheckBoxList can have any number of items selected, but how can you retrieve them?
Any list control has an Items property that contains the collection of the child items. The Items property is implemented through the ListItemCollection class and makes each contained item accessible via a ListItem object. The following code loops through the items stored in a CheckBoxList control and checks the Selected property of each of them:
foreach (var item in chkList.Items)
{
if (item.Selected) {
// This item is selected
var itemValue = item.Value;
...
}
}
Figure 10-3 shows a sample page that lets you select some country/region names and composes an ad hoc query to list all the customers from those countries/regions.
Figure 10-3. A horizontally laid out CheckBoxList control in action.
Note that the SelectedXXX properties work in a slightly different manner for a CheckBoxList control. The SelectedIndex property indicates the lowest index of a selected item. By setting SelectedIndex to a given value, you state that no items with a lower index should be selected any longer. As a result, the control automatically deselects all items with an index lower than the new value of SelectedIndex. Likewise, SelectedItem returns the first selected item, and SelectedValue returns the value of the first selected item.
The RadioButtonList Control
The RadioButtonList control acts as the parent control for a collection of radio buttons. Each of the child items is rendered through a RadioButton control. By design, a RadioButtonList can have zero or one item selected. The SelectedItem property returns the selected element as a ListItem object. Note, though, that there is nothing to guarantee that only one item is selected at any time. For this reason, be extremely careful when you access the SelectedItem of a RadioButtonList control—it could be null.
if (radioButtons.SelectedValue != null)
{
// Process the selection here
...
}
The control supports the same set of properties as the CheckBoxList control and, just like it, accepts some layout directives. In particular, you can control the rendering process of the list with the RepeatLayout and RepeatDirection properties. By default, the list items are rendered within a table, which ensures the vertical alignment of the companion text. The property that governs the layout is RepeatLayout. The alternative is to display the items as free HTML text, using blanks and breaks to guarantee some sort of minimal structure. RepeatDirection is the property that controls the direction in which, with or without a tabular structure, the items flow. Feasible values are Vertical (the default) and Horizontal. RepeatColumns is the property that determines how many columns the list should have. By default, the value is 0, which means all the items will be displayed in a single row, vertical or horizontal, according to the value of RepeatDirection.
The ListBox Control
The ListBox control represents a vertical sequence of items displayed in a scrollable window. The ListBox control allows single-item or multiple-item selection and exposes its contents through the usual Items collection, as shown in the following code:
You can decide the height of the control through the Rows property. The height is measured in number of rows rather than pixels or percentages. When it comes to data binding, the ListBox control behaves like the controls discussed earlier in the chapter. Two properties make this control slightly different than other list controls—the Rows property, which represents the number of visible rows in the control, and the SelectionMode property, which determines whether one or multiple items can be selected. The programming interface