C# 4 and .NET 4

standard Controls and Components ❘ 1127
The Appearance property takes an Appearance enumeration value. This can be either Button or Normal.
When you choose Normal, the radio button looks like a small circle with a label beside it. Selecting the
button fills the circle; selecting another button deselects the currently selected button and makes the
circle look empty. When you choose Button, the control looks like a standard button, but it works like a
toggle — selected is the in position, and deselected is the normal, or out, position.
The CheckedAlign property determines where the circle is in relation to the label text. It could be on top of
the label, on either side, or below it.
The CheckedChanged event is raised whenever the value of the Checked property changes. This way, you
can perform other actions based on the new value of the control.
Combobox, listbox, and Checkedlistbox
ComboBox, ListBox, and CheckedListBox are all derived from the ListControl class. This class provides
some of the basic list management functionality. The most important aspects of using list controls are
adding data to and selecting data from the list. Which list is used is generally determined by how the list is
used and the type of data that is going to be in the list. If there is a need to have multiple selections or if the
user needs to be able to see several items in the list at any time, the ListBox or CheckedListBox is the best
choice. If only a single item is ever selected in the list at any time, a ComboBox may be a good choice.
Data must be added to a list box before it can be useful. You add data by adding objects to the ListBox
.ObjectCollection. This collection is exposed by the list’s Items property. Because the collection
stores objects, any valid .NET type can be added to the list. In order to identify the items, two important
properties need to be set. The first is the DisplayMember property. This setting tells the ListControl what
property of your object should be displayed in the list. The other is ValueMember, which is the property of
your object that you want to return as the value. If strings have been added to the list, by default the string
value is used for both of these properties. The ListExample form in the sample application shows how data
can be loaded into a list box. The example uses Vendor objects for the list data. The Vendor object contains
just two properties: Name and Id. The DisplayMember property is set to the Name property. This tells the list
control to display the value from the Name property in the list to the user.
Once the data is loaded in the list, the SelectedItem and SelectedIndex properties can be used to get at
the data. The SelectedItem property returns the object that is currently selected. If the list is set to allow
multiple selections, there is no guarantee which of the selected items will be returned. In this case, the
SelectedItems collection should be used. This contains a list of all of the currently selected items in the list.
If the item at a specific index is needed, the Items property can be used to access the ListBox
.ObjectCollection. Because this is a standard .NET collection class, the items in the collection can be
accessed in the same way as any other collection class.
If DataBinding is used to populate the list, the SelectedValue property will return the property value of the
selected object that was set to the ValueMember property. If Id is set to ValueMember, the SelectedValue
will return the Id value from the selected item. In order to use ValueMember and SelectedValue the list
must be loaded by way of the DataSource property. An ArrayList or any other IList-based collection must
be loaded with the objects first, and then the list can be assigned to the DataSource property.
The Items property of the ComboBox returns ComboBox.ObjectCollection. A ComboBox is a combination
of an edit control and a list box. You set the style of the ComboBox by passing a DropDownStyle enumeration
value to the DropDownStyle property. The following table lists the various DropDownStyle values.
Value
DropDown
DropDownList
Simple
desCriPTion
The text portion of the combo box is editable, and users can enter a value. They can
also click the arrow button to show the drop-down list of values.
The text portion is not editable. Users must make a selection from the list.
This is similar to DropDown except that the list is always visible.
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