Visual Basic.NET How to Program (PDF)

Chapter 13 Graphical User Interfaces Concepts: Part 2 543
None and One. One allows multiple selection, because checkboxes imply that there are no
logical restrictions on the items—the user can select as many items as required. Thus, the
only choice is whether to give the user multiple selection or no selection at all. This keeps
the CheckedListBox’s behavior consistent with that of CheckBoxes. The programmer
is unable to set the last two SelectionMode values, MultiSimple and Multi-
Extended, because the only logical two selection modes are handled by None and One.
Common properties and events of CheckedListBoxes appear in Fig. 13.13.
Common Programming Error 13.1
The IDE displays an error message if the programmer attempts to set the SelectionMode
property to MultiSimple or MultiExtended in the Properties window of a
CheckedListBox; If this value is set programmatically, a runtime error occurs. 13.1
Event ItemCheck is generated whenever a user checks or unchecks a
CheckedListBox item. Event argument properties CurrentValue and NewValue
return CheckState values for the current and new state of the item, respectively. A comparison
of these values allows the programmer to determine whether the
CheckedListBox item was checked or unchecked. The CheckedListBox control
retains the SelectedItems and SelectedIndices properties (it inherits them from
class ListBox). However, it also includes properties CheckedItems and Checked-
Indices, which return information about the checked items and indices.
CheckedListBox
properties, methods
and events Description / Delegate and Event Arguments
Common Properties (All the ListBox properties and events are inherited by
CheckedListBox.)
CheckedItems Contains the collection of items that are checked. This is distinct from
the selected item, which is highlighted (but not necessarily checked).
[Note: There can be at most one selected item at any given time.]
CheckedIndices Returns indices for all checked items. This is not the same as the
selected index.
SelectionMode Determines how many items can be checked. Only possible values are
One (allows multiple checks to be placed) or None (does not allow any
checks to be placed).
Common Method
GetItemChecked Takes an index and returns True if the corresponding item is checked.
Common Event (Delegate ItemCheckEventHandler, event arguments Item-
CheckEventArgs)
ItemCheck Generated when an item is checked or unchecked.
ItemCheckEventArgs Properties
CurrentValue Indicates whether the current item is checked or unchecked. Possible
values are Checked, Unchecked and Indeterminate.
Fig. 13.13 CheckedListBox properties, methods and events (part 1 of 2).