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Class ListItem from Listing 915 is a simple class with two member variables a string ItemName and an integer ItemValue Properties are de ned to expose these members (Name and Value read-only properties) and the class also has a constructor and overrides the default ToString() member to return the ItemName (by default, the ToString() member of a derived class returns the class name only) The handler for the Add button asks the user to enter a name and value (eg Ten and 10), and then creates a new ListItem initialized to these The item is then added to the ListBox The two CheckBox controls determine how newly added items will be displayed If Display Names is checked, lstDemo DisplayMember is an empty string, indicating that the ToString() member of the ListItem will dictate what is seen When Display Values is checked, radNames_CheckedChanged is red (since its check will be cleared), and lstDemoDisplayMember will be set to Value indicating that the Value property of the ListItem object will be displayed Note that in the above example, the ListItem class is de ned within the Form class It has default (Friend) scope, although it could have Private scope in this case since the class is only used from form code Generally you need a good reason to place one class inside another in this way since we are only able to use the class pre xed with a reference to the form class (frmListsListItem), or not at all if the scope was made Private However, it is a perfectly respectable thing to do for the purpose of this demo, and is a good use of scope where we know that a class will only ever be used in conjunction with (with Friend scope), or wholly within (with Private scope) another class or form ListBox types can also have items added in bulk, which can be useful in a number of ways For example, we can quickly build a combo box to provide the names of the months of the year (Listing 916)
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Private Sub FillCombo() Dim MonthNames() As String = {"January", "February", _ "March", "April", "May", _ "June", "July", "August", _ "September", "October", _ "November", "December"} cboMonthsItemsAddRange(MonthNames) End Sub Listing 916: Filling a list or combo box in a single step
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Note that in Listing 916, the AddRange() method of the Items property is used to assign a whole array of strings to the ComboBox A collection object (eg an ArrayList) can also be used to set the contents of a ListBox type, by setting the Listbox s DataSource property to refer to the collection, as shown in Listing 917
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Private Sub FillComboFromCollection() Dim DayNames As New ArrayList() DayNamesAdd("Monday") DayNamesAdd("Tuesday")
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DayNamesAdd("Wednesday") DayNamesAdd("Thursday") DayNamesAdd("Friday") DayNamesAdd("Saturday") DayNamesAdd("Sunday") cboDaysDataSource = DayNames End Sub Listing 917: An alternative method for lling a List control
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In either case, the text displayed in the list or combo box will depend on either the ToString() method of the objects added, or the ListBox s DisplayMember setting if one is supplied
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952 Accessing the Selected Item(s)
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The purpose of a List control is to allow the user to select an item or text rather than entering one via the keyboard Since errors in typing are eliminated, this makes a List control an ideal way of getting the user to enter information where the information is an item from a range While a ComboBox can only have a single selected item, ListBox and CheckedListBox controls can be con gured via properties so that one or many items can be selected, using the SelectionMode property This can be set to SelectionModeOne (only a single item can be selected), SelectionModeMultiExtended (a range of items can be selected if the user holds down the Shift key on the keyboard after the rst selection clicking on a second item will select all of the items from the rst to the second), or SelectionModeMultiSimple (many individual items can be selected and deselected by clicking on them) For a List control with the SelectionModeOne property setting, a selected item can be accessed using the SelectedItem property If one of the MultiSelectionMode properties is set, the SelectedItems property returns a collection containing all of the selected items To get the single item selected in Figure 97, the following code would suf ce:
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