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Closely associated with a list box is the combo box, which is a single control combining a text box and a list box. The list box component behaves as any other list box (see below). The text box component can be used either to enter an input string or to display an output string (e.g., a label or a heading for the list box), as described in Sec. 4.7. Initial list entries can be entered as strings in the same manner as other control properties. (Press Ctrl-Enter at the end of each list entry, in order to drop down to the next line.) In addition, list entries can be changed (i.e., reassigned) or added during program execution using the AddItem method or the List function; e.g.,
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List1.AddItem("Red") List1.AddItem("White") List1.AddItem("Blue")
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List1.List(0) = "Red" List1.List(1) = "White" List1.List(2) = "Blue"
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etc. Typically, these list modification instructions will appear within a Form_Load() event procedure. The ListCount and ListIndex properties are also useful in many situations. ListCount represents the number of entries within the list (beginning with 1, not 0). It is often used as a stopping condition for a looping structure; e.g.,
For Count = 0 To (ListCount 1) . . . . . Next Count ListIndex represents the index value of the most recently selected list entry (The value corresponding to the first entry is 0, not 1). The use of ListIndex often provides a convenient expression for a Select Case structure; e.g., Select Case ListIndex Case 0 . . . . . Case 1 . . . . . . . . . . End Select 'First entry
'Second entry
'Last entry
Neither ListCount nor ListIndex can be assigned an initial value. Hence, neither property appears in the Properties Window during the initial program design.
EXAMPLE 4.9 SELECTING FROM A LIST (MULTILINGUAL HELLO REVISITED)
Let us return to the situation described in Example 4.6, in which we display the greeting Hello in one of several different languages. In this example, we will use a combo box to display the choice of languages, and a text box to display the appropriate greeting. (There are other ways to accomplish the same thing; for example, a text box to display the choice of languages, and a label to display the greeting.) Since the selection will be made from a combo box (or from a text box, if we had so chosen), only one greeting will be displayed at any one time. Fig. 4.28 shows the preliminary Form Design Window layout.
VISUAL BASIC CONTROL FUNDAMENTALS
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Fig. 4.28
Fig. 4.29
Once the preliminary layout has been completed, we assign the following initial values to the properties: Object Form1 Label1 Combo1 Property Caption Caption Font Text List Value Multilingual Hello 2 Say Hello, in . . . MS Sans Serif, 10-point Language . . . French (press Control-Enter after each list entry) German Hawaiian Hebrew Italian Japanese Spanish MS Sans Serif, 10-point
Font
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Object Text1
Property Text BackColor BorderStyle Font Caption Font
Value (blank) Gray 0 None MS Sans Serif, 14-point Quit MS Sans Serif, 10-point
Command1
Fig. 4.29 shows the appearance of the Form Design Window after assigning these property values. In order to complete this project, we must associate an event procedures with a combo box click event, and another event procedure with the command button. For the combo box click event, we will use a Select Case structure, based upon the value returned by the ListIndex property when the user clicks on one of the combo box list entries. (We could also have used a series of If-Then-Else structures instead.) The command button event procedure is very simple, consisting only of the End command inserted between the opening Sub and the closing End Sub statements. The two event procedures are shown below.
Private Sub Combo1_Click() Select Case Combo1.ListIndex Case 0 Text1.Text = "Bonjour" Case 1 Text1.Text = "Guten Tag" Case 2 Text1.Text = "Aloha" Case 3 Text1.Text = "Shalom" Case 4 Text1.Text = "Buon Giorno" Case 5 Text1.Text = "Konichihua" Case 6 Text1.Text = "Buenos Dias" End Select End Sub Private Sub Command1_Click() End End Sub
When the program is executed, the combo box shows the title Language . . . in the text-box area. The list of languages can be seen in the drop-down window by clicking on the adjacent down-arrow (see Fig. 4.30). When the user clicks on one of these entries, the corresponding greeting appears within the text box, to the right of the drop-down window. Fig. 4.31 shows what happens when the user selects German from the list in the drop-down window. Thus, we see that the hello greeting in German is Guten Tag.
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