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Combo Boxes

Combo boxes work much like list boxes except that the user can add items to a combo box at runtime, whereas the user can only scroll and select items from a list box at runtime. Visual Basic supports three kinds of combo boxes, and the kind you select depends on the combo box you want to display on the form and on the ability you want the user to have. All the list box methods that you learned about in the previous section apply to combo boxes.

Here are the three kinds of combo boxes:

Drop-down combo box—Takes up only a single line on the form unless the user opens the combo box (by pressing the combo box's down arrow) to see additional values. The user can enter additional items at the top of the drop-down combo box and select items from the combo box.
Simple combo box—Displays items as if they were in a list box. The user can add items to the combo box list (whereas the user cannot with a normal list box).
Drop-down list box—Doesn't let the user enter new items, so it's similar to a list box. Unlike a list box, however, the drop-down list box normally appears closed to a single line until the user clicks the down arrow button to open the list box to its full size. Technically, drop-down list boxes are not combo box controls but work more like list boxes. The reason drop-down list boxes fall in the combo box control family is that you place drop-down list boxes on forms by clicking the combo box control and setting the Style combo box property.

Figure 10.5 shows the three kinds of combo boxes. Each combo box contains the names of people that you saw in Figure 10.4. The first combo box, the drop-down combo box, is normally closed; when the user clicks its down arrow, the combo box opens. The third combo box, the drop-down list box, is left unopened. If the user opens the drop-down list box, the user will see a list of people's names but will not be able to add to the names because no data entry is possible in drop-down list boxes.

Figure 10.5 Use Style to change the combo box appearance.

Table 10.3 describes some of the combo box properties.

Table 10.3. The fundamental combo box properties.

Property	Description
`BackColor`	The combo box's background color.
`ForeColor`	The combo box's foreground text color.
`Height`	The height, in twips, of the closed combo box.
`IntegralHeight`	Determines whether the combo box can display partial items, such as the upper half of an item that falls toward the bottom of the combo box.
`List`	A drop-down property list box where you can enter values into the combo box at design time. You can enter only one at a time, and most programmers prefer to initialize the combo box at runtime.
`Sorted`	Determines whether the combo box values are automatically sorted. If `False` (the default value), the values appear in the same order in which the program added the items to the combo box.
`Style`	Determines the type of combo box your application needs. If `0-DropDown Combo`, the combo box is a drop-down combo box. If `1-Simple Combo`, the combo box turns into a simple combo box that remains open to the height you used at design time. If `2-DropDown List`, the combo box turns into a drop-down list box that remains closed until the user is ready to see more of the list.

CAUTION

The user's entered value doesn't add to the Drop-down Combo Box control or to the Simple Combo Box control unless you add the capability to capture the user's entry. The combo box by itself, without code, cannot handle the addition of items automatically. You need to write, in the combo's Change or LostFocus event procedure, enough code to add the new item (that always appears in the combo's Text property) to the combo box list, like this:

cboBox.AddItem cboBox.Text  ' Adds user's value to the box

You can also add a command button that adds the combo box value entered by the user if the user clicks the command button.

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