Tip 202: Forcing a Combo Box Control to Drop Down with the ENTER Key

February 28, 1996

Abstract
This article explains how to force a Combo Box control to drop down its
list box when the ENTER key is pressed in a Microsoft(r) Visual Basic(r)
application.

Monitoring the KeyPress Event
The Microsoft(r) Visual Basic(r) Combo Box control lets your user select
from
its list box or optionally add a new item to the list box. A Combo Box
control has three possible stylesDropdown Combo, Simple Combo, and
Dropdown List.

The Dropdown Combo style of the Combo Box control is displayed with the
arrow and text box portion of the control slightly separated. You must
click the down arrow to select items from the Combo Box control.

The Simple Combo style of the Combo Box control displays its list box
immediately. The disadvantage of this style is that it takes up more
space in your window.

The Dropdown List style of the Combo Box control is almost identical to
the Dropdown Combo style. However, this style does not allow you to modify
the Text property of this control.

Note that most applications use a Combo Box control with its Style
property set to Dropdown Combo. Instead of clicking on the arrow to
cause the Combo Box control to display its list box, it is more
convenient to press the ENTER key.

Each time a key is pressed, the Combo Box control's KeyPress event is
triggered (providing that the Combo Box control has the focus, of course).
By monitoring this event, you can determine whether a user pressed ENTER.
You know the user pressed ENTER if the KeyAscii value is 13.

After you have determined that ENTER was indeed pressed, you send a
CB_SHOWDROPDOWN message to the Combo Box control by using the Microsoft
Windows(r) application programming interface (API) SendMessage function.
If this message is sent to the control with a parameter of True, the
Combo Box control drops down its list box. If this message is sent to
the Combo Box control with a parameter of False, however, the list box
would not be dropped down.

Example Program
This program shows how to force a Combo Box control to drop down when the
ENTER key is pressed.

 1. Create a new project in Visual Basic. Form1 is created by default.
 2. Add the following Constant and Declare statements to the General
    Declarations section of Form1 (note that the Declare statement must
    be typed as a single line of code):

Const WM_USER = &H400
Const CB_SHOWDROPDOWN = &H14F

Private Declare Function SendMessage Lib "user32" Alias "SendMessageA"
   (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As
   Long) As Long

 3. Add the following code to the Form_Load event for Form1:

Private Sub Form_Load()
    Combo1.AddItem "Oranges"
    Combo1.AddItem "Peaches"
    Combo1.AddItem "Apples"
End Sub

 4. Add a Combo Box control to Form1. Combo1 is created by default.
 5. Add the following code to the KeyPress event for Combo1:

Private Sub Combo1_KeyPress(KeyAscii As Integer)
    Dim I As Long
    If KeyAscii = 13 Then
        I = SendMessage(Combo1.hwnd, CB_SHOWDROPDOWN, True, 0&)
    End If
End Sub

Run the example program by pressing F5. While the Combo Box control has
the focus, press ENTER. The Combo Box control displays its list box.