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Tip of the Day
Language: Visual Basic
Expertise: Intermediate
Mar 12, 1999

Extend Intrinsics in UserControl Wrappers

Sometimes you might want to create autoselecting combo boxes, like those in Intuit Quicken or Microsoft Access, where items in the list are selected as you type. Typically, techniques to do this require code in the KeyPress and KeyDown events, which selects the items from the list. I'll show you how to create a more intuitive user-interface design than using a standard combo box.

Instead of repeating this code (or calls to a generic function) in the KeyPress event of every combo box in every project, create your own combo box control with this functionality built in. Create a new UserControl, add a combo box, and use the ActiveX Control Interface Wizard to create Property Let and Get procedures to set and retrieve the combo box's standard properties. Add this code to make sure the combo box always sizes to the same size as the control:

 
Private Sub UserControl_Resize()
	cbo.Move 0, 0, ScaleWidth
	'  The Combo Box height determines the height
	'  of the control
	UserControl.Height = Screen.TwipsPerPixelY * cbo.Height  
End Sub
Add an AutoSelect property as a Boolean value, so the developer can decide when to use the autoselecting functionality. The Property Let procedure sets the value of the module-level Boolean variable mfAutoSelect. Check the value of mfAutoSelect in the KeyPress event:
 
Private Sub cbo_KeyPress (KeyAscii as Integer)
	If mfAutoSelect Then
		'  Call generic function to select the first matching 
		'  value in the list.
		Call ComboBox_AutoSelect(cbo, KeyAscii)
	End if
End Sub
You should also make a number of simple but useful improvements to the standard combo box control. First, add a variant ItemData property, create a LimitToList property similar to combo boxes in Access, and select the text in the GotFocus event. If you set the Appearance property to Flat, the combo box still appears in 3-D, so work around this bug. By wrapping this code in an ActiveX control, you reduce the code in your applications.
Craig Randall
 
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