Multi-Column ComboBox

• Download Source Code - 32.9 Kb

Introduction

After submersing myself into the Internet in the endless search of a MultiColumn ComboBox, and trying for 2 years to develop one (in VB6), I have finally got something half-way decent. Some thanks goes out to Carlos H. Perez whose code has shown me how to do some very interesting things! There is not one routine though that has been "copied" from any Project out there, everything has been re-written (mainly because I wanted to learn) and optimized as best I knew how.

Background

The Multi-Column ComboBox is an Owner Drawn control derived from the System.Windows.Forms.ComboBox. However, the control does not use the standard "Items" array within the normal ComboBox. This is where this control excels. Since I was writing my own Control Suite, I needed a way for every Control that had a need for a Collection of Items to be universal among all Controls. Also, I hated Microsoft for taking away Keys from the Collection Classes...so I wrote my own using ideas from the CollectionGen utility. The Collections used within the ComboBox (and also the ListView shown above) are Strongly-Typed and very fast, this enables a fast lookup on the Control using either the Index or the Key of any Item.

One of the harder things to figure out was the HorizontalExtent of the drop-down List. When Items exceed the Horizontal Extent, the Horizontal Scrollbar should appear. I did not find anything anywhere on how to do this, so I did some hacking and came up with a very easy method using the CreateParams property of the control. By setting the WS_HSCROLL style bit, it gives the Control the ability (not the functionality) to have a Horizontal Scrollbar. The tough part is determining how much of a HorizontalExtent to give the drop-down list! I was able to set the HExtent by Overriding the OnDropDown Event of the ComboBox.

Calculating the Horizontal Extent

The Control needed an auto-magic way of Calculating the Horizontal Extent. Since I hate relying on Users to do the "right thing" (or other programmers in this case), I decided to make the Control "smart" enough to handle things where someone leaves off. When adding a ListColumn object to the Control (the Control NEEDS at least 1 ListColumn in order to Display data), if the programmer omits the "Width" Property, the Width will remain at "-1". By leaving the Column with a Width of -1, that Column's Width will be auto-magically calculated based on the Widest Text in that particular Column. If the Column is set to "Visible = False", then it's width will not be added into the Extent.

Using the code

The Control uses a very simple Object Model. It basically works the same way as the ListView does in "Details" mode. First, the ListColumns are added, then the ListItems are added. The ListItem Object supports a ListSubItems Collection which is used to access the other Columns within the Control (again, exactly like the ListView).

'Load the Form
Private Sub FormLoad(ByVal sender As Object, 
    ByVal e As System.EventArgs) Handles MyBase.Load
  With cboBase
    .ListColumns.Add("State ID") 'The Column's Width 
                                 'will be Automatically Calculated
    .ListColumns.Add("State Name") 
				
    For i As Integer = 0 To 19
      .ListItems.Add("Item" & i.ToString, "Item " & i.ToString)
      .ListItems(i).ListSubItems.Add("Item" & i.ToString & ":SubItem 0")
    Next i
  End With
End Sub

Points of Interest

Unfortunately, this control does not support DataBinding.  There is one thing that may be very Interesting though.  The Control has a Special Event "FocusChanging".  I needed an Event in one of my interfaces that told me which Direction the User was Tabbing through the Controls.  This allowed me to control which Tab got focus on a TabbedDialog control so that Data Entry would be a lot easier.  When the Event is Raised, it tells which Direction the User has Tabbed.  If you want to Handle that Event, and set Focus to another Control, simply tell the EventArgs that you Handled it (i.e.

e.Handled =
					True

Private Sub cboBox3_FocusChanging(ByVal Sender As Object, 
       ByVal e As CodeSamples.Controls.FocusChangeEventArgs) 
       Handles cboBox3.FocusChanging
  If (e.Direction = CodeSamples.Controls.FocusDirection.Forward) Then
    e.Handled = True
    cboBox1.Focus()
  ElseIf (e.Direction = CodeSamples.Controls.FocusDirection.Backward) Then
    e.Handled = True
    cboBox4.Focus()
  End If
End Sub		

The other Point of Interest, is that the Control fully supports AutoTyping!  It will only allow Items to be Selected that are in the List.  The Good news is that in VS.NET 2003, Microsoft has FINALLY got the "e.Handled" Property of the "KeyPressEvents" working!  This makes it so that the AutoTyping feature does not flicker anymore!  I have also tried in the past to be able to set a Property so the Developer could choose which Column was Displayed in the Text area.  However, due to some weird functionality of the CombBox (and Inheriting from it), this will have to come when I re-write the entire ComboBox from scratch LOL!

History

• Fixed the Horizontal Extent not display correctly (make sure you have Installed the GDI fix from Microsoft!)
• GDI fix for Win2k (Windows 2000 Patch: A Lock Occurs Between Two Threads of System GDI in Windows 2000)
• GDI fix for WinXP (Windows XP Patch: GDI+ Graphics Rendering Issues)
• Autotyping feature enhanced.