Hi Parameswaran,

I have spent about two hours researching on your problem. I found that the
exception is caused in the following situation:

1. At first, I type a char in the derived combobox and the drop down list
appears with some items in it.
2. I type some other chars in the derived combobox and there's no data
matching the text in the combobox so there's no item in the drop down list
now (but the drop down list is still opened at this time).
3. Click somewhere else on the form. The drop down list is to be closed and
an exception occurs "InvalidArgument=Value of '0' is not valid for
'index'.".

That is, if there're items in the drop down list when the drop down list is
opened, but these items are all removed before the drop down list is
closed, an exception would occur when the drop down list is to be closed. I
think this exception is caused by the internal implementation of ComboBox.
After all, changing the items in the drop down list when the drop down list
is opened is not a common usage.

To avoid this problem, I suggest that you don't show the drop down list
when the user typies into the derived combobox, but show the drop down list
when the user presses the Enter key after the input finishes. When the drop
down list is opened, the user could select an item from the drop down list.

The following is a sample.

Public Class Form1

   Dim orads As New DataTable
   Dim binding As New BindingSource

   Private Sub Form1_Load(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.Load
   
       AddHandler MultiColumnCombo1.TextChanged, AddressOf
MultiColumnCombo1_TextChanged
       AddHandler MultiColumnCombo1.KeyDown, AddressOf
MultiColumnCombo1_KeyDown

       Dim col As New DataColumn("PROD_CODE", GetType(Integer))
       orads.Columns.Add(col)
       col = New DataColumn("PROD_DESC", GetType(String))
       orads.Columns.Add(col)

       Dim row As DataRow = orads.NewRow()
       row(0) = 1
       row(1) = "a"
       orads.Rows.Add(row)
       row = orads.NewRow()
       row(0) = 2
       row(1) = "b"
       orads.Rows.Add(row)
       row = orads.NewRow()
       row(0) = 3
       row(1) = "aa"
       orads.Rows.Add(row)
       row = orads.NewRow()
       row(0) = 4
       row(1) = "bb"
       orads.Rows.Add(row)

       binding.DataSource = orads

       Me.MultiColumnCombo1.DataSource = binding
       Me.MultiColumnCombo1.DisplayMember = "PROD_DESC"
       Me.MultiColumnCombo1.ValueMember = "PROD_CODE"
       Me.MultiColumnCombo1.ColumnWidths = "250;80"

   End Sub

   Private Sub MultiColumnCombo1_TextChanged(ByVal sender As Object, ByVal
e As System.EventArgs)

       Me.MultiColumnCombo1.DroppedDown = False
       If (Me.MultiColumnCombo1.Text = "") Then
           Me.binding.Filter = ""
           Me.MultiColumnCombo1.SelectedIndex = -1
       Else
           RemoveHandler MultiColumnCombo1.TextChanged, AddressOf
MultiColumnCombo1_TextChanged
           Dim currenttext As String = Me.MultiColumnCombo1.Text
           Me.binding.Filter = "prod_desc like '%" &
Me.MultiColumnCombo1.Text & "%'"
           Me.MultiColumnCombo1.Text = currenttext
           SendKeys.Send("{END}")
           AddHandler MultiColumnCombo1.TextChanged, AddressOf
MultiColumnCombo1_TextChanged

       End If

   End Sub

   Private Sub MultiColumnCombo1_KeyDown(ByVal sender As Object, ByVal e
As System.Windows.Forms.KeyEventArgs)
       If (e.KeyCode = Keys.Enter) Then
           Me.MultiColumnCombo1.DroppedDown = True
           e.Handled = True
       End If
   End Sub
End Class

Hope this helps.
If you have any question, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support