'Die beiden Comboboxen
Dim WithEvents cboKat As New ComboBox With {.Parent = Me, .Left = 400}
Dim cboUnterKat As New ComboBox With {.Parent = Me, .Left = 400, .Top = 50}
 
'Tabelle mit Daten aus der Datenbank
Dim dt As New DataTable
Dim bs As New BindingSource
 
 
'Im Load-Event der Form
 
'Kategorien ermitteln (als Liste)
Dim klst As List(Of String) = _
           (From row As Object In dt.Rows _
            Select DirectCast(row, DataRow).Field(Of String)("Kategorie") _
            Distinct).ToList
 
cboKat.DataSource = klst
 
'Unterkategorie-Spalte an Combo binden
bs.DataSource = dt
cboUnterKat.DisplayMember = "Unterkategorie"
cboUnterKat.DataSource = bs
 
 
'Und dann ....
Private Sub cboKat_SelectedValueChanged(sender As Object, _
        e As System.EventArgs) Handles cboKat.SelectedValueChanged
 
    If bs.Count = 0 Then Exit Sub
 
    Dim filterstring As String = "Kategorie = '" & cboKat.Text & "'"
    bs.Filter = filterstring 'Filtersting gemäß Kategorienauswahl setzen
End Sub