Class MainWindow

  Sub New()
    ' This call is required by the designer.
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.
    Dim source As New ObservableCollection(Of String)()
    source.Add("AAA")
    source.Add("BBB")
    source.Add("CCC")
    cmbTest.ItemsSource = source
  End Sub

  Private Sub ComboBox_SelectionChanged(sender As System.Object, e As System.Windows.Controls.SelectionChangedEventArgs)
    Keyboard.Focus(cmbTest)
  End Sub

End Class