Imports System.Xml
Imports System.Xml.XPath

Public Class Form1
  Dim sXmlFile As String = "..\..\resources\kanzler.xml"

  Private Sub XPathNavigator_SelectElement(ByVal element As String)
    'Element mit Unterelementen gezielt auslesen
    Dim document As XPathDocument = New XPathDocument(sXmlFile)
    Dim navigator As XPathNavigator = document.CreateNavigator()

    Dim query As XPathExpression = _
        navigator.Compile("/bundeskanzler/kanzler")
    Dim nodes As XPathNodeIterator = navigator.Select(query)
    Dim nodesNavigator As XPathNavigator = nodes.Current
    Dim nodesText As XPathNodeIterator = _
        nodesNavigator.SelectDescendants(XPathNodeType.Text, False)
    Dim i As Integer

    If CheckBox1.Checked = False Then
       ListBox1.Items.Clear()
    End If

    While nodesText.MoveNext()
       If i > 0 AndAlso i < 3 Then
          ListBox1.Items.Add(nodesText.Current.Value)
          i += 1
       End If

       If nodesText.Current.Value = element Then
          ListBox1.Items.Add(nodesText.Current.Value)
          i += 1
       End If
    End While
  End Sub

  Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As _
       System.Object, ByVal e As System.EventArgs) Handles _
       ComboBox1.SelectedIndexChanged
    XPathNavigator_SelectElement(ComboBox1.SelectedItem.ToString)
  End Sub

  Private Sub XPathNavigator_SelectAll()
     'Alle Elemente auslesen
    Dim document As XPathDocument = New XPathDocument(sXmlFile)
    Dim navigator As XPathNavigator = document.CreateNavigator()

    Dim query As XPathExpression = _
        navigator.Compile("/bundeskanzler/kanzler")
    Dim nodes As XPathNodeIterator = navigator.Select(query)
    Dim nodesNavigator As XPathNavigator = nodes.Current
    Dim nodesText As XPathNodeIterator = _
        nodesNavigator.SelectDescendants(XPathNodeType.Text, False)
    ListBox1.Items.Clear()

    While nodesText.MoveNext()
       ListBox1.Items.Add(nodesText.Current.Value)
    End While
  End Sub

  Private Sub Button2_Click(ByVal sender As System.Object, _
       ByVal e As System.EventArgs) Handles Button2.Click
    XPathNavigator_SelectAll()
  End Sub

  Private Sub Form1_Load(ByVal sender As System.Object, _
       ByVal e As System.EventArgs) Handles MyBase.Load
    With ComboBox1
      .Items.Add("Konrad Adenauer")
      .Items.Add("Ludwig Erhard")
      .Items.Add("Kurt Georg Kiesinger")
      .Items.Add("Willy Brandt")
      .Items.Add("Helmut Schmidt")
      .Items.Add("Helmut Kohl")
      .Items.Add("Gerhard Schroeder")
      .Items.Add("Angela Merkel")
      .SelectedIndex = 0
    End With
    Label1.Text = "Lebens- und Regierungszeiten der Bundeskanzler"
  End Sub

  Private Sub Button1_Click(ByVal sender As System.Object,_
       ByVal e As System.EventArgs) Handles Button1.Click
    Me.Close()
  End Sub

  Private Sub Button3_Click(ByVal sender As System.Object, _
       ByVal e As System.EventArgs) Handles Button3.Click
    ListBox1.Items.Clear()
  End Sub

End Class