Public Class Form1
  Inherits System.Windows.Forms.Form

Vom Windows Form Designer generierter Code

  Private Function IsDrin(ByVal lst As ListBox, _
        ByVal newitem As String) As Boolean
    Dim item As String
    Dim i As Integer
    For Each item In lst.Items
      If item = newitem Then
        lst.SelectedIndex = i
        Return True
        Exit For
      End If
      i += 1
    Next
    Return False
  End Function

  Private Function IsDrin(ByVal cbo As ComboBox, _
        ByVal newitem As String) As Boolean
    Dim item As String
    Dim i As Integer
    For Each item In cbo.Items
      If item = newitem Then
        cbo.SelectedIndex = i
        Return True
        Exit For
      End If
      i += 1
    Next
    Return False
  End Function

  Private Sub Button1_Click(ByVal sender As System.Object, _
          ByVal e As System.EventArgs) Handles Button1.Click
    If Not IsDrin(ListBox1, TextBox1.Text) Then
      ListBox1.Items.Add(TextBox1.Text)
      Label2.ForeColor = Color.Blue
      Label2.Text = "Neuen Eintrag eingefügt!"
      ListBox1.SelectedIndex = ListBox1.Items.Count - 1
    Else
      Label2.ForeColor = Color.Red
      Label2.Text = "Eintrag schon vorhanden!"
    End If
  End Sub

  Private Sub Button2_Click(ByVal sender As System.Object, _
          ByVal e As System.EventArgs) Handles Button2.Click
    If Not IsDrin(ComboBox1, TextBox1.Text) Then
      ComboBox1.Items.Add(TextBox1.Text)
      Label2.ForeColor = Color.Blue
      Label2.Text = "Neuen Eintrag eingefügt!"
      ComboBox1.SelectedIndex = ComboBox1.Items.Count - 1
    Else
      Label2.ForeColor = Color.Red
      Label2.Text = "Eintrag schon vorhanden!"
    End If
  End Sub
End Class