Private liste As AutoCompleteStringCollection

 liste = New AutoCompleteStringCollection
                    Dim datei As String = Application.StartupPath + "\" + _
                      "Source.txt"
                    If File.Exists(datei) = True Then
                        liste.AddRange(File.ReadAllLines(datei))
                    End If

 With TextBox1
            .AutoCompleteCustomSource = liste
            .AutoCompleteSource = AutoCompleteSource.CustomSource
            .AutoCompleteMode = AutoCompleteMode.SuggestAppend
        End With