Public Class StringParts
 
    Dim _stb As New System.Text.StringBuilder
    Dim _upperbounds() As Integer
    Dim _lowerbounds() As Integer
    Dim _lengths() As Integer
 
 
    Public Sub New(ParamArray lengths() As Integer)
 
        Array.Resize(_lowerbounds, lengths.Length)
        Array.Resize(_lengths, lengths.Length)
        Array.Resize(_upperbounds, lengths.Length)
 
        _lowerbounds(0) = 0
        _upperbounds(0) = lengths(0) - 1
        _lengths(0) = lengths(0)
 
 
        For i As Integer = 1 To lengths.Length - 1
            If lengths(i) < 1 Or lengths(i - 1) < 1 Then
                Throw New ArgumentException
            End If
            _lowerbounds(i) = _upperbounds(i - 1) + 1
            _upperbounds(i) = _upperbounds(i - 1) + lengths(i)
            _lengths(i) = lengths(i)
        Next i
 
        _stb = New System.Text.StringBuilder _
            (New String(" "c, _upperbounds _
             (_upperbounds.GetUpperBound(0)) + 1))
    End Sub
 
 
    Default Public Property Part(ByVal index As Integer) As String
 
        Get
            Return _stb.ToString(_lowerbounds(index), _lengths(index))
        End Get
 
        Set(value As String)
            Dim sb As New System.Text.StringBuilder(value)
            sb.Length = _lengths(index)
            _stb.Remove(_lowerbounds(index), _lengths(index))
            _stb.Insert(_lowerbounds(index), sb)
        End Set
 
    End Property
 
 
    Public ReadOnly Property Total() As String
        Get
            Return _stb.ToString
        End Get
    End Property
 
End Class

'Im Konstruktor 
'Zahl der Elemente und Länge der Elemente angeben
Dim stp As New StringParts(2, 3, 4)
 
'Elemente belegen (automatische Längenanpassung)
stp(0) = "AA"
stp(1) = "BBB"
stp(2) = "CCCC"
 
Dim str As String = stp.Total 'AABBBCCCC
 
stp(1) = "XXXXX" 'Überlänge wird abgeschnitten!
 
str = stp(1) 'XXX
str = stp(2) 'CCCC
 
str = stp.Total 'AAXXXCCCC