Public Class TeilnehmerEnumerator
    Implements IEnumerator(Of Teilnehmer)
 
    Dim _binaryreader As IO.BinaryReader = Nothing
    Public Sub New(ByVal binaryreader As IO.BinaryReader)
        _binaryreader = binaryreader
        _binaryreader.BaseStream.Position = 0
    End Sub
 
    Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
        Return _binaryreader.BaseStream.Position < _
          _binaryreader.BaseStream.Length
    End Function
    Public Sub Reset() Implements IEnumerator.Reset
        _binaryreader.BaseStream.Position = 0
    End Sub
 
    Public ReadOnly Property Current1 As Teilnehmer _
        Implements System.Collections.Generic.IEnumerator(Of Teilnehmer).Current
        Get
            Try
                'muss mit den Write-Anweisungen korrespondieren 
                Dim el As New Teilnehmer
                With el
                    .name = _binaryreader.ReadString
                    .matrnr = _binaryreader.ReadInt32
                    .Fachrichtung = _binaryreader.ReadString
                    Dim nl As String = _binaryreader.ReadString()
                End With
                Return el
            Catch ex As IndexOutOfRangeException
                Throw New InvalidOperationException()
            End Try
        End Get
    End Property
 
    Public ReadOnly Property Current As Object Implements _
      System.Collections.IEnumerator.Current
        Get
            Return Current1
        End Get
    End Property
    Private disposedValue As Boolean
    Protected Overridable Sub Dispose(ByVal disposing As Boolean)
        If Not Me.disposedValue Then
            If disposing Then
                _binaryreader.Close()
            End If
        End If
        Me.disposedValue = True
    End Sub
    Public Sub Dispose() Implements IDisposable.Dispose
        Dispose(True)
        GC.SuppressFinalize(Me)
    End Sub
End Class

Public Class ReadAsEnumerable
    Implements IEnumerable(Of Teilnehmer)
 
    Dim _binaryreader As IO.BinaryReader
    Dim _filestream As IO.FileStream
 
    Public Sub New(ByVal filename As String)
        _filestream = New IO.FileStream(filename, IO.FileMode.Open, _
          IO.FileAccess.Read)
        _binaryreader = New IO.BinaryReader(_filestream)
    End Sub
 
    Public Function GetEnumerator1() As System.Collections.IEnumerator _
       Implements System.Collections.IEnumerable.GetEnumerator
        Return New TeilnehmerEnumerator(_binaryreader)
    End Function
 
    Public Function GetEnumerator() As System.Collections.Generic.IEnumerator( _
      Of Teilnehmer) _
      Implements System.Collections.Generic.IEnumerable(Of _
      Teilnehmer).GetEnumerator
        Return New TeilnehmerEnumerator(_binaryreader)
    End Function
End Class

'DataFile = Pfad/Name der Datei
Dim ras As New ReadAsEnumerable(DataFile)
 
Dim x As IEnumerable(Of Object) = (From n As Teilnehmer In ras _
    Group By n.Fachrichtung Into Count()).tolist