Use Iterators in VB Now: VB: Extend the Streamreader class: Listing 1

Most of what you see in the class is boilerplate code, with the two methods of interest being GetEnumerator and the MoveNext method. This sample also illustrates how to add an extension method to make getting the enumerator a lot simpler. The extension method will show up on an instance of the StreamReader class as a Lines function.

 Module StreamExtensions

   <Extension()> _
   Function Lines(ByVal rdr As TextReader) _
      As IEnumerable(Of String)
      Return New StreamReaderLineEnumerator(rdr)
   End Function

End Module


Public Class StreamReaderLineEnumerator
   Implements IEnumerable(Of String)
   Implements IEnumerator(Of String)

   Private _rdr As TextReader
   Private _current As String

   Sub New(ByVal rdr As TextReader)
      _rdr = rdr
   End Sub


   ''' <summary>
   '''    Get's an enumerator to read lines from a stream
   ''' </summary>
   ''' <returns>IEnumerator(Of String)</returns>
   ''' <remarks>Can only be called once due to the 
   ''' underlying stream.
   '''  Not thread safe.</remarks>
   ''' <exception cref="InvalidOperationException">
   ''' thrown if GetEnumerator is called more 
   ''' than once</exception>
   Public Function GetEnumerator() _
      As IEnumerator(Of String) _
      Implements IEnumerable(Of String).GetEnumerator
      Static iFirstTime As Int32
      If iFirstTime = 0 AndAlso _
         Threading.Interlocked.Increment( _
         iFirstTime) = 1 Then
         Return Me
      Else
         Throw New InvalidOperationException( _
            "GetEnumerator can only be called " & _
            "once on a stream")
      End If
   End Function


   Private Function IEnumerator_GetEnumerator() _
      As IEnumerator _
      Implements IEnumerable.GetEnumerator
      Return GetEnumerator()
   End Function


   Public ReadOnly Property Current() As String _
      Implements IEnumerator(Of String).Current
      Get
         Return _current
      End Get
   End Property

   Private ReadOnly Property IEnumerator_Current() _
      As Object Implements IEnumerator.Current
      Get
         Return Current
      End Get
   End Property


   Public Function MoveNext() As Boolean _
      Implements IEnumerator.MoveNext
      _current = _rdr.ReadLine()
      Return _current IsNot Nothing
   End Function

   Private Sub Reset() Implements IEnumerator.Reset
      Throw New NotImplementedException()
   End Sub

   Public Sub Dispose() Implements _
      IDisposable.Dispose
      'not implemented
   End Sub

End Class
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