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
         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
         Return _current
      End Get
   End Property

   Private ReadOnly Property IEnumerator_Current() _
      As Object Implements IEnumerator.Current
         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 _
      'not implemented
   End Sub

End Class
comments powered by Disqus


  • .NET Core Ranks High Among Frameworks in New Dev Survey

    .NET Core placed high in a web-dominated ranking of development frameworks published by CodinGame, which provides a tech hiring platform.

  • Here's a One-Stop Shop for .NET 5 Improvements

    Culled from reams of Microsoft documentation, here's a high-level summary of what's new for performance, networking, diagnostics and more, along with links to the nitty-gritty details for those wanting to dig in more.

  • Azure SQL Database Ranked Among Top 3 Databases of 2020

    Microsoft touted the inclusion of Azure SQL Database among the top three databases of 2020 in a popularity ranking by DB-Engines, which collects and manages information about database management systems, updating its lists monthly.

  • Time Tracker Says VS Code Is No. 1 Editor for Devs, Some Working 15+ Hours Per Day

    WakaTime, which does time tracking for programmers, released data for 2020 showing that Visual Studio Code is by far the top editor/IDE used by its coders, some of whom are hacking away for more than 15 hours per day.

Upcoming Events