Jason N. Gaylord's Blog

# re: VB.NET's "yield return"

If the function returns IEnumerable(Of String) your return statement (which is the equivalent of yield return "MyReturnValue") would be:

Return New String() {"MyReturnValue"}

Or, instead of String it could be any enumerable type.

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# re: VB.NET's "yield return"

I'm new to .NET technology and came across the code below. However I don't know how to apply the suggestions you above to the following code:

public static IEnumerable<string> ReadLinesFromFile(string filename)

{

   using (StreamReader reader = new StreamReader(filename))

   {

       while (true)

       {

           string s = reader.ReadLine();

           if (s == null)

               break;

           yield return s;

       }

   }

}

********

var products = from line in ReadLinesFromFile(@"c:\import.txt")

              let item = line.Split('\t')

              select new

              {

                  Product = item[0],

                  Quantity = Convert.ToInt32(item[1]),

                  Price = Convert.ToDecimal(item[2]),

                  Total = Convert.ToInt32(item[1]) * Convert.ToDecimal(item[2])

              };GridView1.DataSource = products;

GridView1.DataBind();

*******

Please help in converting C# to VB. Thanks!

# re: VB.NET's "yield return"

Public Shared Function ReadLinesFromFile(ByVal fileName As String) As IEnumerable(Of String)

       Dim stringList As New List(Of String)

       Using reader As StreamReader = New StreamReader(fileName)

           Dim s As String

           While True

               s = reader.ReadLine

               If s Is Nothing Then

                   Exit While

               Else

                   stringList.Add(s)

               End If

           End While

       End Using

       Return stringList

   End Function

# re: VB.NET's "yield return"

That works if you're returning an IEnumerable(Of T), but not if you're returning an IEnumerator(Of T). In that case, return your list but tack on the GetEnumerator() function to the end. i.e. Return myList.GetEnumerator()

# re: VB.NET's "yield return"

isnt that defeating the purpose of "yield return" for lazy evaluating.

# re: VB.NET's "yield return"

C# code

public IEnumerator<mList> GetEnumerator()

{            

foreach (mList column in mItems)                

yield return column;            

yield break;        

}

VB.NET code

Public Function GetEnumerator() As IEnumerator(Of mList)_ Implements IEnumerable(Of mList).GetEnumerator             Return mItems.GetEnumerator        

End Function

# re: VB.NET's "yield return"

In the VB.NET 10, you could use :

Return New List(Of mList) From {mItems}

# re: VB.NET's "yield return"

Vb net s quot yield return quot.. Bully :)

# re: VB.NET's "yield return"

Yield is syntactic sugar. Behind the scenes it returns an IEnumerator instance that handles the callbacks into your function. To do the lazy enumeration, you have to implement it yourself unfortunately.

# re: VB.NET's "yield return"

And how does that mean? I do not understand anything.

# re: VB.NET's "yield return"

The Compiler generates a class for yield return you could decompile it and build it to use it.

 It would look like this..

Public ReadOnly Property Properties As IEnumerable(Of IProperty)

           Get

               Dim x = New get_Propertiesd__18(-2)

               x._This = Me

               Return x

           End Get

       End Property

Private NotInheritable Class get_Propertiesd__18

           Implements IEnumerable(Of IProperty), IEnumerator(Of IProperty)

           ' Fields

           Private _State As Integer

           Private _Current As IProperty

           Public _This As DefaultResolvedTypeDefinition

           Private _InitialThreadId As Integer

           Public i5__1a As Integer

           Public members5__19 As MemberList

           ' Methods

           Public Sub New(ByVal state As Integer)

               _State = state

               _InitialThreadId = Thread.CurrentThread.ManagedThreadId

           End Sub

           '

#Region "IEnumerable(Of IProperty) Support"

           ' Methods

           Private Function GetEnumerator2() As System.Collections.Generic.IEnumerator(Of IProperty) Implements System.Collections.Generic.IEnumerable(Of IProperty).GetEnumerator

               If ((Thread.CurrentThread.ManagedThreadId = _InitialThreadId) AndAlso (_State = -2)) Then

                   _State = 0

                   Return Me

               End If

               Return New get_Propertiesd__18(0) With { _

                   ._This = Me._This _

               }

           End Function

           '

#End Region

           '

#Region "IEnumerable Support"

           ' Methods

           Private Function GetEnumerator1() As System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator

               Return DirectCast(Me, IEnumerable(Of IProperty)).GetEnumerator()

           End Function

           '

#End Region

           '

#Region "IEnumerator(Of IProperty) Support"

           ' Properties

           Private ReadOnly Property Current2 As IProperty Implements System.Collections.Generic.IEnumerator(Of IProperty).Current

               Get

                   Return _Current

               End Get

           End Property

           '

#End Region

           '

#Region "IEnumerator Support"

           ' Methods

           Public Function MoveNext1() As Boolean Implements System.Collections.IEnumerator.MoveNext

               Select Case _State

                   Case 0

                       _State = -1

                       Me.members5__19 = Me._This.GetMemberList

                       Me.i5__1a = 0

                   Case 1

                       _State = -1

                       Me.i5__1a += 1

                   Case Else

                       Return False

               End Select

               '

               Do

                   If (Me.i5__1a < Me.members5__19.FUnresolvedMembers.Length) Then

                       Select Case Me.members5__19.FUnresolvedMembers(Me.i5__1a).EntityType

                           Case EntityType.Property, EntityType.Indexer

                               _Current = DirectCast(Me.members5__19.Item(Me.i5__1a), IProperty)

                               _State = 1

                               Return True

                       End Select

                       Me.i5__1a += 1

                   Else

                       Exit Do

                   End If

               Loop

               '

               Return False

           End Function

           Public Sub Reset1() Implements System.Collections.IEnumerator.Reset

               Throw New NotSupportedException

           End Sub

           ' Properties

           Public ReadOnly Property Current1 As Object Implements System.Collections.IEnumerator.Current

               Get

                   Return _Current

               End Get

           End Property

           '

#End Region

           '

#Region "IDisposable Support"

           ' Methods

           Private Sub Dispose1() Implements IDisposable.Dispose

           End Sub

           '

#End Region

           '

       End Class