Iterators (Visual Basic)

An iterator can be used to step through collections such as lists and arrays.

An iterator method or get accessor performs a custom iteration over a collection. An iterator method uses the Yield statement to return each element one at a time. When a Yield statement is reached, the current location in code is remembered. Execution is restarted from that location the next time the iterator function is called.

You consume an iterator from client code by using a For Each…Next statement, or by using a LINQ query.

In the following example, the first iteration of the For Each loop causes execution to proceed in the SomeNumbers iterator method until the first Yield statement is reached. This iteration returns a value of 3, and the current location in the iterator method is retained. On the next iteration of the loop, execution in the iterator method continues from where it left off, again stopping when it reaches a Yield statement. This iteration returns a value of 5, and the current location in the iterator method is again retained. The loop completes when the end of the iterator method is reached.

Sub Main()  
    For Each number As Integer In SomeNumbers()  
        Console.Write(number & " ")  
    Next  
    ' Output: 3 5 8  
    Console.ReadKey()  
End Sub  
  
Private Iterator Function SomeNumbers() As System.Collections.IEnumerable  
    Yield 3  
    Yield 5  
    Yield 8  
End Function  

The return type of an iterator method or get accessor can be IEnumerable, IEnumerable<T>, IEnumerator, or IEnumerator<T>.

You can use an Exit Function or Return statement to end the iteration.

A Visual Basic iterator function or get accessor declaration includes an Iterator modifier.

Iterators were introduced in Visual Basic in Visual Studio 2012.

In this topic

Note

For all examples in the topic except the Simple Iterator example, include Imports statements for the System.Collections and System.Collections.Generic namespaces.

Simple Iterator

The following example has a single Yield statement that is inside a For…Next loop. In Main, each iteration of the For Each statement body creates a call to the iterator function, which proceeds to the next Yield statement.

Sub Main()  
    For Each number As Integer In EvenSequence(5, 18)  
        Console.Write(number & " ")  
    Next  
    ' Output: 6 8 10 12 14 16 18  
    Console.ReadKey()  
End Sub  
  
Private Iterator Function EvenSequence(  
ByVal firstNumber As Integer, ByVal lastNumber As Integer) _  
As System.Collections.Generic.IEnumerable(Of Integer)  
  
    ' Yield even numbers in the range.  
    For number As Integer = firstNumber To lastNumber  
        If number Mod 2 = 0 Then  
            Yield number  
        End If  
    Next  
End Function  

Creating a Collection Class

In the following example, the DaysOfTheWeek class implements the IEnumerable interface, which requires a GetEnumerator method. The compiler implicitly calls the GetEnumerator method, which returns an IEnumerator.

The GetEnumerator method returns each string one at a time by using the Yield statement, and an Iterator modifier is in the function declaration.

Sub Main()  
    Dim days As New DaysOfTheWeek()  
    For Each day As String In days  
        Console.Write(day & " ")  
    Next  
    ' Output: Sun Mon Tue Wed Thu Fri Sat  
    Console.ReadKey()  
End Sub  
  
Private Class DaysOfTheWeek  
    Implements IEnumerable  
  
    Public days =  
        New String() {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}  
  
    Public Iterator Function GetEnumerator() As IEnumerator _  
        Implements IEnumerable.GetEnumerator  
  
        ' Yield each day of the week.  
        For i As Integer = 0 To days.Length - 1  
            Yield days(i)  
        Next  
    End Function  
End Class  

The following example creates a Zoo class that contains a collection of animals.

The For Each statement that refers to the class instance (theZoo) implicitly calls the GetEnumerator method. The For Each statements that refer to the Birds and Mammals properties use the AnimalsForType named iterator method.

Sub Main()  
    Dim theZoo As New Zoo()  
  
    theZoo.AddMammal("Whale")  
    theZoo.AddMammal("Rhinoceros")  
    theZoo.AddBird("Penguin")  
    theZoo.AddBird("Warbler")  
  
    For Each name As String In theZoo  
        Console.Write(name & " ")  
    Next  
    Console.WriteLine()  
    ' Output: Whale Rhinoceros Penguin Warbler  
  
    For Each name As String In theZoo.Birds  
        Console.Write(name & " ")  
    Next  
    Console.WriteLine()  
    ' Output: Penguin Warbler  
  
    For Each name As String In theZoo.Mammals  
        Console.Write(name & " ")  
    Next  
    Console.WriteLine()  
    ' Output: Whale Rhinoceros  
  
    Console.ReadKey()  
End Sub  
  
Public Class Zoo  
    Implements IEnumerable  
  
    ' Private members.  
    Private animals As New List(Of Animal)  
  
    ' Public methods.  
    Public Sub AddMammal(ByVal name As String)  
        animals.Add(New Animal With {.Name = name, .Type = Animal.TypeEnum.Mammal})  
    End Sub  
  
    Public Sub AddBird(ByVal name As String)  
        animals.Add(New Animal With {.Name = name, .Type = Animal.TypeEnum.Bird})  
    End Sub  
  
    Public Iterator Function GetEnumerator() As IEnumerator _  
        Implements IEnumerable.GetEnumerator  
  
        For Each theAnimal As Animal In animals  
            Yield theAnimal.Name  
        Next  
    End Function  
  
    ' Public members.  
    Public ReadOnly Property Mammals As IEnumerable  
        Get  
            Return AnimalsForType(Animal.TypeEnum.Mammal)  
        End Get  
    End Property  
  
    Public ReadOnly Property Birds As IEnumerable  
        Get  
            Return AnimalsForType(Animal.TypeEnum.Bird)  
        End Get  
    End Property  
  
    ' Private methods.  
    Private Iterator Function AnimalsForType( _  
    ByVal type As Animal.TypeEnum) As IEnumerable  
        For Each theAnimal As Animal In animals  
            If (theAnimal.Type = type) Then  
                Yield theAnimal.Name  
            End If  
        Next  
    End Function  
  
    ' Private class.  
    Private Class Animal  
        Public Enum TypeEnum  
            Bird  
            Mammal  
        End Enum  
  
        Public Property Name As String  
        Public Property Type As TypeEnum  
    End Class  
End Class  

Try Blocks

Visual Basic allows a Yield statement in the Try block of a Try...Catch...Finally Statement. A Try block that has a Yield statement can have Catch blocks, and can have a Finally block.

The following example includes Try, Catch, and Finally blocks in an iterator function. The Finally block in the iterator function executes before the For Each iteration finishes.

Sub Main()  
    For Each number As Integer In Test()  
        Console.WriteLine(number)  
    Next  
    Console.WriteLine("For Each is done.")  
  
    ' Output:  
    '  3  
    '  4  
    '  Something happened. Yields are done.  
    '  Finally is called.  
    '  For Each is done.  
    Console.ReadKey()  
End Sub  
  
Private Iterator Function Test() As IEnumerable(Of Integer)  
    Try  
        Yield 3  
        Yield 4  
        Throw New Exception("Something happened. Yields are done.")  
        Yield 5  
        Yield 6  
    Catch ex As Exception  
        Console.WriteLine(ex.Message)  
    Finally  
        Console.WriteLine("Finally is called.")  
    End Try  
End Function  

A Yield statement cannot be inside a Catch block or a Finally block.

If the For Each body (instead of the iterator method) throws an exception, a Catch block in the iterator function is not executed, but a Finally block in the iterator function is executed. A Catch block inside an iterator function catches only exceptions that occur inside the iterator function.

Anonymous Methods

In Visual Basic, an anonymous function can be an iterator function. The following example illustrates this.

Dim iterateSequence = Iterator Function() _  
                      As IEnumerable(Of Integer)  
                          Yield 1  
                          Yield 2  
                      End Function  
  
For Each number As Integer In iterateSequence()  
    Console.Write(number & " ")  
Next  
' Output: 1 2  
Console.ReadKey()  

The following example has a non-iterator method that validates the arguments. The method returns the result of an anonymous iterator that describes the collection elements.

Sub Main()  
    For Each number As Integer In GetSequence(5, 10)  
        Console.Write(number & " ")  
    Next  
    ' Output: 5 6 7 8 9 10  
    Console.ReadKey()  
End Sub  
  
Public Function GetSequence(ByVal low As Integer, ByVal high As Integer) _  
As IEnumerable  
    ' Validate the arguments.  
    If low < 1 Then  
        Throw New ArgumentException("low is too low")  
    End If  
    If high > 140 Then  
        Throw New ArgumentException("high is too high")  
    End If  
  
    ' Return an anonymous iterator function.  
    Dim iterateSequence = Iterator Function() As IEnumerable  
                              For index = low To high  
                                  Yield index  
                              Next  
                          End Function  
    Return iterateSequence()  
End Function  

If validation is instead inside the iterator function, the validation cannot be performed until the start of the first iteration of the For Each body.

Using Iterators with a Generic List

In the following example, the Stack(Of T) generic class implements the IEnumerable<T> generic interface. The Push method assigns values to an array of type T. The GetEnumerator method returns the array values by using the Yield statement.

In addition to the generic GetEnumerator method, the non-generic GetEnumerator method must also be implemented. This is because IEnumerable<T> inherits from IEnumerable. The non-generic implementation defers to the generic implementation.

The example uses named iterators to support various ways of iterating through the same collection of data. These named iterators are the TopToBottom and BottomToTop properties, and the TopN method.

The BottomToTop property declaration includes the Iterator keyword.

Sub Main()  
    Dim theStack As New Stack(Of Integer)  
  
    ' Add items to the stack.  
    For number As Integer = 0 To 9  
        theStack.Push(number)  
    Next  
  
    ' Retrieve items from the stack.  
    ' For Each is allowed because theStack implements  
    ' IEnumerable(Of Integer).  
    For Each number As Integer In theStack  
        Console.Write("{0} ", number)  
    Next  
    Console.WriteLine()  
    ' Output: 9 8 7 6 5 4 3 2 1 0  
  
    ' For Each is allowed, because theStack.TopToBottom  
    ' returns IEnumerable(Of Integer).  
    For Each number As Integer In theStack.TopToBottom  
        Console.Write("{0} ", number)  
    Next  
    Console.WriteLine()  
    ' Output: 9 8 7 6 5 4 3 2 1 0  
  
    For Each number As Integer In theStack.BottomToTop  
        Console.Write("{0} ", number)  
    Next  
    Console.WriteLine()  
    ' Output: 0 1 2 3 4 5 6 7 8 9   
  
    For Each number As Integer In theStack.TopN(7)  
        Console.Write("{0} ", number)  
    Next  
    Console.WriteLine()  
    ' Output: 9 8 7 6 5 4 3  
  
    Console.ReadKey()  
End Sub  
  
Public Class Stack(Of T)  
    Implements IEnumerable(Of T)  
  
    Private values As T() = New T(99) {}  
    Private top As Integer = 0  
  
    Public Sub Push(ByVal t As T)  
        values(top) = t  
        top = top + 1  
    End Sub  
  
    Public Function Pop() As T  
        top = top - 1  
        Return values(top)  
    End Function  
  
    ' This function implements the GetEnumerator method. It allows  
    ' an instance of the class to be used in a For Each statement.  
    Public Iterator Function GetEnumerator() As IEnumerator(Of T) _  
        Implements IEnumerable(Of T).GetEnumerator  
  
        For index As Integer = top - 1 To 0 Step -1  
            Yield values(index)  
        Next  
    End Function  
  
    Public Iterator Function GetEnumerator1() As IEnumerator _  
        Implements IEnumerable.GetEnumerator  
  
        Yield GetEnumerator()  
    End Function  
  
    Public ReadOnly Property TopToBottom() As IEnumerable(Of T)  
        Get  
            Return Me  
        End Get  
    End Property  
  
    Public ReadOnly Iterator Property BottomToTop As IEnumerable(Of T)  
        Get  
            For index As Integer = 0 To top - 1  
                Yield values(index)  
            Next  
        End Get  
    End Property  
  
    Public Iterator Function TopN(ByVal itemsFromTop As Integer) _  
        As IEnumerable(Of T)  
  
        ' Return less than itemsFromTop if necessary.  
        Dim startIndex As Integer =  
            If(itemsFromTop >= top, 0, top - itemsFromTop)  
  
        For index As Integer = top - 1 To startIndex Step -1  
            Yield values(index)  
        Next  
    End Function  
End Class  

Syntax Information

An iterator can occur as a method or get accessor. An iterator cannot occur in an event, instance constructor, static constructor, or static destructor.

An implicit conversion must exist from the expression type in the Yield statement to the return type of the iterator.

In Visual Basic, an iterator method cannot have any ByRef parameters.

In Visual Basic, "Yield" is not a reserved word and has special meaning only when it is used in an Iterator method or get accessor.

Technical Implementation

Although you write an iterator as a method, the compiler translates it into a nested class that is, in effect, a state machine. This class keeps track of the position of the iterator as long the For Each...Next loop in the client code continues.

To see what the compiler does, you can use the Ildasm.exe tool to view the Microsoft intermediate language code that is generated for an iterator method.

When you create an iterator for a class or struct, you do not have to implement the whole IEnumerator interface. When the compiler detects the iterator, it automatically generates the Current, MoveNext, and Dispose methods of the IEnumerator or IEnumerator<T> interface.

On each successive iteration of the For Each…Next loop (or the direct call to IEnumerator.MoveNext), the next iterator code body resumes after the previous Yield statement. It then continues to the next Yield statement until the end of the iterator body is reached, or until an Exit Function or Return statement is encountered.

Iterators do not support the IEnumerator.Reset method. To re-iterate from the start, you must obtain a new iterator.

For additional information, see the Visual Basic Language Specification.

Use of Iterators

Iterators enable you to maintain the simplicity of a For Each loop when you need to use complex code to populate a list sequence. This can be useful when you want to do the following:

  • Modify the list sequence after the first For Each loop iteration.

  • Avoid fully loading a large list before the first iteration of a For Each loop. An example is a paged fetch to load a batch of table rows. Another example is the EnumerateFiles method, which implements iterators within the .NET Framework.

  • Encapsulate building the list in the iterator. In the iterator method, you can build the list and then yield each result in a loop.

See also