Yield Statement (Visual Basic)

 

Updated: July 20, 2015

Sends the next element of a collection to a For Each...Next statement.

Yield expression  

Parameters

TermDefinition
expressionRequired. An expression that is implicitly convertible to the type of the iterator function or Get accessor that contains the Yield statement.

The Yield statement returns one element of a collection at a time. The Yield statement is included in an iterator function or Get accessor, which perform custom iterations over a collection.

You consume an iterator function by using a For Each...Next Statement or a LINQ query. Each iteration of the For Each loop calls the iterator function. When a Yield statement is reached in the iterator function, expression is returned, and the current location in code is retained. Execution is restarted from that location the next time that the iterator function is called.

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

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

"Yield" is not a reserved word and has special meaning only when it is used in an Iterator function or Get accessor.

For more information about iterator functions and Get accessors, see Iterators.

The declaration of an iterator function or Get accessor must meet the following requirements:

An iterator function cannot occur in an event, instance constructor, static constructor, or static destructor.

An iterator function can be an anonymous function. For more information, see Iterators.

A Yield statement can be inside a 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.

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

If the For Each body (outside of the iterator function) 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.

The following code returns an IEnumerable (Of String) from an iterator function and then iterates through the elements of the IEnumerable (Of String).

Dim elements As IEnumerable(Of String) = MyIteratorFunction()  
    …  
For Each element As String In elements  
Next  

The call to MyIteratorFunction doesn't execute the body of the function. Instead the call returns an IEnumerable(Of String) into the elements variable.

On an iteration of the For Each loop, the MoveNext method is called for elements. This call executes the body of MyIteratorFunction until the next Yield statement is reached. The Yield statement returns an expression that determines not only the value of the element variable for consumption by the loop body but also the Current property of elements, which is an IEnumerable (Of String).

On each subsequent iteration of the For Each loop, the execution of the iterator body continues from where it left off, again stopping when it reaches a Yield statement. The For Each loop completes when the end of the iterator function or a Return or Exit Function statement is reached.

The following example has a Yield statement that is inside a For…Next loop. Each iteration of the For Each statement body in Main creates a call to the Power iterator function. Each call to the iterator function proceeds to the next execution of the Yield statement, which occurs during the next iteration of the For…Next loop.

The return type of the iterator method is IEnumerable<T>, an iterator interface type. When the iterator method is called, it returns an enumerable object that contains the powers of a number.

    Sub Main()
        For Each number In Power(2, 8)
            Console.Write(number & " ")
        Next
        ' Output: 2 4 8 16 32 64 128 256
        Console.ReadKey()
    End Sub

    Private Iterator Function Power(
    ByVal base As Integer, ByVal highExponent As Integer) _
    As System.Collections.Generic.IEnumerable(Of Integer)

        Dim result = 1

        For counter = 1 To highExponent
            result = result * base
            Yield result
        Next
    End Function

The following example demonstrates a Get accessor that is an iterator. The property declaration includes an Iterator modifier.

    Sub Main()
        Dim theGalaxies As New Galaxies
        For Each theGalaxy In theGalaxies.NextGalaxy
            With theGalaxy
                Console.WriteLine(.Name & "  " & .MegaLightYears)
            End With
        Next
        Console.ReadKey()
    End Sub

    Public Class Galaxies
        Public ReadOnly Iterator Property NextGalaxy _
        As System.Collections.Generic.IEnumerable(Of Galaxy)
            Get
                Yield New Galaxy With {.Name = "Tadpole", .MegaLightYears = 400}
                Yield New Galaxy With {.Name = "Pinwheel", .MegaLightYears = 25}
                Yield New Galaxy With {.Name = "Milky Way", .MegaLightYears = 0}
                Yield New Galaxy With {.Name = "Andromeda", .MegaLightYears = 3}
            End Get
        End Property
    End Class

    Public Class Galaxy
        Public Property Name As String
        Public Property MegaLightYears As Integer
    End Class

For additional examples, see Iterators.

Visual Studio 2012

Iterators
Statements

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