Iterator

An Iterator returns values when repeatedly called—it maintains state between calls. The Yield keyword returns a value from an Iterator. Control then resumes next after the Yield.

In VB.NET, the Yield and Iterator keywords use code generation. Special classes are generated. In these classes, the state of Iterators are maintained.

An example

The ComputePower Function generates a series of numbers with an increasing exponent value. We pass, as arguments, 2 values—the base number and the highest exponent required.

Detail In ComputePower we iterate through the exponents. We compute the running total and store it in the local variables.

Detail With the Yield keyword we "yield" control of ComputePower to Main. When next called, we resume after Yield.

Module Module1

    Sub Main()
        ' Loop over first 10 exponents.
        For Each value As Integer In ComputePower(2, 10)
            Console.WriteLine(value)
        Next
    End Sub

    Public Iterator Function ComputePower(
        ByVal number As Integer,
        ByVal exponent As Integer) As IEnumerable(Of Integer)

        Dim exponentNum As Integer = 0
        Dim numberResult As Integer = 1

        ' Yield all numbers with exponents up to the argument value.
        While exponentNum < exponent
            numberResult *= number
            exponentNum += 1
            Yield numberResult
        End While
    End Function

End Module2
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For performance, a simple For-loop or List is a better choice. Generate values, place into an array, and return. This avoids much of the excess classes that an Iterator will require.

Important Iterators are not low-level features. They use keywords and short syntax, but a significant amount of code is generated.

The Iterator and Yield features in VB.NET are advanced—complex code is generated. We often call Iterators within a For Each loop. Values can be returned based on any logic.