for-each-extension-get-enumerator.vb

Module Program

    Sub Main()

        Dim list = New List(Of Integer) From {1, 2, 3, 4, 5}

        Dim info = list.FirstAndRest()

        If info.First IsNot Nothing Then
            Console.Write(info.First.GetValueOrDefault())

            For Each other In info.Additional
                Console.Write(", ")
                Console.Write(other)
            Next

            Console.WriteLine()
        End If
    End Sub

    <Runtime.CompilerServices.Extension>
    Function FirstAndRest(Of T As Structure)(sequence As IEnumerable(Of T)) As (First As T?, Additional As IEnumerator(Of T))
        Dim enumerator = sequence.GetEnumerator()

        If enumerator.MoveNext() Then
            Return (enumerator.Current, enumerator)
        Else
            Return (Nothing, enumerator)
        End If
    End Function

    <Runtime.CompilerServices.Extension>
    Function GetEnumerator(Of T)(enumerator As IEnumerator(Of T)) As IEnumerator(Of T)
        Return enumerator
    End Function
End Module