Question

It looks like passing a list’s enumerator to a function “byval” is quite different than passing it “byref”. Essentially, regular “byval” passing will NOT change the caller’s “enumerator.Current value”, even if the function advances the enumerator. I was wondering if anyone knows why this is the case? Is an enumerator a primitive like an integer, without an object reference, and hence changes to it don’t get reflected in the caller?

Here is the sample code:

This function is byval, and gets stuck in an infinite loop, spitting out “1” message boxes, because the enumerator’s “current” never advances past 5:

Public Sub listItemsUsingByValFunction()
    Dim list As New List(Of Integer)(New Integer() {1, 2, 3, 4, 5, 6, 7, 8, 9, 10})

    Dim enumerator = list.GetEnumerator()
    enumerator.MoveNext()
    While enumerator.Current <= 5
        listFirstItemByVal(enumerator)
    End While
End Sub
Private Sub listFirstItemByVal(ByVal enumerator As List(Of Integer).Enumerator)
    MsgBox(enumerator.Current)
    enumerator.MoveNext()
End Sub

This, on the other hand, works just as one would expect:

Public Sub listItemsUsingByRefFunction()
    Dim list As New List(Of Integer)(New Integer() {1, 2, 3, 4, 5, 6, 7, 8, 9, 10})

    Dim enumerator = list.GetEnumerator()
    enumerator.MoveNext()
    While enumerator.Current <= 5
        listFirstItemByRef(enumerator)
    End While
End Sub
Private Sub listFirstItemByRef(ByRef enumerator As List(Of Integer).Enumerator)
    MsgBox(enumerator.Current)
    enumerator.MoveNext()
End Sub

The difference between the two functions is only whether the listFirstItem__ function accepts a byval or a byref enumerator.

Answer 1

The reason why you’re seeing this behavior is that List(Of T).Enumerator is a Struct and not a Class as is commonly expected. So when you pass the enumerator you pass a copy of it and hence only that copy gets updated when you call MoveNext