无法在VBA中创建新枚举

Question

我在VBA中创建一个Immutable Linked List类.它提供ToArray和ToCollection方法,我已经验证了它的工作.但是该Get NewEnum() As IUnknown物业不起作用,我不知道为什么.

Public Property Get NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4

    Set NewEnum = ToCollection.[_NewEnum]

End Property

通过下面的代码与步进sequence作为SList与调试器

Public Function Copy(ByVal sequence As Variant) As SList

    Dim made As New SList

    Dim element As Variant
    For Each element In sequence
       Set made = made.Cons(element)
    Next

    Set Copy = made.Reverse

End Function

显示正确构建集合的For Each element In sequence调用Get NewEnum,然后Copy在不执行迭代且没有错误之后返回并退出循环.我唯一的猜测是它NewEnum是一个变量的迭代器,它在退出时被销毁Get NewEnum.那是怎么回事？

Answer 1

Public Property Get NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4

    Set NewEnum = ToCollection.[_NewEnum]

End Property

ToCollection每次调用时都会返回一个新的集合; 这可能会奏效:

Public Property Get NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4

    Static internalCollection As Collection
    If internalCollection Is Nothing Then Set internalCollection = ToCollection

    Set NewEnum = internalCollection.[_NewEnum]

End Property

......但它相当难看.理想情况下,您需要一些实例级别encapsulated As Collection来返回[_NewEnum]值.