Question

I’m playing with the following:

Public MustInherit Class TempTable
    Public Sub New()
        For Each f As FieldInfo In Me.GetType().GetFields
            Dim l As TypedLeaf = CType(f.GetValue(Me), TypedLeaf)
            Console.WriteLine(l.Name)
        Next
    End Sub
End Class

Public Class JMTempTable
    Inherits TempTable

    Public KeyIndex As New TypedLeaf(Me, "KeyIndex", OQL.Type.AUTONUMBER)
    Public Debit As New TypedLeaf(Me, "Debit", OQL.Type.DECIMAL(16, 2))

    Public Sub New()
        MyBase.New()
    End Sub
End Class

but getting Nothing for the values retrieved. The reason seems to be that the derived class’ fields do not get initialised until after the base class’ constructor is called… to further complicate matters, if I were to do:

Public Class JMTempTable
    Inherits TempTable

    Public KeyIndex As TypedLeaf
    Public Debit As TypedLeaf

    Public Sub New()
        KeyIndex = New TypedLeaf(Me, "KeyIndex", OQL.Type.AUTONUMBER)
        Debit = New TypedLeaf(Me, "Debit", OQL.Type.DECIMAL(16, 2))

        MyBase.New()
    End Sub
End Class

The compiler will complain that the base class constructor must be called in the first line of the derived class’ constructor…

Is there a way I can delay the base class constructor from running until after the derived class’ fields have been initialised?

Answer 1

Here’s one way (perhaps the way) to do it:

Public MustInherit Class TempTable
    Public Sub New()
        Initialize()
        For Each f As FieldInfo In Me.GetType().GetFields
            Dim l As TypedLeaf = CType(f.GetValue(Me), TypedLeaf)
            Console.WriteLine(l.Name)
        Next
    End Sub

    Protected MustOverride Sub Initialize()
End Class

Public Class JMTempTable
    Inherits TempTable

    Public KeyIndex As TypedLeaf()
    Public Debit As TypedLeaf()

    Public Sub New() ' Optional block. You don't have to explicitly define a default constructor.
        MyBase.New()
    End Sub

    Protected Overrides Sub Initialize()
        KeyIndex = New TypedLeaf(Me, "KeyIndex", OQL.Type.AUTONUMBER)
        Debit = New TypedLeaf(Me, "Debit", OQL.Type.DECIMAL(16, 2))
    End Sub
End Class

The abstract Initialize() method forces inheritors to have a method called Initialize(). This method is implicitly called when you call MyBase.New(). That means you can now move your initialization logic out of the constructor and into the Initialize() method to get the effect you’re looking for.