Question

I’m a c# developer and have not enought experience in VB.NET.

the scenario:

Namespace Presenters
    Public Class BaseFooPresenter

        ' assuming the public default parameterless constructor

        Public Sub New(ByVal strvar As String)
            ' TODO with strvar variabile
        End Sub

    End Class

    Public Class FooPresenter
           Inherits BaseFooPresenter

        Public Sub New(ByVal boolvar As Boolean)
            ' TODO with boolvar variabile
        End Sub

        Public Sub New(ByVal boolvar As Boolean, _
                       ByVal objvar As Object)
            MyBase.New(String.Empty)
            Me.New(true)
            ' TODO with objvar variabile
        End Sub

    End Class
End Namespace

With this code at the second FooPresenter constructor i get an error

“Constructor call is valid only at the first statement in an instance constructor.”

at:

            Me.New(true)

If i invert the order i get the error at:

            MyBase.New(String.Empty)

I can create a method SetValues( … parameters … ) and call it from the two constructors but does someone knwos a workaround to avoid this error?, why the compiler do not validate the possibility to call the base constructor before the overloaded constructor?.

Does someone knows how to justify logically the fact that it’s not possible to call the base class constructor and another class level constructor from one class level constructor at the same time?

Answer 1

The issue is that once you specify a parametric constructor, the parameterless constructor becomes private unless explicitly specified otherwise by you.

So, modifying ken2K’s code:

Namespace Presenters
    Public Class BaseFooPresenter

        ' SPECIFYING the protected default parameterless constructor
        ' can also be public
        Protected Sub New()

        End Sub

        Public Sub New(ByVal strvar As String)
            ' TODO with strvar variabile
        End Sub

    End Class

    Public Class FooPresenter
        Inherits BaseFooPresenter

        Public Sub New(ByVal boolvar As Boolean)
            MyBase.New()
            ' TODO with boolvar variabile
        End Sub

        Public Sub New(ByVal boolvar As Boolean, _
                       ByVal objvar As Object)
            Me.New(boolvar)
            ' TODO with objvar variabile
        End Sub

    End Class
End Namespace