16

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?

Improve this question
3
  • sorry @Moonlight because i'm very new in stackoverflow and i don't have sufficient reputation scores (at now 14) to vote the answers :( Commented Dec 29, 2011 at 11:09
  • 3
    you can mark the answer on your own questions (not the voting thin, mark as answer) Commented Dec 29, 2011 at 11:12
  • 2
    thanx @Moonlight for your constructive aproach, i'm marking the answers now ;) Commented Dec 29, 2011 at 11:27

3 Answers 3

Reset to default
24

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
Improve this answer
Sign up to request clarification or add additional context in comments.

1 Comment

Hello @M.A. Hanin, i need the first FooPresenter constructor to call the parameterless base constructor and the second FooPresenter constructor to call first the first FooPresenter constructor before/after call the base overloaded base constructor, i see it's not possible.
7

Just like with C#, you can't call this() and base() at the same time. Here's what you should do:

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)
            MyBase.New(String.Empty)
            ' 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
Improve this answer

5 Comments

Hi @ken2k, it's important that the first constructor not to call the overloaded base class constructor, it's imperative that the first constructor calls the implicit parameterless base class constructor, thanx in advance.
@manuellt: 'it's imperative that the first constructor calls the implicit parameterless base class constructor' Why? It looks like you're just making a rod for your own back with this requirement.
@Robin Brown, thanx for your observation, the FooPresenter consumes an object created in the base class if constructed with the parameterless OR a static object from an stack if called with the overloaded constructor passing it a key to the base class, several existing classes ereditates from this base class and i only want to find the less-impacting way to implement the new behavior at some existing classes and the new ones.
@manuellt: I think the problem is that you're trying to make one item do two things, hence the 'OR' in your comment above. Would it be possible to revisit your overall design and make a change so that you're not relying on inheritance? Maybe use an interface to enforce behaviour, or possibly a singleton/factory class that creates your objects? If you haven't done so I recommend (re)reading 'Head First Design Patterns' as it helps clarify a lot of these problems
@Robin Brown, I agree, actually there's one object for all presenters, i need some presenters to have a singleton object (yet implemented), maybe i've to review the design, thank you for your greatfull recommendation :)
2

You can't call multiple constructors like that. Just like in C# you can only call one initializer.

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.