CODEWITHSUNDEEP
vb.net
w0051977
w0051977

Reputation: 15787

VB.NET - reference to an object under construction is not valid when calling another constructor

Is there any difference between doing this:

Public Class Class1
    Protected Test1 As String
    Public Sub New(ByVal test2 As String)
        Test1 = test2
    End Sub
End Class

Public Class Class2
    Inherits Class1

    Public Sub New()
        MyBase.New("called from class 2")
    End Sub

End Class

and this:

Public Class Class1
    Protected Test1 As String
End Class

Public Class Class2
    Inherits Class1

    Public Sub New()
        Test1 = "Called from class 2"
    End Sub
End Class

In the first example, the superclass instance variable is initialised in the constructor. In the second example, the superclass instance variable is initialised in the subclass.

The reason I ask is because I am trying to do this from the subclass:

Public Sub New()
        MyBase.New(                   System.Configuration.ConfigurationManager.AppSettings.Item("PurgeFile" & Me.GetType.Name), & _
               System.Configuration.ConfigurationManager.AppSettings.Item("PurgeHeader" & Me.GetType.Name) )
        End Sub

and I am getting an error: "reference to an object under construction is not valid when calling another constructor".

Upvotes: 0

Views: 913

Answers (1)

MarcinJuraszek
MarcinJuraszek

Reputation: 125620

You can't use Me within MyBase.New() call, so following part of your code is invalid:

Me.GetType.Name

Update

There is huge difference your 2 samples: First one doesn't allow Class1 initialization without constructor parameter and the second one does.

I would rather think about something like:

Public MustInherit Class Class1
    Public MustOverride ReadOnly Property Test1 As String
End Class

Public Class Class2
    Inherits Class1

    Private _Test1 As String = "Called from class 2"

    Public Overrides ReadOnly Property Test1 As String
        Get
            Return _Test1
        End Get
    End Property
End Class

Reference from MSDN

Upvotes: 2

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