Question

Could someone explain to me why I get a compiler error when I try to call a base class’ constructor from an inherited object? I’ve included a brief example of what I’m referring to.

Public Class Person
    Public name As String

    Public Sub New()
        name = "John Doe"
    End Sub

    Public Sub New(Name As String)
        name = Name
    End Sub

End Class

Public Class NamedPerson
    Inherits Person

    Public Sub New(FirstName As String, LastName As String)
        name = FirstName & " " & LastName
    End Sub

    'adding this makes it work
    Public Sub New(Name As String)
       MyBase.New(Name)
    End Sub

End Class

'Valid
Dim guy1 As Person = New Person()

'Valid 
Dim guy2 As Person = New Person("John Smith")

'Valid
Dim guy3 As NamedPerson = New NamedPerson("John", "Smith")

'Compiler Error
Dim guy4 As NamedPerson = New NamedPerson("John Smith")

Answer 1

Child classes do not inherit constructors from their base types. A child class is responsible for defining it’s own constructors. Additionally it must ensure that each constructor it defines either implicitly or explicitly calls into a base class constructor or chains to another constructor in the same type.

For more info See: Instance Constructors

From your sample classes,

Public Class NamedPerson
    Inherits Person

    Public Sub New(Name As String)
        MyBase.New(Name)
    End Sub

    Public sub New(FirstName As String, LastName As String)
        name = FirstName & " " & LastName
    End Sub
End Class