Why aren't Constructor Inherited?

Question

As a follow up Question, to Base Class Only Methods

Parent Methods are inherited by their descendants.

How come then Constructors (eg New()) aren't? Which to me seem to break inheritance.

Is there an attribute somewhere that marks it as special? (If so what is it?)

Could so one please explain what going on.

Answer 1

I suspect the real reason is twofold:

First of all, no static (Shared) members are inherited in VB# (or C#). While this is in line with what most OO languages do, it’s not a necessary design. It could conceivably have been implemented differently.

Secondly, it often makes sense to restrict the set of constructors that can construct a child object. In particular, since a child class often has additional members that need to be initialised, invoking a parent class constructor would leave the child in an uninitialised state. Imagine constructors could be inherited and tell me what the following code is supposed to do:

Class Base
    Public Sub New()
    End Sub
End Class

Class Derived : Inherits Base
    Public Property X() As Integer

    Public Sub New(ByVal value As Integer)
        X = value
    End Sub
End Class

' …

Dim foo As New Derived()
Console.WriteLine(foo.X) ' = ???

Given this, it makes a good deal of sense to forbid inheritance of constructors.

Answer 2

Inheritance (or, to be precise, a subtype relationship) guarantees that if S is a subtype of T, you can use an object of type S whenever an object of type T is required.

Constructors can never be executed on an object -- they are executed to create an object. They are special in that way:

myS.SomeMethodOfT()    ' works
myS.New()              ' doesn't work -- constructors are special.

In other words, a constructor could be seen as a static (Shared in Visual Basic) method returning an object of a specific type (and some languages actually implement it this way):

Dim myS = New S()      ' can be seen as syntactic sugar for 
                       ' something like myS = S.CreateNew()

Answer 3

In the constructor you should only construct the elements for that class, not its base class. By calling the contructor of the base class those elements will be constructed.