instantiate object ONLY from other class in VB.NET - vb.net

Question

I have 2 classes A and B (in VB.NET).
I want the only way to create an object of B class was by using a mehtod of A.
Examples:
You could do:
Dim objectA as new A
Dim objectB as B = objectA.getAobject()
BUT you couldn´t do:
Dim objectB as new B
Thanks!
Edit: in "You could do" section I wanna mean "Dim objectB as B = objectA.getAobject()"

Answer 1

You can make a private constructor in B:
Private Sub New()
End Sub
If there are no public constructors, this will block you from writing code like this anywhere but within B itself:
Dim objectB as new B
However, this requires you to write code like that somewhere in B, or you won't be able to ever create an instance of B anywhere. Typically a Shared method is the place to do this:
Friend Shared Function Create() As B
'...
Return New B
End Function
Note the Friend access modifier. Now, if you have an assembly (class library project) containing only A and B, only code inside that assembly (only A and B) will be able to use that function. Add to that a method in A that looks like this:
Public Shared Function getObject() As B
Return B.Create()
End Function
And we've met all your stated objectives. Code that references this assembly will be able to do this:
Dim objectB as B = A.getAobject()
But will not be able to do either of these:
Dim objectB as new B
Dim objectB As B = B.Create()

Answer 2

You could just check in the constructor of B, if your calling class is of type A
Class B
Sub New ()
' get parent frame and calling method
Dim frame As StackFrame = new StackFrame( 1, True )
Dim method As MethodBase = frame.GetMethod()
' for debug purposes
Dim methodName As String = method.DeclaringType.Name & "->" & method.Name
Console.WriteLine(methodName)
' throw exception
If (method.DeclaringType.Equals(typeof(A)) = False) {
throw new Exception("Not called from class A!!")
End If
End Sub
End Class

Answer 3

Create a new Project of type Class Library and add code similar to this for Class A and Class B
Public Class A
Public Function getObject() As B
Return New B()
End Function
End Class
Public Class B
Protected Friend Sub New()
Console.WriteLine("Constructor B called")
Console.ReadLine()
End Sub
End Class
Now the Constructor of Class B could only be called from inside code present in this project not from other assemblies
In your main project you need to add a reference to this new project and Imports its namespace.
Module Module1
Sub Main()
Dim a1 = New A()
' Dim b1 = New B() 'This will not compile at all'
Dim b2 = a1.getObject()
End Sub
End Module
Of course now you have to distribute two files instead of one....
The key for this to work is the different assembly and the keyword Protected Friend applied to the constructor of class B