How To MustOverride Shared/Constructor Function Visual Basic - database

Question

I'm currently using Visual Basic for a College Project which requires us to make a simple database system. For my system I have a base(abstract) class called Record which is inherited by the different types of records there are in my database e.g. Member, User, Role.
I am saving my data in csv files and have already written a CSVHandler class. However, I want an elegant way of constructing an instance of a class derived from Record with a string from the CSVHandler.
This is where the problem occurs. The only way I can think of doing this is by making a Constrcutor or Shared Function in each class derived from Record. However, Visual Basic does not allow you make Constructors or Shared Functions also MustOverride.
Here is the code I would expect to write:
' Base Class
Public MustInherit Class Record
Public MustOverride Shared Function fromString(ByVal str as String) As Record
End Class
' Example Of Class Derived From Record
Public Class User
Inherits Record
Private _id As String
Private _name As String
Public Sub New(ByVal id As String, ByVal name As String)
_id = id
_name = name
End Sub
Public Overrides Shared Function fromString(ByVal str as String) As Record
Dim strs() As String = str.Split(",")
Return New User(strs(0), strs(1))
End Function
End Class
' Example Of Creating Instacnce Of User
Dim user1 = User.fromString("1671,Kappeh")
Is there a way to achieve this effect?

Answer 1

Have your constructor call a Protected MustOverride method that does the initialisation.
Public MustInherit Class Record
'This is required because each derived constructor must be able to implicitly invoke a parameterless
'base constructor if it doesn't explicitly invoke a base constructor with parameters.
Protected Sub New()
End Sub
Public Sub New(csv As String)
Init(csv)
End Sub
Protected MustOverride Sub Init(csv As String)
End Class
Public Class User
Inherits Record
Private Property Id As String
Private Property Name As String
'This is still required because you can use a base constructor directly to create a derived instance.
Public Sub New(csv As String)
MyBase.New(csv)
End Sub
Public Sub New(id As String, name As String)
Id = id
Name = name
End Sub
Protected Overrides Sub Init(csv As String)
'Add your type-specific implementation here.
End Sub
End Class
This "solution" doesn't actually do what I thought it would because, while it forces you to override Init in a derived class, you still have to provide a derived constructor that invokes the base constructor that calls Init and you still can't enforce that. I think that I'll leave this as an answer though, because, while it doesn't actually provide a solution to your problem, it demonstrates further why (as far as I can tell) there is no such solution.

Answer 2

The following is similar to the answer from #jmcilhinney in that it forces the derived class to implement an initialization method. However it makes use of a generic shared function and uses the little known GetUninitializedObject method to get around using the generic New constraint and it's requirement of an accessible parameter-less constructor.
Public MustInherit Class Record
Public Shared Function fromString(Of T As {Record})(ByVal str As String) As T
' create an unintialized instance of T
Dim ret As T = DirectCast(System.Runtime.Serialization.FormatterServices.GetUninitializedObject(GetType(T)), T)
ret.Initialize(str)
Return ret
End Function
Protected MustOverride Sub Initialize(source As String)
End Class
The User class then would be something like this:
Public Class User : Inherits Record
Private _id As String
Private _name As String
Public Sub New(ByVal id As String, ByVal name As String)
_id = id
_name = name
End Sub
Protected Overrides Sub Initialize(source As String)
Dim strs() As String = source.Split(","c)
_id = strs(0)
_name = strs(1)
End Sub
End Class
Example usage:
Dim userRecord As User = Record.fromString(Of User)("1,2")