How To MustOverride Shared/Constructor Function Visual Basic

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?
Thanks in advanced for the help :)

edited Nov 21 '18 at 16:44

asked Nov 21 '18 at 2:34

Kieran Powell

305

1

Unfortunately, there is no way in .NET to enforce the existence of a Shared Sub or Function. I've felt the lack before. In one case, it's a factory method, and since I'm already using reflection to implement the factory, I just require the Shared member to be provided by convention (and get it via reflection). In another case, a routine that logically should be shared ends up being a member function just so that I can require it to be present on a generic argument.

– Craig
Nov 21 '18 at 15:32

1

Why would your college ask you to write a database application without a database?

– Mary
Nov 21 '18 at 22:30

add a comment |

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?
Thanks in advanced for the help :)

edited Nov 21 '18 at 16:44

asked Nov 21 '18 at 2:34

Kieran Powell

305

1

Unfortunately, there is no way in .NET to enforce the existence of a Shared Sub or Function. I've felt the lack before. In one case, it's a factory method, and since I'm already using reflection to implement the factory, I just require the Shared member to be provided by convention (and get it via reflection). In another case, a routine that logically should be shared ends up being a member function just so that I can require it to be present on a generic argument.

– Craig
Nov 21 '18 at 15:32

1

Why would your college ask you to write a database application without a database?

– Mary
Nov 21 '18 at 22:30

add a comment |

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?
Thanks in advanced for the help :)

edited Nov 21 '18 at 16:44

asked Nov 21 '18 at 2:34

Kieran Powell

305

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?
Thanks in advanced for the help :)

database vb.net inheritance constructor shared

edited Nov 21 '18 at 16:44

asked Nov 21 '18 at 2:34

Kieran Powell

305

edited Nov 21 '18 at 16:44

asked Nov 21 '18 at 2:34

Kieran Powell

305

edited Nov 21 '18 at 16:44

asked Nov 21 '18 at 2:34

Kieran Powell

305

asked Nov 21 '18 at 2:34

Kieran Powell

305

asked Nov 21 '18 at 2:34

Kieran Powell

305

1

Unfortunately, there is no way in .NET to enforce the existence of a Shared Sub or Function. I've felt the lack before. In one case, it's a factory method, and since I'm already using reflection to implement the factory, I just require the Shared member to be provided by convention (and get it via reflection). In another case, a routine that logically should be shared ends up being a member function just so that I can require it to be present on a generic argument.

– Craig
Nov 21 '18 at 15:32

1

Why would your college ask you to write a database application without a database?

– Mary
Nov 21 '18 at 22:30

add a comment |

1

Unfortunately, there is no way in .NET to enforce the existence of a Shared Sub or Function. I've felt the lack before. In one case, it's a factory method, and since I'm already using reflection to implement the factory, I just require the Shared member to be provided by convention (and get it via reflection). In another case, a routine that logically should be shared ends up being a member function just so that I can require it to be present on a generic argument.

– Craig
Nov 21 '18 at 15:32

1

Why would your college ask you to write a database application without a database?

– Mary
Nov 21 '18 at 22:30

Unfortunately, there is no way in .NET to enforce the existence of a Shared Sub or Function. I've felt the lack before. In one case, it's a factory method, and since I'm already using reflection to implement the factory, I just require the Shared member to be provided by convention (and get it via reflection). In another case, a routine that logically should be shared ends up being a member function just so that I can require it to be present on a generic argument.

– Craig
Nov 21 '18 at 15:32

Why would your college ask you to write a database application without a database?

– Mary
Nov 21 '18 at 22:30

add a comment |

2 Answers
2

active

oldest

votes

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")

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

I do like this solution. Although I have a working code, I may give this approach a shot as it is more elegant than mine. It also ties in well with how some of my other code works. Thank you very much :)

– Kieran Powell
Nov 21 '18 at 14:04

add a comment |

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.

edited Nov 21 '18 at 2:56

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

Thanks for your time and help. I think what I will do is make a MustOverride Sub like setFromString(ByVal str As String) Which I can use after the instance of the object has been created. It seems to be the only way I can think of to make it enforced.

– Kieran Powell
Nov 21 '18 at 3:50

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

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")

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

I do like this solution. Although I have a working code, I may give this approach a shot as it is more elegant than mine. It also ties in well with how some of my other code works. Thank you very much :)

– Kieran Powell
Nov 21 '18 at 14:04

add a comment |

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")

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

I do like this solution. Although I have a working code, I may give this approach a shot as it is more elegant than mine. It also ties in well with how some of my other code works. Thank you very much :)

– Kieran Powell
Nov 21 '18 at 14:04

add a comment |

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")

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

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")

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

answered Nov 21 '18 at 4:43

TnTinMn

6,32931027

I do like this solution. Although I have a working code, I may give this approach a shot as it is more elegant than mine. It also ties in well with how some of my other code works. Thank you very much :)

– Kieran Powell
Nov 21 '18 at 14:04

add a comment |

I do like this solution. Although I have a working code, I may give this approach a shot as it is more elegant than mine. It also ties in well with how some of my other code works. Thank you very much :)

– Kieran Powell
Nov 21 '18 at 14:04

I do like this solution. Although I have a working code, I may give this approach a shot as it is more elegant than mine. It also ties in well with how some of my other code works. Thank you very much :)

– Kieran Powell
Nov 21 '18 at 14:04

add a comment |

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

edited Nov 21 '18 at 2:56

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

Thanks for your time and help. I think what I will do is make a MustOverride Sub like setFromString(ByVal str As String) Which I can use after the instance of the object has been created. It seems to be the only way I can think of to make it enforced.

– Kieran Powell
Nov 21 '18 at 3:50

add a comment |

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

edited Nov 21 '18 at 2:56

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

Thanks for your time and help. I think what I will do is make a MustOverride Sub like setFromString(ByVal str As String) Which I can use after the instance of the object has been created. It seems to be the only way I can think of to make it enforced.

– Kieran Powell
Nov 21 '18 at 3:50

add a comment |

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

edited Nov 21 '18 at 2:56

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

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

edited Nov 21 '18 at 2:56

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

edited Nov 21 '18 at 2:56

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

answered Nov 21 '18 at 2:43

jmcilhinney

25.7k22032

Thanks for your time and help. I think what I will do is make a MustOverride Sub like setFromString(ByVal str As String) Which I can use after the instance of the object has been created. It seems to be the only way I can think of to make it enforced.

– Kieran Powell
Nov 21 '18 at 3:50

add a comment |

Thanks for your time and help. I think what I will do is make a MustOverride Sub like setFromString(ByVal str As String) Which I can use after the instance of the object has been created. It seems to be the only way I can think of to make it enforced.

– Kieran Powell
Nov 21 '18 at 3:50

Thanks for your time and help. I think what I will do is make a MustOverride Sub like setFromString(ByVal str As String) Which I can use after the instance of the object has been created. It seems to be the only way I can think of to make it enforced.

– Kieran Powell
Nov 21 '18 at 3:50

add a comment |

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