Singleton classes and interfaces

Question

Hi

I am trying to create an interface and then about 3 or 4 singleton classes that would be accessed via the interface.

I know how to get interfaces to work and I know how to create concrete singleton classes and access them via GetInstance(). But when I put the two together, failure results.

My problem is in the GetInstance method of the concrete singletons as in

Public Class MovingList
implements ISelectedList

Public Shared Function GetInstance ()  as MovingList _ Implements ISelectedList.GetInstance()

end function

end class

ISelectedList is the interface. This fails because ISelectedList.GetInstance is not of the type MovingList. Plus there is a problem with the use of Shared that I think is solved by simply deleting it (interfaces implicitly share).

I can't define the type of ISelectedList.GetInstance because each of my singleton classes is of a different type even though they implement exactly the same properties and methods.

How best to implement such singleton classes? I could do away with the interface and simply deal directly with the concrete singletons but that feels like bad practice. Programming to an abstraction not an implementation and all that.

Any and all help gratefully appreciated.

Doug

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VBDataStarter

Answer 1

You can't implement interface with shared members. Either you should re-consider using singletons or re-consider using interfaces.

If you want to have different return types for your methods that implement a common interface, make the interface generic:

Public Interface IGenericInterface(Of T)
    Function Method() As T

End Interface

Public Class ImplementationA
    Implements IGenericInterface(Of ImplementationA)

    Public Function Method() As ImplementationA Implements IGenericInterface(Of ImplementationA).Method

    End Function
End Class

Public Class ImplementationB
    Implements IGenericInterface(Of String)

    Public Function Method() As String Implements IGenericInterface(Of String).Method

    End Function
End Class

Public Class ImplementationC
    Implements IGenericInterface(Of ImplementationB)

    Public Function Method() As ImplementationB Implements IGenericInterface(Of ImplementationB).Method

    End Function
End Class

Answer 2

 3 or 4 singleton classes ??

This don't sound right to start with.  If there is 3 or 4, they are not singleton

What is your goal exactly??

Answer 3

Thanks for this. I see how it works.

So there is no way I can use a singleton class with an interface? I have to program to the concrete singleton class?

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VBDataStarter

Answer 4

 3 or 4 singleton classes ??

This don't sound right to start with.  If there is 3 or 4, they are not singleton

What is your goal exactly??

Thanks Crazypennie

Well they are singletons in that I want one and only instance of the class to exist at any given time. However, I do need each of the different class to exist at the same time.

The reason for making them singletons is that they are data classes that hold different data that is used by my application at many differrent points. Quite often the data will be used by a process, that process will need to be interrupted while another action is completed which changes the data held in one or more of the data classes. Then the original process will continue using the changed data. Given the unpredicatability of these interruptions (perhaps better put as given the high variability of when these interruptions take place and what changes are made to the data), I am very worried about data integrity and the potential for creating multiple, differing versions of the data.

So my thought was to hold specific data in singleton classes so that only one copy could exist and all changes would be made to the same instance of the data.

The same instance of these classes does not persist throughout the life of the application. The singleton classes will constantly be created and destroyed as particular records from the database are used and changed by the application. At the end of the processing, the data in the data classes is written back to the database. I don't want read/write to the database when every change is made as the application requires frequent changing of data, often multiple times within a method.

 That's my problem. Perhaps using singleton classes is a bad solution. Other ideas?

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VBDataStarter

Answer 5

In that case, you can use an interface to access the different singleton via the interface

Look at this

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim somecondition As Boolean

        Dim Data As ISelectedList

        If SomeCondition Then
            Data = MovingList_1.GetInstance
        Else
            Data = MovingList_2.GetInstance
        End If

    End Sub
End Class




Public Interface ISelectedList
    'Just put nothing here - Empty interface
End Interface



Public Class MovingList_1 : Implements ISelectedList


    Private Shared Instance As MovingList_1

    Private Sub New()
    End Sub

    Public Shared Function GetInstance() As MovingList_1
        If Instance Is Nothing Then Instance = New MovingList_1
        Return Instance
    End Function

    Public Property data() As String

End Class




Public Class MovingList_2 : Implements ISelectedList

    Private Shared Instance As MovingList_2

    Private Sub New()
    End Sub

    Public Shared Function GetInstance() As MovingList_2
        If Instance Is Nothing Then Instance = New MovingList_2
        Return Instance
    End Function

    Public Property data() As String

End Class

and you can also use an interface like this if some property or method need to be access

Public Interface ISelectedList
    Property data() As String
End Interface

Answer 6

Hi Crazypennie

OK. Thanks, I see how this works. If I understand it properly, this would allow me to access methods and properties via the interface, with all the benefits that this brings. It requires me to instantiate the concrete classes directly rather than via the interface. This would certainly work. It feels like a bit of a workaround rather than programming elegance but I guess sometimes that is what is required.

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VBDataStarter