singleton pattern in vb

Question

I am normally a c# programmer but am now working in VB for this one project when I use to set up a singleton class I would follow the Jon Skeet model

public sealed class Singleton
{
    static Singleton instance = null;
    static readonly object padlock = new object();

    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            lock (padlock)
            {
                if (instance == null)
                {
                    instance = new Singleton();
                }
                return instance;
            }
        }
    }

    //Added to illustrate the point
    public static void a()
    {
    }

    public void b()
    {
    }

}

or one of the variations now if I write the statement in c#

Singleton.Instance What procedures is all of the members that are not static, b but not a.

Now when I do the same in VB

Private Shared _instance As StackTracker
Private Shared ReadOnly _lock As Object = New Object()
Private Sub New()
    _WorkingStack = New Stack(Of MethodObject)
    _HistoryStack = New Queue(Of MethodObject)
End Sub

Public Shared ReadOnly Property Instance() As StackTracker
    Get
        SyncLock _lock
            If (_instance Is Nothing) Then
                _instance = New StackTracker()
            End If
        End SyncLock

        Return _instance
    End Get

End Property

I get StackTracker.Instance.Instance and it keeps going, while it is not the end of the world it looks bad.

Question is there a way in VB to hide the second instance so the user can not recursively call Instance?

user_5818981 · Answer

Maybe I'm missing something but I just do some variation on this, depending on what else is going on in the class:

Class MySingleton

    'The instance initializes once and persists (provided it's not intentionally destroyed)
    Private Shared oInstance As MySingleton = New MySingleton

    'A property initialized via the Create method
    Public Shared Property SomeProperty() As Object = Nothing

   'Constructor cannot be called directly so prevents external instantiation
    Private Sub New()
        'Nothing to do
    End Sub

    'The property returns the single instance
    Public Shared ReadOnly Property Instance As MySingleton
        Get
            Return oInstance
        End Get
    End Property

    'The method returns the single instance while also initializing SomeProperty
    Public Shared Function Create(
        ByVal SomeParam As Object) As MySingleton

        _SomeProperty = SomeParam
        Return oInstance
    End Function
End Class

Obviously, you would usually only provide either the Instance property or the Create method, not both (though you could if you wanted to for some reason).

In its simplest form it's:

Class MySingleton

    Private Shared oInstance As MySingleton = New MySingleton

    Private Sub New()
    End Sub

    Public Shared ReadOnly Property Instance As MySingleton
        Get
            Return oInstance
        End Get
    End Property
End Class

singleton pattern in vb

4 Answers