how to initialize a Singleton?

Sometimes there is a need to initialize the singleton class with some helper values. But we can't "publish" a constructor for it. What is the workaround for this?

Attention! overloading the GetInstance or setting a color is not my idea. The color should be set only once. I would like to be sure that MyPainter p开发者_运维问答aints ONLY with the initialized color. Any default color should ever be used.

For more clarity I provide a sample:

''' <summary>
''' Singleton class MyPainter
''' </summary>
Public Class MyPainter
  Private Shared _pen As Pen
  Private Shared _instance As MyPainter = Nothing

  Private Sub New()
  End Sub

  ''' <summary>
  ''' This method should be called only once, like a constructor!
  ''' </summary>
  Public Shared Sub InitializeMyPainter(ByVal defaultPenColor As Color)
    _pen = New Pen(defaultPenColor)
  End Sub


  Public Shared Function GetInstance() As MyPainter
    If _instance Is Nothing Then
      _instance = New MyPainter
    End If

    Return _instance
  End Function

  Public Sub DrawLine(ByVal g As Graphics, ByVal pointA As Point, ByVal pointB As Point)
    g.DrawLine(_pen, pointA, pointB)
  End Sub

End Class

Thanks.

If you want to initialize it only once upon creation, why not to do that inside the constructor by calling some method, that will pull parameters from somewhere? If this initialization will be called several times - transform it into separate method like setOptions.

Could you not just overload your GetInstance method?

Dim _IsInitialized as boolean = false    
Public Shared ReadOnly Property IsInitialized() As Boolean
        Get
            Return _IsInitialized
        End Get
    End Property


Public Shared Function GetInstance() As MyPainter
        If _instance Is Nothing Then
            _instance = New MyPainter
        End If

        _IsInitialized = True
        Return _instance
    End Function

    Public Overloads Shared Function GetInstance(ByVal DefaultPenColor As Color) As MyPainter
        If _instance Is Nothing Then
            _instance = New MyPainter
            InitializeMyPainter(DefaultPenColor)
        End If

        _IsInitialized = True
        Return _instance
    End Function

Before calling the GetInstance method, check to see if it has been intialized yet. If not give it a pen color.

If MyPainter.IsInitialized then
     Dim Painter as MyPainter = MyPainter.GetInstance
Else
     Dim Painter as MyPainter - MyPainter.GetInstance(System.Drawing.Color.Red)
EndIf

You can declare Shared constructors but they must be parameterless:

Public Class MyPainter
    Private Shared _pen As Pen

    Shared Sub New()
        _pen = New Pen(Color.White)
    End Sub

    ....

End Class

You could set the color to a sensible default in the constructor and then add a helper method to change the color if desired, much like you have in your sample:

Public Shared Sub SetColor(ByVal defaultPenColor As Color)
    _pen = New Pen(defaultPenColor)
End Sub

You could use an optional parameter in GetInstance

Public Shared Function GetInstance(Optional ByVal defaultPenColor As Color = Colors.Red) As MyPainter
    If _instance Is Nothing Then
      If defaultPenColor is nothing then
          defaultPenColor = Colors.Red
      end if
      _pen = New Pen(defaultPenColor)
      _instance = New MyPainter
    End If

    Return _instance
End Function

Create a CreateInstance method that takes the required parameters. And remove the creation ability from GetInstance. Not totally a singleton then, but at least you KNOW things are going to be initialized correctly.

You could use an Init-method: (It's an C# example, I just ran into the same issue with initializable singletons)

public sealed class XSingleton
{
    private static XSingleton instance = null;
    int value = -1;

    private XSingleton(int v)
    {
        value = v;
    }

    public static void Init(int v)
    {
        if (instance != null) throw new Exception("Init() must be called only once!");
        instance = new XSingleton(v);
    }

    public static XSingleton Instance
    {
        get
        {
            if (instance == null) throw new Exception("Call Init() before!");
            return instance;
        }
    }

    public int Value { get { return value; } }
}

If you try to use XSingleton unitinialized or try to initialize more than once, you get an exception.

So you initialize the singleton with

XSingleton.Init(123);

Now you can use the initialized Value:

Console.WriteLine(XSingleton.Instance.Value);