单例模式确保某对象只能实例化一次，因此可以确保程序中所有对象访问同一个对象，但是也存在一个弊端，如，不能解决删除单个对象的问题等，因为缺少实际的开发运用，对性能以及单例的优缺点就不会太明白，这里重点总结一下单例模式的5种拓展实现。

原文详见：http://www.cnblogs.com/psunny/archive/2010/06/18/1760133.html

1 、单例模式的简单实现：（这种方式创建是线程不安全的）

Public NotInheritable Class UserDataReaderToEntityStrategy

    Shared singleInstance As UserDataReaderToEntityStrategy = Nothing

    '私有化构造函数
    Private Sub New()

    End Sub
    '创建静态方法，用于取得全局唯一实例

    Public Shared ReadOnly Property GetInstance() As UserDataReaderToEntityStrategy
        Get
            If singleInstance Is Nothing Then
                singleInstance = New UserDataReaderToEntityStrategy
            End If
            Return singleInstance
        End Get
    End Property

End Class

2、安全线程的实现

Public NotInheritable Class UserDataReaderToEntityStrategy

    Shared singleInstance As UserDataReaderToEntityStrategy = Nothing
    Shared ReadOnly padlock As New Object()

    '私有化构造函数
    Private Sub New()

    End Sub

    Public Shared ReadOnly Property GetInstance() As UserDataReaderToEntityStrategy
        Get
            '这里添加一个锁，保证了线程的安全
            SyncLock padlock
                If singleInstance Is Nothing Then
                    singleInstance = New UserDataReaderToEntityStrategy
                End If
            End SyncLock
            Return singleInstance
        End Get
    End Property

End Class

3、双重锁定

Public NotInheritable Class UserDataReaderToEntityStrategy

    Shared singleInstance As UserDataReaderToEntityStrategy = Nothing
    Shared ReadOnly padlock As New Object()

    '私有化构造函数
    Private Sub New()
    End Sub

    Public Shared ReadOnly Property GetInstance() As UserDataReaderToEntityStrategy
        Get
            '这里添加一个锁，保证了线程的安全

            If singleInstance Is Nothing Then
                SyncLock padlock
                    If singleInstance Is Nothing Then
                        singleInstance = New UserDataReaderToEntityStrategy
                    End If
                End SyncLock
            End If
            Return singleInstance
        End Get
    End Property

End Class

4、静态初始化（首选方式）

Public NotInheritable Class UserDataReaderToEntityStrategy

    Shared singleInstance As UserDataReaderToEntityStrategy = Nothing

    Shared Sub New()

    End Sub
    '私有化构造函数
    Private Sub New()
    End Sub

    Public Shared ReadOnly Property GetInstance() As UserDataReaderToEntityStrategy
        Get
            Return singleInstance
        End Get
    End Property

End Class

5、延迟初始化（比较常用）

Public NotInheritable Class UserDataReaderToEntityStrategy

    Shared singleInstance As UserDataReaderToEntityStrategy = Nothing

   
    '私有化构造函数
    Private Sub New()
    End Sub

    Public Shared ReadOnly Property GetInstance() As UserDataReaderToEntityStrategy
        Get
            Return Nested.singleInstance
        End Get
    End Property
    Private Class Nested
        Shared Sub New()

        End Sub
        Friend Shared ReadOnly singleInstance As New UserDataReaderToEntityStrategy()
    End Class
End Class