Lazy(Of T) Constructor (Func(Of T))

lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject)

' The following lines show how to use other constructors to achieve exactly the 
' same result as the previous line:  
'lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject, True) 
'lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject, LazyThreadSafetyMode.ExecutionAndPublication)

Try 
    Dim large As LargeObject = lazyLargeObject.Value

    ' IMPORTANT: Lazy initialization is thread-safe, but it doesn't protect the   
    '            object after creation. You must lock the object before accessing it, 
    '            unless the type is thread safe. (LargeObject is not thread safe.) 
    SyncLock large
        large.Data(0) = Thread.CurrentThread.ManagedThreadId
        Console.WriteLine("Initialized by thread {0}; last used by thread {1}.", _
            large.InitializedBy, large.Data(0))
    End SyncLock 
Catch aex As ApplicationException
    Console.WriteLine("Exception: {0}", aex.Message)
End Try

Imports System
Imports System.Threading

Friend Class Program
    Private Shared lazyLargeObject As Lazy(Of LargeObject) = Nothing 

    Private Shared Function InitLargeObject() As LargeObject
        Return New LargeObject()
    End Function 


    Shared Sub Main()
        ' The lazy initializer is created here. LargeObject is not created until the  
        ' ThreadProc method executes.
        lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject)

        ' The following lines show how to use other constructors to achieve exactly the 
        ' same result as the previous line:  
        'lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject, True) 
        'lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject, LazyThreadSafetyMode.ExecutionAndPublication)


        Console.WriteLine(vbCrLf _
            & "LargeObject is not created until you access the Value property of the lazy" _
            & vbCrLf & "initializer. Press Enter to create LargeObject.")
        Console.ReadLine()

        ' Create and start 3 threads, each of which tries to use LargeObject. 
        Dim threads() As Thread = { New Thread(AddressOf ThreadProc), _
            New Thread(AddressOf ThreadProc), New Thread(AddressOf ThreadProc) }
        For Each t As Thread In threads
            t.Start()
        Next t

        ' Wait for all 3 threads to finish. (The order doesn't matter.) 
        For Each t As Thread In threads
            t.Join()
        Next t

        Console.WriteLine(vbCrLf & "Press Enter to end the program")
        Console.ReadLine()
    End Sub 


    Private Shared Sub ThreadProc(ByVal state As Object)
        Try 
            Dim large As LargeObject = lazyLargeObject.Value

            ' IMPORTANT: Lazy initialization is thread-safe, but it doesn't protect the   
            '            object after creation. You must lock the object before accessing it, 
            '            unless the type is thread safe. (LargeObject is not thread safe.) 
            SyncLock large
                large.Data(0) = Thread.CurrentThread.ManagedThreadId
                Console.WriteLine("Initialized by thread {0}; last used by thread {1}.", _
                    large.InitializedBy, large.Data(0))
            End SyncLock 
        Catch aex As ApplicationException
            Console.WriteLine("Exception: {0}", aex.Message)
        End Try 
    End Sub 
End Class 

Friend Class LargeObject
    Private initBy As Integer = 0
    Public ReadOnly Property InitializedBy() As Integer 
        Get 
            Return initBy
        End Get 
    End Property 

    Private Shared instanceCount As Integer = 0
    Public Sub New()
        If 1 = Interlocked.Increment(instanceCount) Then 
            Throw New ApplicationException("Throw only ONCE.")
        End If

        initBy = Thread.CurrentThread.ManagedThreadId
        Console.WriteLine("LargeObject was created on thread id {0}.", initBy)
    End Sub 
    Public Data(99999999) As Long 
End Class 

' This example produces output similar to the following: 
' 
'LargeObject is not created until you access the Value property of the lazy 
'initializer. Press Enter to create LargeObject. 
' 
'Exception: Throw only ONCE. 
'Exception: Throw only ONCE. 
'Exception: Throw only ONCE. 
' 
'Press Enter to end the program 
'