Lazy<T> Lazy<T> Lazy<T> Lazy<T> Constructors

Definition

Overloads

Lazy<T>() Lazy<T>() Lazy<T>()

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the default constructor of the target type is used.

Lazy<T>(Boolean) Lazy<T>(Boolean) Lazy<T>(Boolean) Lazy<T>(Boolean)

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the default constructor of the target type and the specified initialization mode are used.

Lazy<T>(Func<T>) Lazy<T>(Func<T>) Lazy<T>(Func<T>) Lazy<T>(Func<T>)

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the specified initialization function is used.

Lazy<T>(LazyThreadSafetyMode) Lazy<T>(LazyThreadSafetyMode) Lazy<T>(LazyThreadSafetyMode) Lazy<T>(LazyThreadSafetyMode)

Initializes a new instance of the Lazy<T> class that uses the default constructor of T and the specified thread-safety mode.

Lazy<T>(T) Lazy<T>(T) Lazy<T>(T) Lazy<T>(T)

Initializes a new instance of the Lazy<T> class that uses a preinitialized specified value.

Lazy<T>(Func<T>, Boolean) Lazy<T>(Func<T>, Boolean) Lazy<T>(Func<T>, Boolean) Lazy<T>(Func<T>, Boolean)

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the specified initialization function and initialization mode are used.

Lazy<T>(Func<T>, LazyThreadSafetyMode) Lazy<T>(Func<T>, LazyThreadSafetyMode) Lazy<T>(Func<T>, LazyThreadSafetyMode) Lazy<T>(Func<T>, LazyThreadSafetyMode)

Initializes a new instance of the Lazy<T> class that uses the specified initialization function and thread-safety mode.

Lazy<T>() Lazy<T>() Lazy<T>()

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the default constructor of the target type is used.

public:
 Lazy();
public Lazy ();
Public Sub New ()

Examples

The following example demonstrates the use of this constructor. It also illustrates the use of the Lazy<T>(Boolean) constructor (specifying true for isThreadSafe) and the Lazy<T>(LazyThreadSafetyMode) constructor (specifying LazyThreadSafetyMode.ExecutionAndPublication for mode). To switch to a different constructor, just change which constructors are commented out.

The example defines a LargeObject class that will be initialized lazily by one of several threads. The two key lines of code in this example are the creation of the initializer and the actual initialization. At the beginning of the Main method, the example creates the thread-safe lazy initializer for LargeObject:

lazyLargeObject = new Lazy<LargeObject>();

// The following lines show how to use other constructors to achieve exactly the
// same result as the previous line: 
//lazyLargeObject = new Lazy<LargeObject>(true);
//lazyLargeObject = new Lazy<LargeObject>(LazyThreadSafetyMode.ExecutionAndPublication);
lazyLargeObject = New Lazy(Of LargeObject)()

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

The example creates and starts three threads that block on a ManualResetEvent object, so that the example can release the threads all at once. The ThreadProc method that's used by all three threads calls the Value property to get the LargeObject instance:

LargeObject large = lazyLargeObject.Value;
Dim large As LargeObject = lazyLargeObject.Value

The Lazy<T> class provides locking, so that only one thread is allowed to create the LargeObject instance. The example demonstrates that the other threads all get the same instance.

Note

For simplicity, this example uses a global instance of Lazy<T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.

using System;
using System.Threading;

class Program
{
    static Lazy<LargeObject> lazyLargeObject = null;

    static void Main()
    {
        // The lazy initializer is created here. LargeObject is not created until the 
        // ThreadProc method executes.
        lazyLargeObject = new Lazy<LargeObject>();

        // The following lines show how to use other constructors to achieve exactly the
        // same result as the previous line: 
        //lazyLargeObject = new Lazy<LargeObject>(true);
        //lazyLargeObject = new Lazy<LargeObject>(LazyThreadSafetyMode.ExecutionAndPublication);


        Console.WriteLine(
            "\r\nLargeObject is not created until you access the Value property of the lazy" +
            "\r\ninitializer. Press Enter to create LargeObject.");
        Console.ReadLine();

        // Create and start 3 threads, passing the same blocking event to all of them.
        ManualResetEvent startingGate = new ManualResetEvent(false);
        Thread[] threads = { new Thread(ThreadProc), new Thread(ThreadProc), new Thread(ThreadProc) };
        foreach (Thread t in threads)
        {
            t.Start(startingGate);
        }

        // Give all 3 threads time to start and wait, then release them all at once.
        Thread.Sleep(100);
        startingGate.Set();

        // Wait for all 3 threads to finish. (The order doesn't matter.)
        foreach (Thread t in threads)
        {
            t.Join();
        }

        Console.WriteLine("\r\nPress Enter to end the program");
        Console.ReadLine();
    }


    static void ThreadProc(object state)
    {
        // Wait for the signal.
        ManualResetEvent waitForStart = (ManualResetEvent) state;
        waitForStart.WaitOne();

        LargeObject large = lazyLargeObject.Value;

        // The following line introduces an artificial delay, to exaggerate the race 
        // condition.
        Thread.Sleep(5); 

        // 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.)
        lock(large)
        {
            large.Data[0] = Thread.CurrentThread.ManagedThreadId;
            Console.WriteLine("Initialized by thread {0}; last used by thread {1}.", 
                large.InitializedBy, large.Data[0]);
        }
    }
}

class LargeObject
{
    int initBy = 0;
    public int InitializedBy { get { return initBy; } }

    public LargeObject()
    {
        initBy = Thread.CurrentThread.ManagedThreadId;
        Console.WriteLine("LargeObject was created on thread id {0}.", initBy);
    }
    public long[] Data = new long[100000000];
}

/* 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.

LargeObject was created on thread id 4.
Initialized by thread 4; last used by thread 3.
Initialized by thread 4; last used by thread 4.
Initialized by thread 4; last used by thread 5.

Press Enter to end the program
 */
Imports System
Imports System.Threading

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

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

        ' The following lines show how to use other constructors to achieve exactly the
        ' same result as the previous line: 
        'lazyLargeObject = New Lazy(Of LargeObject)(True)
        'lazyLargeObject = New Lazy(Of LargeObject)(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, passing the same blocking event to all of them.
        Dim startingGate As New ManualResetEvent(False)
        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(startingGate)
        Next t

        ' Give all 3 threads time to start and wait, then release them all at once.
        Thread.Sleep(100)
        startingGate.Set()

        ' 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)
        ' Wait for the signal.
        Dim waitForStart As ManualResetEvent = CType(state, ManualResetEvent)
        waitForStart.WaitOne()

        Dim large As LargeObject = lazyLargeObject.Value

        ' The following line introduces an artificial delay, to exaggerate the race 
        ' condition.
        Thread.Sleep(5)

        ' 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
    End Sub
End Class

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

    Public Sub New()
        initBy = Thread.CurrentThread.ManagedThreadId
        Console.WriteLine("LargeObject was created on thread id {0}.", initBy)
    End Sub
    Public Data(100000000) 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.
'
'LargeObject was created on thread id 3.
'Initialized by thread 3; last used by thread 5.
'Initialized by thread 3; last used by thread 4.
'Initialized by thread 3; last used by thread 3.
'
'Press Enter to end the program

Remarks

An instance that is created with this constructor may be used concurrently from multiple threads.

The thread safety mode of a Lazy<T> instance that is initialized with this constructor is LazyThreadSafetyMode.ExecutionAndPublication. The thread safety mode describes the behavior when multiple threads try to initialize the Lazy<T> instance.

A Lazy<T> instance that is created with this constructor does not cache exceptions. For more information, see the Lazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

See also

Lazy<T>(Boolean) Lazy<T>(Boolean) Lazy<T>(Boolean) Lazy<T>(Boolean)

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the default constructor of the target type and the specified initialization mode are used.

public:
 Lazy(bool isThreadSafe);
public Lazy (bool isThreadSafe);
new Lazy<'T> : bool -> Lazy<'T>
Public Sub New (isThreadSafe As Boolean)

Parameters

isThreadSafe
Boolean Boolean Boolean Boolean

true to make this instance usable concurrently by multiple threads; false to make the instance usable by only one thread at a time.

Examples

The following example demonstrates the use of this constructor to create a lazy initializer that is not thread safe, for scenarios where all access to the lazily initialized object occurs on the same thread. It also demonstrates the use of the Lazy<T>(LazyThreadSafetyMode) constructor (specifying LazyThreadSafetyMode.None for mode. To switch to a different constructor, just change which constructor is commented out.

Note

For code that demonstrates how to use this constructor in multithreaded scenarios (specifying true for isThreadSafe), see the example for the Lazy<T>() constructor.

The example defines a LargeObject class that will be initialized lazily. In the Main method, the example creates a Lazy<T> instance and then pauses. When you press the Enter key, the example accesses the Value property of the Lazy<T> instance, which causes initialization to occur. The constructor of the LargeObject class displays a console message.

Note

For simplicity, this example uses a global instance of Lazy<T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.

using System;
using System.Threading;

class Program
{
    static Lazy<LargeObject> lazyLargeObject = null;

    static void Main()
    {
        // The lazy initializer is created here. LargeObject is not created until the 
        // ThreadProc method executes.
        lazyLargeObject = new Lazy<LargeObject>(false);

        // The following lines show how to use other constructors to achieve exactly the
        // same result as the previous line: 
        //lazyLargeObject = new Lazy<LargeObject>(LazyThreadSafetyMode.None);


        Console.WriteLine(
            "\r\nLargeObject is not created until you access the Value property of the lazy" +
            "\r\ninitializer. Press Enter to create LargeObject.");
        Console.ReadLine();

        LargeObject large = lazyLargeObject.Value;

        large.Data[11] = 89;

        Console.WriteLine("\r\nPress Enter to end the program");
        Console.ReadLine();
    }
}

class LargeObject
{
    public LargeObject()
    {
        Console.WriteLine("LargeObject was created on thread id {0}.", 
            Thread.CurrentThread.ManagedThreadId);
    }
    public long[] Data = new long[100000000];
}

/* 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.

LargeObject was created on thread id 1.

Press Enter to end the program
 */
Imports System
Imports System.Threading

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

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

        ' The following lines show how to use other constructors to achieve exactly the
        ' same result as the previous line: 
        'lazyLargeObject = new Lazy<LargeObject>(LazyThreadSafetyMode.None);


        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()

        Dim large As LargeObject = lazyLargeObject.Value

        large.Data(11) = 89

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

Friend Class LargeObject
    Public Sub New()
        Console.WriteLine("LargeObject was created on thread id {0}.", _
            Thread.CurrentThread.ManagedThreadId)
    End Sub
    Public Data(100000000) 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.
'
'LargeObject was created on thread id 1.
'
'Press Enter to end the program

Remarks

The thread safety mode of a Lazy<T> instance that is initialized with this constructor is LazyThreadSafetyMode.ExecutionAndPublication if isThreadSafe is true; otherwise, the mode is LazyThreadSafetyMode.None. The thread safety mode describes the behavior when multiple threads try to initialize the Lazy<T> instance. To specify the LazyThreadSafetyMode.PublicationOnly mode, use the Lazy<T>(Func<T>, LazyThreadSafetyMode) or Lazy<T>(LazyThreadSafetyMode) constructor.

A Lazy<T> instance that is created with this constructor does not cache exceptions. For more information, see the Lazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

See also

Lazy<T>(Func<T>) Lazy<T>(Func<T>) Lazy<T>(Func<T>) Lazy<T>(Func<T>)

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the specified initialization function is used.

public:
 Lazy(Func<T> ^ valueFactory);
public Lazy (Func<T> valueFactory);
new Lazy<'T> : Func<'T> -> Lazy<'T>
Public Sub New (valueFactory As Func(Of T))

Parameters

valueFactory
Func<T>

The delegate that is invoked to produce the lazily initialized value when it is needed.

Exceptions

Examples

The following example demonstrates the use of this constructor to provide lazy initialization with exception caching. It also demonstrates the use of the Lazy<T>(Func<T>, Boolean) constructor (specifying true for isThreadSafe) and the Lazy<T>(Func<T>, LazyThreadSafetyMode) constructor (specifying LazyThreadSafetyMode.ExecutionAndPublication for mode). To switch to a different constructor, just change which constructors are commented out.

The example defines a LargeObject class that will be initialized lazily by one of several threads. The three key sections of code illustrate the creation of the initializer, the actual initialization, and the constructor of the LargeObject class, which demonstrates exception caching. At the beginning of the Main method, the example creates the thread-safe lazy initializer for LargeObject:

lazyLargeObject = new Lazy<LargeObject>(InitLargeObject);

// The following lines show how to use other constructors to achieve exactly the
// same result as the previous line: 
//lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, true);
//lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, LazyThreadSafetyMode.ExecutionAndPublication);
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)

The example creates and starts three threads. The ThreadProc method that's used by all three threads calls the Value property to get the LargeObject instance:

try
{
    LargeObject large = 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.)
    lock(large)
    {
        large.Data[0] = Thread.CurrentThread.ManagedThreadId;
        Console.WriteLine("Initialized by thread {0}; last used by thread {1}.", 
            large.InitializedBy, large.Data[0]);
    }
}
catch (ApplicationException aex)
{
    Console.WriteLine("Exception: {0}", aex.Message);
}
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

In the constructor of the LargeObject class, the third key section of code throws an exception the first time a LargeObject instance is created, but thereafter allows instance creation to occur:

static int instanceCount = 0;
public LargeObject()
{
    if (1 == Interlocked.Increment(ref instanceCount))
    {
        throw new ApplicationException("Throw only ONCE.");
    }

    initBy = Thread.CurrentThread.ManagedThreadId;
    Console.WriteLine("LargeObject was created on thread id {0}.", initBy);
}
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

When the example is run, the first thread that tries to create an instance of LargeObject fails, and the exception is caught. You might expect that the next thread would successfully create an instance, but the Lazy<T> object has cached the exception. Because of this, all three threads throw the exception.

Note

For simplicity, this example uses a global instance of Lazy<T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.

using System;
using System.Threading;

class Program
{
    static Lazy<LargeObject> lazyLargeObject = null;

    static LargeObject InitLargeObject()
    {
        return new LargeObject();
    }


    static void Main()
    {
        // The lazy initializer is created here. LargeObject is not created until the 
        // ThreadProc method executes.
        lazyLargeObject = new Lazy<LargeObject>(InitLargeObject);

        // The following lines show how to use other constructors to achieve exactly the
        // same result as the previous line: 
        //lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, true);
        //lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, LazyThreadSafetyMode.ExecutionAndPublication);


        Console.WriteLine(
            "\r\nLargeObject is not created until you access the Value property of the lazy" +
            "\r\ninitializer. Press Enter to create LargeObject.");
        Console.ReadLine();

        // Create and start 3 threads, each of which tries to use LargeObject.
        Thread[] threads = { new Thread(ThreadProc), new Thread(ThreadProc), new Thread(ThreadProc) };
        foreach (Thread t in threads)
        {
            t.Start();
        }

        // Wait for all 3 threads to finish. (The order doesn't matter.)
        foreach (Thread t in threads)
        {
            t.Join();
        }

        Console.WriteLine("\r\nPress Enter to end the program");
        Console.ReadLine();
    }


    static void ThreadProc(object state)
    {
        try
        {
            LargeObject large = 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.)
            lock(large)
            {
                large.Data[0] = Thread.CurrentThread.ManagedThreadId;
                Console.WriteLine("Initialized by thread {0}; last used by thread {1}.", 
                    large.InitializedBy, large.Data[0]);
            }
        }
        catch (ApplicationException aex)
        {
            Console.WriteLine("Exception: {0}", aex.Message);
        }
    }
}

class LargeObject
{
    int initBy = 0;
    public int InitializedBy { get { return initBy; } }

    static int instanceCount = 0;
    public LargeObject()
    {
        if (1 == Interlocked.Increment(ref instanceCount))
        {
            throw new ApplicationException("Throw only ONCE.");
        }

        initBy = Thread.CurrentThread.ManagedThreadId;
        Console.WriteLine("LargeObject was created on thread id {0}.", initBy);
    }
    public long[] Data = new long[100000000];
}

/* 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
 */
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
' 

Remarks

An instance that is created with this constructor may be used concurrently from multiple threads.

The thread safety mode of a Lazy<T> instance that is initialized with this constructor is LazyThreadSafetyMode.ExecutionAndPublication. The thread safety mode describes the behavior when multiple threads try to initialize the Lazy<T> instance.

Exceptions that are thrown by valueFactory are cached. For more information, see the Lazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

See also

Lazy<T>(LazyThreadSafetyMode) Lazy<T>(LazyThreadSafetyMode) Lazy<T>(LazyThreadSafetyMode) Lazy<T>(LazyThreadSafetyMode)

Initializes a new instance of the Lazy<T> class that uses the default constructor of T and the specified thread-safety mode.

public:
 Lazy(System::Threading::LazyThreadSafetyMode mode);
public Lazy (System.Threading.LazyThreadSafetyMode mode);
new Lazy<'T> : System.Threading.LazyThreadSafetyMode -> Lazy<'T>
Public Sub New (mode As LazyThreadSafetyMode)

Parameters

mode
LazyThreadSafetyMode LazyThreadSafetyMode LazyThreadSafetyMode LazyThreadSafetyMode

One of the enumeration values that specifies the thread safety mode.

Exceptions

Examples

The following example demonstrates the use of this constructor to create a lazy initializer that enables multiple threads to race to create an object lazily. Multiple threads might succeed in creating instances, but all threads use the instance that was created first.

Note

For an example that demonstrates how to use this constructor in single-threaded scenarios (specifying LazyThreadSafetyMode.None for mode), see the Lazy<T>(Boolean) constructor. For an example that demonstrates how to use this constructor to provide locking instead of race conditions in multithreaded scenarios (specifying LazyThreadSafetyMode.ExecutionAndPublication for mode), see the Lazy<T>() constructor.

The example defines a LargeObject class that will be initialized lazily by any of several threads. The three key sections of code illustrate the creation of the initializer, the actual initialization, and the constructor and finalizer of the LargeObject class. At the beginning of the Main method, the example creates the Lazy<T> object that performs lazy initialization of the LargeObject:

lazyLargeObject = new Lazy<LargeObject>(LazyThreadSafetyMode.PublicationOnly);
lazyLargeObject = New Lazy(Of LargeObject)(LazyThreadSafetyMode.PublicationOnly)

The example creates and starts three threads that block on a ManualResetEvent object, so that the example can release the threads all at once. In the ThreadProc method that's used by all three threads, calling the Value property creates the LargeObject instance:

LargeObject large = lazyLargeObject.Value;
Dim large As LargeObject = lazyLargeObject.Value

Because the constructor for the Lazy<T> instance specified LazyThreadSafetyMode.PublicationOnly, all three threads are allowed to create LargeObject instances. The example demonstrates this by displaying console messages in the constructor and in the finalizer of the LargeObject class:

public LargeObject() 
{ 
    initBy = Thread.CurrentThread.ManagedThreadId;
    Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy);
}

~LargeObject()
{
    Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy);
}
Public Sub New()
    initBy = Thread.CurrentThread.ManagedThreadId
    Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy)
End Sub

Protected Overrides Sub Finalize()
    Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy)
End Sub

However, the Lazy<T> object ensures that only one instance is used by all threads. The output from the example shows that all three threads use the same instance, and also shows that the other two instances can be reclaimed by garbage collection.

Note

For simplicity, this example uses a global instance of Lazy<T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.

using System;
using System.Threading;

class Program
{
    static Lazy<LargeObject> lazyLargeObject = null;

    static void Main()
    {
        // The lazy initializer is created here. LargeObject is not created until the 
        // ThreadProc method executes.
        lazyLargeObject = new Lazy<LargeObject>(LazyThreadSafetyMode.PublicationOnly);


        // Create and start 3 threads, passing the same blocking event to all of them.
        ManualResetEvent startingGate = new ManualResetEvent(false);
        Thread[] threads = { new Thread(ThreadProc), new Thread(ThreadProc), new Thread(ThreadProc) };
        foreach (Thread t in threads)
        {
            t.Start(startingGate);
        }

        // Give all 3 threads time to start and wait, then release them all at once.
        Thread.Sleep(50);
        startingGate.Set();

        // Wait for all 3 threads to finish. (The order doesn't matter.)
        foreach (Thread t in threads)
        {
            t.Join();
        }

        Console.WriteLine(
            "\r\nThreads are complete. Running GC.Collect() to reclaim the extra instances.");

        GC.Collect();

        // Allow time for garbage collection, which happens asynchronously.
        Thread.Sleep(100);

        Console.WriteLine( 
            "\r\nNote that all three threads used the instance that was not collected.");
        Console.WriteLine("Press Enter to end the program");
        Console.ReadLine();
    }


    static void ThreadProc(object state)
    {
        // Wait for the signal.
        ManualResetEvent waitForStart = (ManualResetEvent) state;
        waitForStart.WaitOne();

        LargeObject large = lazyLargeObject.Value;

        // The following line introduces an artificial delay, to exaggerate the race 
        // condition.
        Thread.Sleep(5); 

        // 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.)
        lock(large)
        {
            large.Data[0] = Thread.CurrentThread.ManagedThreadId;
            Console.WriteLine("LargeObject was initialized by thread {0}; last used by thread {1}.", 
                large.InitializedBy, large.Data[0]);
        }
    }
}

class LargeObject
{
    int initBy = -1;
    public int InitializedBy { get { return initBy; } }

    public LargeObject() 
    { 
        initBy = Thread.CurrentThread.ManagedThreadId;
        Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy);
    }

    ~LargeObject()
    {
        Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy);
    }
    
    public long[] Data = new long[100000000];
}

/* This example produces output similar to the following:

Constructor: Instance initializing on thread 4
Constructor: Instance initializing on thread 3
Constructor: Instance initializing on thread 5
LargeObject was initialized by thread 4; last used by thread 4.
LargeObject was initialized by thread 4; last used by thread 5.
LargeObject was initialized by thread 4; last used by thread 3.

Threads are complete. Running GC.Collect() to reclaim the extra instances.
Finalizer: Instance was initialized on 3
Finalizer: Instance was initialized on 5

Note that all three threads used the instance that was not collected.
Press Enter to end the program

Instance finalizing; initialized on 4
 */
Imports System
Imports System.Threading

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

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


        ' Create and start 3 threads, passing the same blocking event to all of them.
        Dim startingGate As New ManualResetEvent(False)
        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(startingGate)
        Next t

        ' Give all 3 threads time to start and wait, then release them all at once.
        Thread.Sleep(50)
        startingGate.Set()

        ' 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 & _
            "Threads are complete. Running GC.Collect() to reclaim the extra instances.")

        GC.Collect()

        ' Allow time for garbage collection, which happens asynchronously.
        Thread.Sleep(100)

        Console.WriteLine(vbCrLf & _
            "Note that all three threads used the instance that was not collected.")
        Console.WriteLine("Press Enter to end the program")
        Console.ReadLine()

    End Sub


    Private Shared Sub ThreadProc(ByVal state As Object)
        ' Wait for the signal.
        Dim waitForStart As ManualResetEvent = CType(state, ManualResetEvent)
        waitForStart.WaitOne()

        Dim large As LargeObject = lazyLargeObject.Value

        ' The following line introduces an artificial delay, to exaggerate the race 
        ' condition.
        Thread.Sleep(5)

        ' 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( _
                "LargeObject was initialized by thread {0}; last used by thread {1}.", _
                large.InitializedBy, large.Data(0))
        End SyncLock
    End Sub
End Class

Friend Class LargeObject
    Private initBy As Integer = -1
    Public ReadOnly Property InitializedBy() As Integer
        Get
            Return initBy
        End Get
    End Property

    Public Sub New()
        initBy = Thread.CurrentThread.ManagedThreadId
        Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy)
    End Sub

    Protected Overrides Sub Finalize()
        Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy)
    End Sub

    Public Data(100000000) As Long
End Class

' This example produces output similar to the following:
'
'Constructor: Instance initializing on thread 3
'Constructor: Instance initializing on thread 5
'Constructor: Instance initializing on thread 4
'LargeObject was initialized by thread 3; last used by thread 4.
'LargeObject was initialized by thread 3; last used by thread 3.
'LargeObject was initialized by thread 3; last used by thread 5.
'
'Threads are complete. Running GC.Collect() to reclaim the extra instances.
'Finalizer: Instance was initialized on 5
'Finalizer: Instance was initialized on 4
'
'Note that all three threads used the instance that was not collected.
'Press Enter to end the program
'
'Finalizer: Instance was initialized on 3
' 

Remarks

The thread safety mode of a Lazy<T> instance describes the behavior when multiple threads try to initialize the Lazy<T> instance.

A Lazy<T> instance that is created with this constructor does not cache exceptions. For more information, see the Lazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

See also

Lazy<T>(T) Lazy<T>(T) Lazy<T>(T) Lazy<T>(T)

Initializes a new instance of the Lazy<T> class that uses a preinitialized specified value.

public:
 Lazy(T value);
public Lazy (T value);
new Lazy<'T> : 'T -> Lazy<'T>
Public Sub New (value As T)

Parameters

value
T T T T

The preinitialized value to be used.

Remarks

An instance created with this constructor is usable by multiple threads concurrently.

Lazy<T>(Func<T>, Boolean) Lazy<T>(Func<T>, Boolean) Lazy<T>(Func<T>, Boolean) Lazy<T>(Func<T>, Boolean)

Initializes a new instance of the Lazy<T> class. When lazy initialization occurs, the specified initialization function and initialization mode are used.

public:
 Lazy(Func<T> ^ valueFactory, bool isThreadSafe);
public Lazy (Func<T> valueFactory, bool isThreadSafe);
new Lazy<'T> : Func<'T> * bool -> Lazy<'T>
Public Sub New (valueFactory As Func(Of T), isThreadSafe As Boolean)

Parameters

valueFactory
Func<T>

The delegate that is invoked to produce the lazily initialized value when it is needed.

isThreadSafe
Boolean Boolean Boolean Boolean

true to make this instance usable concurrently by multiple threads; false to make this instance usable by only one thread at a time.

Exceptions

Examples

The following example demonstrates the use of this constructor to provide lazy initialization with exception caching, in a scenario with a single thread. It also demonstrates the use of the Lazy<T> constructor (specifying LazyThreadSafetyMode.None for mode). To switch to that constructor, just change which constructor is commented out.

Note

For code that demonstrates how to use this constructor in multithreaded scenarios (specifying true for isThreadSafe), see the example for the Lazy<T>(Func<T>) constructor.

The example defines a LargeObject class that will be initialized lazily by one of several threads. The three key sections of code illustrate the creation of the initializer, the actual initialization, and the constructor of the LargeObject class, which demonstrates exception caching. At the beginning of the Main method, the example creates the thread-safe lazy initializer for LargeObject:

lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, false);

// The following lines show how to use other constructors to achieve exactly the
// same result as the previous line: 
//lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, LazyThreadSafetyMode.None);
lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject, False)

' 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, LazyThreadSafetyMode.None)

In the call to the constructor, the isThreadSafe parameter is false, so the Lazy<T> is not thread safe. Because it's not thread safe, the example calls the Value property three times on the same thread:

for (int i = 0; i < 3; i++)
{
    try
    {
        LargeObject large = lazyLargeObject.Value;
        large.Data[11] = 89;
    }
    catch (ApplicationException aex)
    {
        Console.WriteLine("Exception: {0}", aex.Message);
    }
}
For i As Integer = 0 To 2
    Try
        Dim large As LargeObject = lazyLargeObject.Value
        large.Data(11) = 89
    Catch aex As ApplicationException
        Console.WriteLine("Exception: {0}", aex.Message)
    End Try
Next i

In the constructor of the LargeObject class, the third key section of code throws an exception the first time a LargeObject instance is created, but thereafter allows instance creation to occur:

static bool pleaseThrow = true;
public LargeObject()
{
    if (pleaseThrow)
    {
        pleaseThrow = false;
        throw new ApplicationException("Throw only ONCE.");
    }

    Console.WriteLine("LargeObject was created on thread id {0}.", 
        Thread.CurrentThread.ManagedThreadId);
}
Private Shared pleaseThrow As Boolean = True
Public Sub New()
    If pleaseThrow Then
        pleaseThrow = False
        Throw New ApplicationException("Throw only ONCE.")
    End If

    Console.WriteLine("LargeObject was created on thread id {0}.", _
        Thread.CurrentThread.ManagedThreadId)
End Sub

When the example is run, the first attempt to create an instance of LargeObject fails, and the exception is caught. You might expect that the next attempt would succeed, but the Lazy<T> object has cached the exception. Because of this, all three attempts throw the exception.

Note

For simplicity, this example uses a global instance of Lazy<T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.

using System;
using System.Threading;

class Program
{
    static Lazy<LargeObject> lazyLargeObject = null;

    static LargeObject InitLargeObject()
    {
        return new LargeObject();
    }


    static void Main()
    {
        // The lazy initializer is created here. LargeObject is not created until the 
        // ThreadProc method executes.
        lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, false);

        // The following lines show how to use other constructors to achieve exactly the
        // same result as the previous line: 
        //lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, LazyThreadSafetyMode.None);


        Console.WriteLine(
            "\r\nLargeObject is not created until you access the Value property of the lazy" +
            "\r\ninitializer. Press Enter to create LargeObject (three tries).");
        Console.ReadLine();

        for (int i = 0; i < 3; i++)
        {
            try
            {
                LargeObject large = lazyLargeObject.Value;
                large.Data[11] = 89;
            }
            catch (ApplicationException aex)
            {
                Console.WriteLine("Exception: {0}", aex.Message);
            }
        }

        Console.WriteLine("\r\nPress Enter to end the program");
        Console.ReadLine();
    }
}

class LargeObject
{
    static bool pleaseThrow = true;
    public LargeObject()
    {
        if (pleaseThrow)
        {
            pleaseThrow = false;
            throw new ApplicationException("Throw only ONCE.");
        }

        Console.WriteLine("LargeObject was created on thread id {0}.", 
            Thread.CurrentThread.ManagedThreadId);
    }
    public long[] Data = new long[100000000];
}

/* 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 (three tries).

Exception: Throw only ONCE.
Exception: Throw only ONCE.
Exception: Throw only ONCE.

Press Enter to end the program
 */
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, False)

        ' 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, LazyThreadSafetyMode.None)


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

        For i As Integer = 0 To 2
            Try
                Dim large As LargeObject = lazyLargeObject.Value
                large.Data(11) = 89
            Catch aex As ApplicationException
                Console.WriteLine("Exception: {0}", aex.Message)
            End Try
        Next i

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

Friend Class LargeObject
    Private Shared pleaseThrow As Boolean = True
    Public Sub New()
        If pleaseThrow Then
            pleaseThrow = False
            Throw New ApplicationException("Throw only ONCE.")
        End If

        Console.WriteLine("LargeObject was created on thread id {0}.", _
            Thread.CurrentThread.ManagedThreadId)
    End Sub
    Public Data(100000000) 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 (three tries).
'
'Exception: Throw only ONCE.
'Exception: Throw only ONCE.
'Exception: Throw only ONCE.
'
'Press Enter to end the program
' 

Remarks

The thread safety mode of a Lazy<T> instance that is initialized with this constructor is LazyThreadSafetyMode.ExecutionAndPublication if isThreadSafe is true; otherwise, the mode is LazyThreadSafetyMode.None. The thread safety mode describes the behavior when multiple threads try to initialize the Lazy<T> instance.

To specify the LazyThreadSafetyMode.PublicationOnly mode, use the Lazy<T>(Func<T>, LazyThreadSafetyMode) or Lazy<T>(LazyThreadSafetyMode) constructor.

Exceptions that are thrown by valueFactory are cached. For more information, see the Lazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

See also

Lazy<T>(Func<T>, LazyThreadSafetyMode) Lazy<T>(Func<T>, LazyThreadSafetyMode) Lazy<T>(Func<T>, LazyThreadSafetyMode) Lazy<T>(Func<T>, LazyThreadSafetyMode)

Initializes a new instance of the Lazy<T> class that uses the specified initialization function and thread-safety mode.

public:
 Lazy(Func<T> ^ valueFactory, System::Threading::LazyThreadSafetyMode mode);
public Lazy (Func<T> valueFactory, System.Threading.LazyThreadSafetyMode mode);
new Lazy<'T> : Func<'T> * System.Threading.LazyThreadSafetyMode -> Lazy<'T>
Public Sub New (valueFactory As Func(Of T), mode As LazyThreadSafetyMode)

Parameters

valueFactory
Func<T>

The delegate that is invoked to produce the lazily initialized value when it is needed.

mode
LazyThreadSafetyMode LazyThreadSafetyMode LazyThreadSafetyMode LazyThreadSafetyMode

One of the enumeration values that specifies the thread safety mode.

Exceptions

Examples

The following example demonstrates the use of this constructor to create a lazy initializer that enables multiple threads to race to create an object lazily. Multiple threads might succeed in creating instances, but all threads use the instance that was created first. In addition, the example demonstrates that exceptions are never cached when you specify LazyThreadSafetyMode.PublicationOnly, even if initialization is performed by a function instead of by the default constructor of the lazily created type.

Note

For an example that demonstrates how to use this constructor in single-threaded scenarios (specifying LazyThreadSafetyMode.None for mode), see the Lazy<T>(Boolean) constructor. For an example that demonstrates how to use this constructor to provide locking instead of race conditions in multithreaded scenarios (specifying LazyThreadSafetyMode.ExecutionAndPublication for mode), see the Lazy<T>() constructor.

The example defines a LargeObject class that will be initialized lazily by any of several threads. The four key sections of code illustrate the creation of the initializer, the actual initialization, the initialization function, and the constructor and finalizer of the LargeObject class. At the beginning of the Main method, the example creates the Lazy<T> object that performs lazy initialization of the LargeObject:

lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, 
                             LazyThreadSafetyMode.PublicationOnly);
lazyLargeObject = New Lazy(Of LargeObject)(AddressOf InitLargeObject, _
     LazyThreadSafetyMode.PublicationOnly)

The lazy initializer uses a function to perform the initialization. In this case, a function is required because there is no default constructor for the LargeObject class.

The example creates and starts three threads that block on a ManualResetEvent object, so that the example can release the threads all at once. In the ThreadProc method that's used by all three threads, calling the Value property creates the LargeObject instance:

LargeObject large = null;
try
{
    large = lazyLargeObject.Value;

    // The following line introduces an artificial delay, to exaggerate the race 
    // condition.
    Thread.Sleep(5); 

    // 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.)
    lock(large)
    {
        large.Data[0] = Thread.CurrentThread.ManagedThreadId;
        Console.WriteLine("LargeObject was initialized by thread {0}; last used by thread {1}.", 
            large.InitializedBy, large.Data[0]);
    }
}
catch (ApplicationException ex)
{
    Console.WriteLine("ApplicationException: {0}", ex.Message);
}
Dim large As LargeObject = Nothing
Try
    large = lazyLargeObject.Value

    ' The following line introduces an artificial delay, to exaggerate the race 
    ' condition.
    Thread.Sleep(5)

    ' 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( _
            "LargeObject was initialized by thread {0}; last used by thread {1}.", _
            large.InitializedBy, large.Data(0))
    End SyncLock
Catch ex As ApplicationException
    Console.WriteLine("ApplicationException: {0}", ex.Message)
End Try

In the third key section of code, the lazy initialization function is called to create the LargeObject instance. The function throws an exception the first time it's called:

static int instanceCount = 0;
static LargeObject InitLargeObject()
{
    if (1 == Interlocked.Increment(ref instanceCount))
    {
        throw new ApplicationException(
            String.Format("Lazy initialization function failed on thread {0}.",
            Thread.CurrentThread.ManagedThreadId));
    }
    return new LargeObject(Thread.CurrentThread.ManagedThreadId);
}
Private Shared instanceCount As Integer = 0
Private Shared Function InitLargeObject() As LargeObject
    If 1 = Interlocked.Increment(instanceCount) Then
        Throw New ApplicationException( _
            "Lazy initialization function failed on thread " & _
            Thread.CurrentThread.ManagedThreadId & ".")
    End If
    Return New LargeObject(Thread.CurrentThread.ManagedThreadId)
End Function

With any other LazyThreadSafetyMode setting, an unhandled exception in the initialization function would be cached. However, LazyThreadSafetyMode.PublicationOnly suppresses exception caching. The output from the example demonstrates that a subsequent attempt to initialize the object succeeds.

Note

The exception message usually appears after messages indicating that other threads have successfully initialized the object. This is because of the delay introduced by throwing and catching the exception.

Because the constructor for the Lazy<T> instance specified LazyThreadSafetyMode.PublicationOnly, all three threads are allowed to create LargeObject instances. The example demonstrates this by displaying console messages in the constructor and in the finalizer of the LargeObject class:

public LargeObject(int initializedBy) 
{ 
    initBy = initializedBy;
    Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy);
}

~LargeObject()
{
    Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy);
}
Public Sub New(ByVal initializedBy As Integer)
    initBy = initializedBy
    Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy)
End Sub

Protected Overrides Sub Finalize()
    Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy)
End Sub

The Lazy<T> object ensures that only one instance is used by all threads (except the thread where the initialization function throws an exception). The output from the example shows this.

Note

For simplicity, this example uses a global instance of Lazy<T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.

using System;
using System.Threading;

class Program
{
    static Lazy<LargeObject> lazyLargeObject = null;

    // Factory function for lazy initialization.
    static int instanceCount = 0;
    static LargeObject InitLargeObject()
    {
        if (1 == Interlocked.Increment(ref instanceCount))
        {
            throw new ApplicationException(
                String.Format("Lazy initialization function failed on thread {0}.",
                Thread.CurrentThread.ManagedThreadId));
        }
        return new LargeObject(Thread.CurrentThread.ManagedThreadId);
    }

    static void Main()
    {
        // The lazy initializer is created here. LargeObject is not created until the 
        // ThreadProc method executes.
        lazyLargeObject = new Lazy<LargeObject>(InitLargeObject, 
                                     LazyThreadSafetyMode.PublicationOnly);


        // Create and start 3 threads, passing the same blocking event to all of them.
        ManualResetEvent startingGate = new ManualResetEvent(false);
        Thread[] threads = { new Thread(ThreadProc), new Thread(ThreadProc), new Thread(ThreadProc) };
        foreach (Thread t in threads)
        {
            t.Start(startingGate);
        }

        // Give all 3 threads time to start and wait, then release them all at once.
        Thread.Sleep(50);
        startingGate.Set();

        // Wait for all 3 threads to finish. (The order doesn't matter.)
        foreach (Thread t in threads)
        {
            t.Join();
        }

        Console.WriteLine(
            "\r\nThreads are complete. Running GC.Collect() to reclaim extra instances.");

        GC.Collect();

        // Allow time for garbage collection, which happens asynchronously.
        Thread.Sleep(100);

        Console.WriteLine("\r\nNote that only one instance of LargeObject was used.");
        Console.WriteLine("Press Enter to end the program");
        Console.ReadLine();
    }


    static void ThreadProc(object state)
    {
        // Wait for the signal.
        ManualResetEvent waitForStart = (ManualResetEvent) state;
        waitForStart.WaitOne();

        LargeObject large = null;
        try
        {
            large = lazyLargeObject.Value;

            // The following line introduces an artificial delay, to exaggerate the race 
            // condition.
            Thread.Sleep(5); 

            // 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.)
            lock(large)
            {
                large.Data[0] = Thread.CurrentThread.ManagedThreadId;
                Console.WriteLine("LargeObject was initialized by thread {0}; last used by thread {1}.", 
                    large.InitializedBy, large.Data[0]);
            }
        }
        catch (ApplicationException ex)
        {
            Console.WriteLine("ApplicationException: {0}", ex.Message);
        }
    }
}

class LargeObject
{
    int initBy = -1;
    public int InitializedBy { get { return initBy; } }

    public LargeObject(int initializedBy) 
    { 
        initBy = initializedBy;
        Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy);
    }

    ~LargeObject()
    {
        Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy);
    }
    
    public long[] Data = new long[100000000];
}

/* This example produces output similar to the following:

Constructor: Instance initializing on thread 5
Constructor: Instance initializing on thread 4
ApplicationException: Lazy initialization function failed on thread 3.
LargeObject was initialized by thread 5; last used by thread 5.
LargeObject was initialized by thread 5; last used by thread 4.

Threads are complete. Running GC.Collect() to reclaim extra instances.
Finalizer: Instance was initialized on 4

Note that only one instance of LargeObject was used.
Press Enter to end the program

Finalizer: Instance was initialized on 5
 */
Imports System
Imports System.Threading

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

    ' Factory function for lazy initialization.
    Private Shared instanceCount As Integer = 0
    Private Shared Function InitLargeObject() As LargeObject
        If 1 = Interlocked.Increment(instanceCount) Then
            Throw New ApplicationException( _
                "Lazy initialization function failed on thread " & _
                Thread.CurrentThread.ManagedThreadId & ".")
        End If
        Return New LargeObject(Thread.CurrentThread.ManagedThreadId)
    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, _
             LazyThreadSafetyMode.PublicationOnly)


        ' Create and start 3 threads, passing the same blocking event to all of them.
        Dim startingGate As New ManualResetEvent(False)
        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(startingGate)
        Next t

        ' Give all 3 threads time to start and wait, then release them all at once.
        Thread.Sleep(50)
        startingGate.Set()

        ' 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 & _
            "Threads are complete. Running GC.Collect() to reclaim extra instances.")

        GC.Collect()

        ' Allow time for garbage collection, which happens asynchronously.
        Thread.Sleep(100)

        Console.WriteLine(vbCrLf & "Note that only one instance of LargeObject was used.")
        Console.WriteLine("Press Enter to end the program")
        Console.ReadLine()
    End Sub


    Private Shared Sub ThreadProc(ByVal state As Object)
        ' Wait for the signal.
        Dim waitForStart As ManualResetEvent = CType(state, ManualResetEvent)
        waitForStart.WaitOne()

        Dim large As LargeObject = Nothing
        Try
            large = lazyLargeObject.Value

            ' The following line introduces an artificial delay, to exaggerate the race 
            ' condition.
            Thread.Sleep(5)

            ' 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( _
                    "LargeObject was initialized by thread {0}; last used by thread {1}.", _
                    large.InitializedBy, large.Data(0))
            End SyncLock
        Catch ex As ApplicationException
            Console.WriteLine("ApplicationException: {0}", ex.Message)
        End Try
    End Sub
End Class

Friend Class LargeObject
    Private initBy As Integer = -1
    Public ReadOnly Property InitializedBy() As Integer
        Get
            Return initBy
        End Get
    End Property

    Public Sub New(ByVal initializedBy As Integer)
        initBy = initializedBy
        Console.WriteLine("Constructor: Instance initializing on thread {0}", initBy)
    End Sub

    Protected Overrides Sub Finalize()
        Console.WriteLine("Finalizer: Instance was initialized on {0}", initBy)
    End Sub

    Public Data(99999999) As Long
End Class

' This example produces output similar to the following:
'
'Constructor: Instance initializing on thread 4
'ApplicationException: Lazy initialization function failed on thread 3.
'Constructor: Instance initializing on thread 5
'LargeObject was initialized by thread 4; last used by thread 4.
'LargeObject was initialized by thread 4; last used by thread 5.
'
'Threads are complete. Running GC.Collect() to reclaim extra instances.
'Finalizer: Instance was initialized on 5
'
'Note that only one instance of LargeObject was used.
'Press Enter to end the program
'
'Finalizer: Instance was initialized on 4
' 

Remarks

The thread safety mode of a Lazy<T> instance describes the behavior when multiple threads try to initialize the Lazy<T> instance.

Exceptions that are thrown by valueFactory are cached, unless mode is LazyThreadSafetyMode.PublicationOnly. For more information, see the Lazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

See also

Applies to