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Lazy<T> Constructor (LazyThreadSafetyMode)

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

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

public Lazy(
	LazyThreadSafetyMode mode
)

Parameters

mode
Type: System.Threading.LazyThreadSafetyMode

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

ExceptionCondition
ArgumentOutOfRangeException

mode contains an invalid value.

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.

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.

NoteNote

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);

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;

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);
}

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.

NoteNote

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
 */

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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