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Lazy<T> Constructor

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

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

public Lazy()

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 theLazy<T> class or the System.Threading.LazyThreadSafetyMode enumeration.

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);

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;

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.

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>();

        // 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
 */

.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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