Lazy<T> 类

参考

定义

命名空间:: System

程序集:: System.Runtime.dll

程序集:: mscorlib.dll

程序集:: netstandard.dll

Source:: Lazy.cs

Source:: Lazy.cs

Source:: Lazy.cs

重要

一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。

提供对延迟初始化的支持。

generic <typename T>
public ref class Lazy

public class Lazy<T>

[System.Runtime.InteropServices.ComVisible(false)]
[System.Serializable]
public class Lazy<T>

type Lazy<'T> = class

[<System.Runtime.InteropServices.ComVisible(false)>]
[<System.Serializable>]
type Lazy<'T> = class

Public Class Lazy(Of T)

类型参数

T

正在延迟初始化的对象的类型。

继承: Object
Lazy<T>

派生: System.Lazy<T,TMetadata>

属性: ComVisibleAttribute SerializableAttribute

示例

以下示例演示如何使用 Lazy<T> 类提供从多个线程访问的延迟初始化。

注意

该示例使用 Lazy<T>(Func<T>) 构造函数。它还演示了使用 Lazy<T>(Func<T>, Boolean) 构造函数（指定 isThreadSafe的 true）和 Lazy<T>(Func<T>, LazyThreadSafetyMode) 构造函数（为 mode指定 LazyThreadSafetyMode.ExecutionAndPublication）。若要切换到其他构造函数，只需更改注释掉哪些构造函数。

有关使用相同构造函数演示异常缓存的示例，请参阅 Lazy<T>(Func<T>) 构造函数。

该示例定义一个 LargeObject 类，该类将由多个线程之一延迟初始化。代码的四个关键部分说明了如何创建初始值设定项、工厂方法、实际初始化和 LargeObject 类的构造函数，该类在创建对象时显示消息。在 Main 方法的开头，该示例为 LargeObject创建线程安全的延迟初始值设定项：

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);

let lazyLargeObject = Lazy<LargeObject> initLargeObject

// The following lines show how to use other constructors to achieve exactly the
// same result as the previous line:
//     let lazyLargeObject = Lazy<LargeObject>(initLargeObject, true)
//     let lazyLargeObject = 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)

工厂方法显示对象的创建，其中占位符用于进一步初始化：

static LargeObject InitLargeObject()
{
    LargeObject large = new LargeObject(Thread.CurrentThread.ManagedThreadId);
    // Perform additional initialization here.
    return large;
}

let initLargeObject () =
    let large = LargeObject Thread.CurrentThread.ManagedThreadId
    // Perform additional initialization here.
    large

Private Shared Function InitLargeObject() As LargeObject
    Dim large As New LargeObject(Thread.CurrentThread.ManagedThreadId)
    ' Perform additional initialization here.
    Return large
End Function

请注意，前两个代码部分可以使用 lambda 函数进行组合，如下所示：

lazyLargeObject = new Lazy<LargeObject>(() =>
{
    LargeObject large = new LargeObject(Thread.CurrentThread.ManagedThreadId);
    // Perform additional initialization here.
    return large;
});

let lazyLargeObject = Lazy<LargeObject>(fun () ->
    let large = LargeObject Thread.CurrentThread.ManagedThreadId
    // Perform additional initialization here.
    large)

lazyLargeObject = New Lazy(Of LargeObject)(Function () 
    Dim large As New LargeObject(Thread.CurrentThread.ManagedThreadId) 
    ' Perform additional initialization here.
    Return large
End Function)

该示例暂停，以指示在延迟初始化发生之前，不确定期可能已过。按 Enter 键时，该示例将创建并启动三个线程。所有三个线程使用的 ThreadProc 方法都会调用 Value 属性。首次发生这种情况时，将创建 LargeObject 实例：

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]);
}

let 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 (fun () ->
    large.Data[0] <- Thread.CurrentThread.ManagedThreadId
    printfn $"Initialized by thread {large.InitializedBy} last used by thread {large.Data[0]}.")

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

LargeObject 类的构造函数（包括代码的最后一个关键部分）显示消息并记录初始化线程的标识。程序输出显示在完整代码列表的末尾。

int initBy = 0;
public LargeObject(int initializedBy)
{
    initBy = initializedBy;
    Console.WriteLine("LargeObject was created on thread id {0}.", initBy);
}

type LargeObject(initBy) =
    do 
        printfn $"LargeObject was created on thread id %i{initBy}."

Private initBy As Integer = 0
Public Sub New(ByVal initializedBy As Integer)
    initBy = initializedBy
    Console.WriteLine("LargeObject was created on thread id {0}.", initBy)
End Sub

注意

为简单起见，此示例使用 Lazy<T>的全局实例，所有方法都 static（Visual Basic 中的Shared）。这些不是使用延迟初始化的要求。

using System;
using System.Threading;

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

    static LargeObject InitLargeObject()
    {
        LargeObject large = new LargeObject(Thread.CurrentThread.ManagedThreadId);
        // Perform additional initialization here.
        return large;
    }

    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 uses LargeObject.
        Thread[] threads = new Thread[3];
        for (int i = 0; i < 3; i++)
        {
            threads[i] = new Thread(ThreadProc);
            threads[i].Start();
        }

        // Wait for all 3 threads to finish.
        foreach (Thread t in threads)
        {
            t.Join();
        }

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

    static void ThreadProc(object state)
    {
        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]);
        }
    }
}

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

    int initBy = 0;
    public LargeObject(int initializedBy)
    {
        initBy = initializedBy;
        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 3.
Initialized by thread 3; last used by thread 3.
Initialized by thread 3; last used by thread 4.
Initialized by thread 3; last used by thread 5.

Press Enter to end the program
 */

open System
open System.Threading

type LargeObject(initBy) =
    do 
        printfn $"LargeObject was created on thread id %i{initBy}."
    member _.InitializedBy = initBy
    member val Data = Array.zeroCreate<int64> 100000000

let initLargeObject () =
    let large = LargeObject Thread.CurrentThread.ManagedThreadId
    // Perform additional initialization here.
    large

// The lazy initializer is created here. LargeObject is not created until the
// ThreadProc method executes.
let lazyLargeObject = Lazy<LargeObject> initLargeObject

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

let threadProc (state: obj) =
    let 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 (fun () ->
        large.Data[0] <- Thread.CurrentThread.ManagedThreadId
        printfn $"Initialized by thread {large.InitializedBy} last used by thread {large.Data[0]}.")

printfn """
LargeObject is not created until you access the Value property of the lazy
initializer. Press Enter to create LargeObject."""
stdin.ReadLine() |> ignore

// Create and start 3 threads, each of which uses LargeObject.

let threads = Array.zeroCreate 3
for i = 0 to 2 do
    threads[i] <- Thread(ParameterizedThreadStart threadProc)
    threads[i].Start()

// Wait for all 3 threads to finish.
for t in threads do
    t.Join()

printfn "\nPress Enter to end the program"
stdin.ReadLine() |> ignore

// 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 3.
//     Initialized by thread 3 last used by thread 4.
//     Initialized by thread 3 last used by thread 5.
//     
//     Press Enter to end the program

Imports System.Threading

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

    Private Shared Function InitLargeObject() As LargeObject
        Dim large As New LargeObject(Thread.CurrentThread.ManagedThreadId)
        ' Perform additional initialization here.
        Return large
    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 uses LargeObject.
        Dim threads(2) As Thread
        For i As Integer = 0 To 2
            threads(i) = New Thread(AddressOf ThreadProc)
            threads(i).Start()
        Next i

        ' Wait for all 3 threads to finish. 
        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)
        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
    End Sub
End Class

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

    Private initBy As Integer = 0
    Public Sub New(ByVal initializedBy As Integer)
        initBy = initializedBy
        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.
'
'LargeObject was created on thread id 3.
'Initialized by thread 3; last used by thread 3.
'Initialized by thread 3; last used by thread 5.
'Initialized by thread 3; last used by thread 4.
'
'Press Enter to end the program
'

注解

使用延迟初始化来延迟创建大型或资源密集型对象，或者执行资源密集型任务，尤其是在程序生存期内可能不会发生此类创建或执行时。

若要准备延迟初始化，请创建 Lazy<T>实例。所创建的 Lazy<T> 对象的 type 参数指定要延迟初始化的对象的类型。用于创建 Lazy<T> 对象的构造函数确定初始化的特征。延迟初始化是在首次访问 Lazy<T>.Value 属性时发生的。

在大多数情况下，选择构造函数取决于两个问题的解答：

是否会从多个线程访问延迟初始化的对象？如果是这样，Lazy<T> 对象可能会在任何线程上创建它。可以使用其默认行为之一的简单构造函数来创建线程安全的 Lazy<T> 对象，因此无论尝试访问该对象多少个线程，只创建一个延迟实例化对象的实例。若要创建非线程安全的 Lazy<T> 对象，必须使用一个构造函数来指定无线程安全性。

谨慎

使 Lazy<T> 对象线程安全不会保护延迟初始化的对象。如果多个线程可以访问延迟初始化的对象，则必须使其属性和方法安全进行多线程访问。
延迟初始化是否需要大量代码，或者延迟初始化的对象是否具有无参数构造函数，该构造函数需要执行所需的一切操作，并且不会引发异常？如果需要编写初始化代码或需要处理异常，请使用采用工厂方法的构造函数之一。在工厂方法中编写初始化代码。

下表根据以下两个因素显示了要选择的构造函数：

对象将由	如果不需要初始化代码（无参数构造函数），请使用	如果需要初始化代码，请使用
多个线程	Lazy<T>()	Lazy<T>(Func<T>)
一个线程	Lazy<T>(Boolean)，`isThreadSafe` 设置为 `false`。	Lazy<T>(Func<T>, Boolean)，`isThreadSafe` 设置为 `false`。

可以使用 lambda 表达式来指定工厂方法。这会将所有初始化代码保留在一个位置。 lambda 表达式捕获上下文，包括传递给延迟初始化对象的构造函数的任何参数。

异常缓存 使用工厂方法时，将缓存异常。也就是说，如果在线程首次尝试访问 Lazy<T> 对象的 Value 属性时，工厂方法将引发异常，则每次后续尝试时都会引发相同的异常。这可确保每次调用 Value 属性都会生成相同的结果，并避免在不同线程获得不同结果时可能出现的细微错误。 Lazy<T> 代表一个实际 T，否则在一些早期点（通常是在启动期间）初始化的。此时的失败通常是致命的。如果可能存在可恢复的故障，我们建议将重试逻辑构建到初始化例程（在本例中为工厂方法），就像不使用延迟初始化一样。

锁定在某些情况下，你可能希望避免 Lazy<T> 对象的默认锁定行为的开销。在极少数情况下，可能存在死锁。在这种情况下，可以使用 Lazy<T>(LazyThreadSafetyMode) 或 Lazy<T>(Func<T>, LazyThreadSafetyMode) 构造函数，并指定 LazyThreadSafetyMode.PublicationOnly。这使 Lazy<T> 对象能够在线程同时调用 Value 属性时，在多个线程上创建延迟初始化对象的副本。 Lazy<T> 对象可确保所有线程都使用相同的延迟初始化对象的实例，并丢弃未使用的实例。因此，降低锁定开销的成本是，程序有时可能会创建并丢弃昂贵的对象的额外副本。在大多数情况下，这不太可能。 Lazy<T>(LazyThreadSafetyMode) 和 Lazy<T>(Func<T>, LazyThreadSafetyMode) 构造函数的示例演示了此行为。

重要

指定 LazyThreadSafetyMode.PublicationOnly时，即使指定工厂方法，也永远不会缓存异常。

等效构造函数 除了启用 LazyThreadSafetyMode.PublicationOnly的使用外，Lazy<T>(LazyThreadSafetyMode) 和 Lazy<T>(Func<T>, LazyThreadSafetyMode) 构造函数还可以复制其他构造函数的功能。下表显示了生成等效行为的参数值。

创建 Lazy<T> 对象	对于具有 `LazyThreadSafetyModemode` 参数的构造函数，请将 `mode` 设置为	对于具有布尔 `isThreadSafe` 参数的构造函数，请将 `isThreadSafe` 设置为	对于没有线程安全参数的构造函数
完全线程安全;使用锁定来确保只有一个线程初始化该值。	ExecutionAndPublication	`true`	所有这些构造函数都是完全线程安全的。
不是线程安全。	None	`false`	不適用。
完全线程安全;线程争用来初始化值。	PublicationOnly	不適用。	不適用。

其他功能 有关将 Lazy<T> 与线程静态字段配合使用或作为属性的后盾存储的信息，请参阅延迟初始化。

构造函数

Lazy<T>()	初始化 Lazy<T> 类的新实例。发生延迟初始化时，将使用目标类型的无参数构造函数。
Lazy<T>(Boolean)	初始化 Lazy<T> 类的新实例。发生延迟初始化时，将使用目标类型的无参数构造函数和指定的初始化模式。
Lazy<T>(Func<T>)	初始化 Lazy<T> 类的新实例。发生延迟初始化时，将使用指定的初始化函数。
Lazy<T>(Func<T>, Boolean)	初始化 Lazy<T> 类的新实例。发生延迟初始化时，将使用指定的初始化函数和初始化模式。
Lazy<T>(Func<T>, LazyThreadSafetyMode)	初始化使用指定的初始化函数和线程安全模式的 Lazy<T> 类的新实例。
Lazy<T>(LazyThreadSafetyMode)	初始化 Lazy<T> 类的新实例，该实例使用 `T` 的无参数构造函数和指定的线程安全模式。
Lazy<T>(T)	初始化使用预初始化指定值的 Lazy<T> 类的新实例。

属性

IsValueCreated	获取一个值，该值指示是否已为此 Lazy<T> 实例创建值。
Value	获取当前 Lazy<T> 实例的延迟初始化值。

方法

Equals(Object)	确定指定的对象是否等于当前对象。 (继承自 Object)
GetHashCode()	用作默认哈希函数。 (继承自 Object)
GetType()	获取当前实例的 Type。 (继承自 Object)
MemberwiseClone()	创建当前 Object的浅表副本。 (继承自 Object)
ToString()	创建并返回此实例的 Value 属性的字符串表示形式。

适用于

线程安全性

默认情况下，Lazy<T> 类的所有公共和受保护成员都是线程安全的，并且可以从多个线程并发使用。可以使用类型构造函数的参数（可选）和每个实例删除这些线程安全保证。

另请参阅

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