Lazy(Func<T>)的异常缓存问题

Lazy可以提供多线程环境下的安全保障，但是用不好也是会跳到坑里。

我这里使用Lazy<t>(Func<T>)来创建一个Lazy实例，然后在需要的地方访问它的Value属性，它可以保证在多线程环境下Func<T>仅执行一次，这看起来十分的美好：需要的时候执行，并且仅执行一次，再翻译下就是延迟加载，线程安全，资源消耗少。

问题

但是程序运行一段时间后出现了诡异的情况：出现一次异常后，程序不能自动恢复，一直抛出异常，直到程序重启，而出现异常的地方就在Func<T>中。

所有的好冥冥之中都是有代价的，查阅官方文档，发现Lazy会缓存异常。

https://docs.microsoft.com/en-us/dotnet/api/system.lazy-1.-ctor?view=net-5.0#System_Lazy_1__ctor_System_Func__0__

Lazy<T>(Func<T>) 等同于 Lazy<T>(Func<T>, true) 或者 Lazy<T>(Func<T>,LazyThreadSafetyMode.ExecutionAndPublication)，后边这两个构造函数的第二个参数的意思是在多线程环境下，委托只执行1次，使用这次的执行结果作为Lazy的值，同时如果委托中发生任何异常，都会被缓存下来。

官方还提供了一个例子可以验证异常缓存的问题，粘贴到这里：

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
 */

解决方案

在提出解决办法前，需要想一下，为什么会缓存异常？

因为要保证多线程环境下只执行一次，如果异常了还允许再次执行，就不能保证只执行一次了，而有些程序多次执行是不可行的。

来看几个解决方案：

1、不使用Lazy，自己加锁处理。

出现问题的程序中Lazy内部也是用了锁。

部分情况下可以用双检锁或则带升级的读写锁，以提高读的性能。

如果发生异常，可以抛到上层，并且再次获取时会重试执行。

2、使用Value时如果有异常，则重新给Lazy赋值。

不过这可能又要求赋值时线程安全。

3、如果经过评估可以多次创建Value，则可以更改线程安全模式为：LazyThreadSafetyMode.PublicationOnly

在这种模式下：多线程时每个线程都会创建，但是只使用第一个创建的，同时不缓存异常，异常发生后再次获取时会重新执行。

哪个适合自己，还需自己选择。