为什么有的人把代码写的如此复杂？

技术群里有人发了一段代码：

附言：兄弟们，这个单例怎么样？

我回复：什么鬼，看不懂啊？！

也有其他小伙伴表示看不懂，看来大家的C#基础和我一样并不全面。

我看不懂，主要是因为我没用过TaskCompletionSource和Interlocked的CompareExchange方法，然后经过我1、2个小时的研究，终于勉强看懂了。

由于上面这段代码只贴了一张图，我没有拿到源码，所以我写了个差不多的Demo用于测试，代码如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SingletonTest
{
    public class Singleton
    {
        private static Task<string> _stringTask;

        /// <summary>
        /// 重置，方便重复测试
        /// </summary>
        public void Reset()
        {
            _stringTask = null;
        }

        public Task<string> InitAsync()
        {
            if (_stringTask != null)
            {
                return _stringTask;
            }

            var inition = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);

            var initonTask = Interlocked.CompareExchange(ref _stringTask, inition.Task, null);

            if (initonTask != null)
            {
                return initonTask;
            }

            _stringTask = CreateContent(inition);
            return inition.Task;
        }

        private async Task<string> CreateContent(TaskCompletionSource<string> inition)
        {
            string content = await TextUtil.GetTextAsync();
            inition.SetResult(content);
            return content;
        }
    }
}

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然后按照我自己的习惯，又写了一版：

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SingletonTest
{
    class Singleton2
    {
        private static string _value;
        private SemaphoreSlim _semaphoreSlim = new SemaphoreSlim(1, 1);

        /// <summary>
        /// 重置，方便重复测试
        /// </summary>
        public void Reset()
        {
            _value = null;
        }

        public async Task<string> InitAsync()
        {
            if (_value != null)
            {
                return _value;
            }

            await _semaphoreSlim.WaitAsync();
            if (_value == null)
            {
                _value = await TextUtil.GetTextAsync();
            }
            _semaphoreSlim.Release();

            return _value;
        }

    }
}

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很容易懂，不是吗？ 

这段代码我好像是理解了，可是我不理解的是，为什么代码会写的这么复杂呢？

最主要的是我不理解下面几行：

var inition = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);

var initonTask = Interlocked.CompareExchange(ref _stringTask, inition.Task, null);

if (initonTask != null)
{
    return initonTask;
}

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我要给它翻译成我能理解的代码，我意思到new的TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously)也是个单例，所以我先写了个TaskCompletionSourceFactory类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SingletonTest
{
    public class TaskCompletionSourceFactory : IDisposable
    {
        private TaskCompletionSource<string> _value;

        private TaskCompletionSourceData _data;

        private SemaphoreSlim _semaphoreSlim = new SemaphoreSlim(1, 1);

        public TaskCompletionSourceData Instance
        {
            get
            {
                _semaphoreSlim.Wait();
                if (_value == null)
                {
                    _data = new TaskCompletionSourceData();
                    _value = new TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously);
                    _data.Value = _value;
                    _data.First = true;
                }
                else
                {
                    _data = new TaskCompletionSourceData();
                    _data.Value = _value;
                    _data.First = false;
                }
                _semaphoreSlim.Release();
                return _data;
            }
        }

        public void Dispose()
        {
            _semaphoreSlim.Dispose();
        }
    }

    public class TaskCompletionSourceData
    {
        public bool First { get; set; }

        public TaskCompletionSource<string> Value { get; set; }
    }
}

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然后把Demo中Singleton这个类改写了一下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SingletonTest
{
    public class Singleton3
    {
        private static Task<string> _stringTask;

        /// <summary>
        /// 重置，方便重复测试
        /// </summary>
        public void Reset()
        {
            _stringTask = null;
        }

        public Task<string> InitAsync(TaskCompletionSourceFactory factory)
        {
            if (_stringTask != null)
            {
                return _stringTask;
            }

            var inition = factory.Instance;
            if (!inition.First)
            {
                return inition.Value.Task;
            }

            _stringTask = CreateContent(inition.Value);
            return inition.Value.Task;
        }

        private async Task<string> CreateContent(TaskCompletionSource<string> inition)
        {
            string content = await TextUtil.GetTextAsync();
            inition.SetResult(content);
            return content;
        }
    }
}

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当我差不多理解了之后，我发现原始代码有一点点小问题，就是TaskCompletionSource<string>是有机率被重复new的。

大家觉得哪种写法好呢？

附：

TextUtil.cs代码，是一个模拟获取文本的方法：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SingletonTest
{
    public class TextUtil
    {
        public static Task<string> GetTextAsync()
        {
            return Task.Run<string>(() =>
            {
                Thread.Sleep(10);
                Random rnd = new Random();
                return rnd.Next(0, 1000).ToString().PadRight(10);
            });
        }
    }
}

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测试代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SingletonTest
{
    class Program
    {
        private static int _count = 200;
        private static Singleton _singleton = new Singleton();
        private static Singleton2 _singleton2 = new Singleton2();
        private static Singleton3 _singleton3 = new Singleton3();

        static void Main(string[] args)
        {
            ThreadPool.SetMinThreads(20, 20);
            Task.Run(() => { }); //Task预热
            Console.WriteLine("输入1测试Singleton，输入2测试Singleton2，如果值都相同，说明单例测试通过，否则不通过");

            while (true)
            {
                var key = Console.ReadKey().Key;

                if (key == ConsoleKey.D1)
                {
                    Console.WriteLine("测试Singleton");
                    Test();
                }

                if (key == ConsoleKey.D2)
                {
                    Console.WriteLine("测试Singleton2");
                    Test2();
                }

                if (key == ConsoleKey.D3)
                {
                    Console.WriteLine("测试Singleton3");
                    Test3();
                }
            }

        }

        public static void Test()
        {
            List<Task> taskList = new List<Task>();
            for (int i = 0; i < _count; i++)
            {
                Task task = Task.Run(async () =>
                {
                    string content = await _singleton.InitAsync();
                    Console.Write(content);
                });
                taskList.Add(task);
            }

            Task.WaitAll(taskList.ToArray());
            _singleton.Reset();
            Console.WriteLine("");
        }

        public static void Test2()
        {
            List<Task> taskList = new List<Task>();
            for (int i = 0; i < _count; i++)
            {
                Task task = Task.Run(async () =>
                {
                    string content = await _singleton2.InitAsync();
                    Console.Write(content);
                });
                taskList.Add(task);
            }

            Task.WaitAll(taskList.ToArray());
            _singleton2.Reset();
            Console.WriteLine("");
        }

        public static void Test3()
        {
            TaskCompletionSourceFactory factory = new TaskCompletionSourceFactory();
            List<Task> taskList = new List<Task>();
            for (int i = 0; i < _count; i++)
            {
                Task task = Task.Run(async () =>
                {
                    string content = await _singleton3.InitAsync(factory);
                    Console.Write(content);
                });
                taskList.Add(task);
            }

            Task.WaitAll(taskList.ToArray());
            _singleton3.Reset();
            factory.Dispose();
            Console.WriteLine("");
        }
    }
}

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