為什麼有的人把程式碼寫的如此複雜？

技術群裡有人發了一段程式碼：

附言：兄弟們，這個單例怎麼樣？

我回復：什麼鬼，看不懂啊？！

也有其他小夥伴表示看不懂，看來大家的C#基礎和我一樣並不全面。

我看不懂，主要是因為我沒用過TaskCompletionSource和Interlocked的CompareExchange方法，然後經過我1、2個小時的研究，終於勉強看懂了。

由於上面這段程式碼只貼了一張圖，我沒有拿到原始碼，所以我寫了個差不多的Demo用於測試，程式碼如下：

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，方便重複測試
        /// </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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然後按照我自己的習慣，又寫了一版：

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，方便重複測試
        /// </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，不是嗎？

這段程式碼我好像是理解了，可是我不理解的是，為什麼程式碼會寫的這麼複雜呢？

最主要的是我不理解下面幾行：

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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我要給它翻譯成我能理解的程式碼，我意思到new的TaskCompletionSource<string>(TaskCreationOptions.RunContinuationsAsynchronously)也是個單例，所以我先寫了個TaskCompletionSourceFactory類：

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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然後把Demo中Singleton這個類改寫了一下：

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，方便重複測試
        /// </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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當我差不多理解了之後，我發現原始程式碼有一點點小問題，就是TaskCompletionSource<string>是有機率被重複new的。

大家覺得哪種寫法好呢？

附：

TextUtil.cs程式碼，是一個模擬獲取文字的方法：

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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測試程式碼：

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預熱
            Console.WriteLine("輸入1測試Singleton，輸入2測試Singleton2，如果值都相同，說明單例測試通過，否則不通過");

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

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

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

                if (key == ConsoleKey.D3)
                {
                    Console.WriteLine("測試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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