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所属分类：.NET技术
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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