在 .NET 4.0 下编写扩展代码以支持 async 异步编程
微软在C# 5中引入了async、await这两个异步编程的关键字，要使用这两个关键字需要你的IDE支持C#5.0语法，也就意味着你需要使用VS 2012版本以上IDE,或者在Vs2010卸载其编译器在重新安装，这并不是本文的重点，有需求的朋友点击这篇博客，现在都快接近2019年，VS2019 预览版都出来了，现在使用Vs2010确实有点更不上时代了，建议大家升级一下vs，其他问题可以自行百度谷歌解决下。
async和await带给我们的方便是巨大的,从代码的简洁或者处理UI阻塞问题，都是很便捷的，而XP系统最高只支持.Net 4.0，也就意味着，假如你编写的Winform程序使用了这两个关键字将不能在XP中，而目前很多系统仍然使用着XP,因此很有必要在.Net 4 环境下使用 async异步编程。
当我们创建一个.Net Framework 4.0的项目时候，在使用async 和 await的时候，代码如下：
static void Main(string[] args) { Console.WriteLine("异步方法调用前：" + Thread.CurrentThread.ManagedThreadId.ToString()); Test(); Console.WriteLine("异步方法调用后：" + Thread.CurrentThread.ManagedThreadId.ToString()); Console.Read(); } public async static void Test() { Console.WriteLine("异步方法等待前：" + Thread.CurrentThread.ManagedThreadId.ToString()); Console.WriteLine("开始等待：" + DateTime.Now.ToString()); await Wait(); Console.WriteLine("异步方法等待后：" + Thread.CurrentThread.ManagedThreadId.ToString()); Console.WriteLine("结束等待：" + DateTime.Now.ToString()); } public static Task Wait() { Action action = () => { Console.WriteLine("任务运行：" + Thread.CurrentThread.ManagedThreadId.ToString()); Thread.Sleep(2000); Console.WriteLine("任务运行后：" + Thread.CurrentThread.ManagedThreadId.ToString()); }; Task task = null; // task= TaskEx.Run(action); task = new Task(action); task.Start(); return task; }
这时候编译器会报一个错误 ：CS1061 “Task”未包含“GetAwaiter”的定义，并且找不到可接受第一个“Task”类型参数的可访问扩展方法“GetAwaiter”(是否缺少 using 指令或程序集引用?)
而在.Net 4.5下，是不会报这样错的。在异步的环境下，正确执行的情况下，Test()方法会单独开辟一个线程，所以整个测试，会在Main方法中Test()方法是异步的，具体执行顺序如下：
回归到.Net 4.0中报的错：“Task”未包含“GetAwaiter”的定义，并且找不到可接受第一个“Task”类型参数的可访问扩展方法“GetAwaiter”。由此我们推断.Net 4.5 跟 .Net 4.0在处理异步的区别是否在于多了Task的扩展方法GetAwaiter()。
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由于涉及到 .net async 状态机，这东西目前不是目前我一下两下可以懂的，先将实现贴出来，以后学得深了在完善此随笔。
新建一个类TaskEx，用如下的代码替换掉类里面的所有内容，会发现.Net 4.0下的报错会消失，也可以正常的使用异步async。有兴趣的同学可以在Github项目参考牛人写的代码。
using System; using System.Collections.Generic; using System.Linq; using System.Runtime.CompilerServices; using System.Security; using System.Text; using System.Threading.Tasks; namespace System.Threading.Tasks { internal static class TaskEx { public static TaskAwaiter GetAwaiter(this Task task) { return new TaskAwaiter(task); } public static TaskAwaiter<T> GetAwaiter<T>(this Task<T> task) { return new TaskAwaiter<T>(task); } public static Task Run(Action action) { return Task.Factory.StartNew(action); } } internal struct TaskAwaiter : INotifyCompletion { readonly Task task; internal TaskAwaiter(Task task) { this.task = task; } internal static TaskScheduler TaskScheduler { get { if (SynchronizationContext.Current == null) return TaskScheduler.Default; else return TaskScheduler.FromCurrentSynchronizationContext(); } } public bool IsCompleted { get { return task.IsCompleted; } } public void OnCompleted(Action continuation) { this.task.ContinueWith( delegate (Task task) { //Console.WriteLine("状态机回调函数信息："+Thread.CurrentThread.ManagedThreadId); //Console.WriteLine("\t名称："+continuation.Method.Name); //Console.WriteLine("\t所属类型：" + continuation.Method.DeclaringType.ToString()); continuation(); }, TaskAwaiter.TaskScheduler); } public void GetResult() { try { task.Wait(); } catch (AggregateException ex) { throw ex.InnerExceptions[0]; } } } internal struct TaskAwaiter<T> : INotifyCompletion { readonly Task<T> task; internal TaskAwaiter(Task<T> task) { this.task = task; } public bool IsCompleted { get { return task.IsCompleted; } } public void OnCompleted(Action continuation) { this.task.ContinueWith( delegate (Task<T> task) { continuation(); }, TaskAwaiter.TaskScheduler); } public T GetResult() { try { return task.Result; } catch (AggregateException ex) { throw ex.InnerExceptions[0]; } } } } namespace System.Runtime.CompilerServices { internal interface INotifyCompletion { void OnCompleted(Action continuation); } internal interface ICriticalNotifyCompletion : INotifyCompletion { [SecurityCritical] void UnsafeOnCompleted(Action continuation); } internal interface IAsyncStateMachine { void MoveNext(); void SetStateMachine(IAsyncStateMachine stateMachine); } internal struct AsyncVoidMethodBuilder { public static AsyncVoidMethodBuilder Create() { return new AsyncVoidMethodBuilder(); } public void SetException(Exception exception) { throw exception; } public void SetResult() { } public void SetStateMachine(IAsyncStateMachine stateMachine) { // Should not get called as we don't implement the optimization that this method is used for. throw new NotImplementedException(); } public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine { stateMachine.MoveNext(); } public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine { awaiter.OnCompleted(stateMachine.MoveNext); } public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine { awaiter.OnCompleted(stateMachine.MoveNext); } } internal struct AsyncTaskMethodBuilder { TaskCompletionSource<object> tcs; public Task Task { get { return tcs.Task; } } public static AsyncTaskMethodBuilder Create() { AsyncTaskMethodBuilder b; b.tcs = new TaskCompletionSource<object>(); return b; } public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine { stateMachine.MoveNext(); } public void SetStateMachine(IAsyncStateMachine stateMachine) { // Should not get called as we don't implement the optimization that this method is used for. throw new NotImplementedException(); } public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine { awaiter.OnCompleted(stateMachine.MoveNext); } public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine { awaiter.OnCompleted(stateMachine.MoveNext); } public void SetResult() { tcs.SetResult(null); } public void SetException(Exception exception) { tcs.SetException(exception); } } internal struct AsyncTaskMethodBuilder<T> { TaskCompletionSource<T> tcs; public Task<T> Task { get { return tcs.Task; } } public static AsyncTaskMethodBuilder<T> Create() { AsyncTaskMethodBuilder<T> b; b.tcs = new TaskCompletionSource<T>(); return b; } public void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine { stateMachine.MoveNext(); } public void SetStateMachine(IAsyncStateMachine stateMachine) { // Should not get called as we don't implement the optimization that this method is used for. throw new NotImplementedException(); } public void AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : INotifyCompletion where TStateMachine : IAsyncStateMachine { awaiter.OnCompleted(stateMachine.MoveNext); } public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) where TAwaiter : ICriticalNotifyCompletion where TStateMachine : IAsyncStateMachine { AwaitOnCompleted(ref awaiter, ref stateMachine); } public void SetResult(T result) { tcs.SetResult(result); } public void SetException(Exception exception) { tcs.SetException(exception); } } }
