【C#】【多线程】【05-使用C#6.0】08-自定义awaitable类型

## awaitable表达式要求

为了与 `await` 操作符保持兼容，类型应当遵守在C#规则说明中规定的一些要求。
我安装的是VS2017，其路径为：
*C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC#\Specifications\1033     
└ [CSharp Language Specification.docx](/Uploads/userfiles/e671fced-86e2-4d56-a20a-d79aab766aa7/files/2017/08/CSharp Language Specification.docx)*

------

### 7.7.7.1 Awaitable expressions
> The task of an await expression is required to be awaitable. An expression t is awaitable if one of the following holds:
> Await表达式的任务被要求是awaitable。如果一个表达式t满足下面任意一条则认为是awaitalbe的：
> - t is of compile time type dynamic
> t是动态编译时的类型
> - t has an accessible instance or extension method called GetAwaiter with no parameters and no type parameters, and a return type A for which all of the following hold:
> t有一个名为GetAwaiter的可访问的实例或扩展方法。该方法没有参数和类型参数，并且返回值类型A满足以下所有条件
>     - A implements the interface System.Runtime.CompilerServices.INotifyCompletion (hereafter known as INotifyCompletion for brevity)
>     A实现了 System.Runtime.CompilerServices.INotifyCompletion 接口（为简单起见，以后简称为 INotifyCompletion）
>     - A has an accessible, readable instance property IsCompleted of type bool
>     A有一个可访问的、可读的类型为bool的实例属性IsCompleted。
>     - A has an accessible instance method GetResult with no parameters and no type parameters
>     A有一个名为GetResult的可访问的实例方法，该方法没有任何参数和类型参数。
>
> The purpose of the GetAwaiter method is to obtain an awaiter for the task. The type A is called the awaiter type for the await expression.
> The purpose of the IsCompleted property is to determine if the task is already complete. If so, there is no need to suspend evaluation.
> The purpose of the INotifyCompletion.OnCompleted method is to sign up a “continuation” to the task; i.e. a delegate (of type System.Action) that will be invoked once the task is complete.
> The purpose of the GetResult method is to obtain the outcome of the task once it is complete. This outcome may be successful completion, possibly with a result value, or it may be an exception which is thrown by the GetResult method.

------

## 示例代码

```
/// <summary>
/// 自定义awaitable类型
/// </summary>
/// <param name="args"></param>
static void Main(string[] args)
{
    Task t = AsynchronousProcessing();
    t.Wait();

Console.ReadLine();
}

static async Task AsynchronousProcessing()
{
    var sync = new CustomAwaitable(true);
    string result = await sync;
    // Completed synchronously
    Console.WriteLine(result);

var async = new CustomAwaitable(false);
    result = await async;
    // Task is running on a thread id 3. Is thread pool thread: True
    Console.WriteLine(result);
}

/// <summary>
/// 类型t
/// </summary>
class CustomAwaitable
{
    private readonly bool _completeSynchronously;

public CustomAwaitable(bool completeSynchronously)
    {
        _completeSynchronously = completeSynchronously;
    }

/// <summary>
    /// t有一个名为GetAwaiter的可访问的实例或扩展方法
    /// </summary>
    /// <returns>类型A</returns>
    public CustomAwaiter GetAwaiter()
    {
        return new CustomAwaiter(_completeSynchronously);
    }
}

/// <summary>
/// 类型A 实现了 System.Runtime.CompilerServices.INotifyCompletion 接口
/// </summary>
class CustomAwaiter : INotifyCompletion
{
    private string _result = "Completed synchronously";
    private readonly bool _completeSynchronously;

/// <summary>
    /// A有一个可访问的、可读的类型为bool的实例属性IsCompleted
    /// 如果IsCompleted属性返回true，则只需同步调用GetResult方法
    /// </summary>
    public bool IsCompleted => _completeSynchronously;

public CustomAwaiter(bool completeSynchronously)
    {
        _completeSynchronously = completeSynchronously;
    }

/// <summary>
    /// A有一个名为GetResult的可访问的实例方法，该方法没有任何参数和类型参数。
    /// </summary>
    /// <returns></returns>
    public string GetResult()
    {
        return _result;
    }

public void OnCompleted(Action continuation)
    {
        ThreadPool.QueueUserWorkItem(state =>
        {
            Thread.Sleep(TimeSpan.FromSeconds(1));
            _result = GetInfo();
            continuation?.Invoke();
        });
    }

private string GetInfo()
    {
        return $"Task is running on a thread id {Thread.CurrentThread.ManagedThreadId}. " +
            $"Is thread pool thread: {Thread.CurrentThread.IsThreadPoolThread}";
    }
}
```

运行结果：

```
Completed synchronously
Task is running on a thread id 3. Is thread pool thread: True
```

## notes

*该实现只适用于教学目的。当你编写异步函数时，最自然的方式好还是使用标准的Task类型。*

【C#】【多线程】【05-使用C#6.0】08-自定义awaitable类型

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