是否有可能等待未声明为异步的IO操作？(Is it possible to await an IO operation that is not declared as async? If not, what should I do?)

 我是C＃中的异步编程新手，我仍然对一些事情感到困惑。 我已经阅读了.NET 4.5之后，APM和EAP不再被推荐用于新开发，因为TAP应该取代它们（ 源代码 ）。 
 
 我想我理解异步/等待如何工作，我可以使用它们来执行具有异步方法的IO操作。 例如，我可以编写一个异步方法，等待HttpWebClient的GetStringAsync结果，因为它被声明为异步方法。 那很棒。 
 
 我的问题是：如果我们在未声明为异步的方法中发生IO操作，该怎么办？ 像这样：假设我有一个有方法的API 
 
 
string GetResultFromWeb()
 
 
 从Web查询某些内容。 我有很多不同的查询要做，我必须使用这种方法来做到这一点。 然后我需要处理每个查询结果。 我明白，如果这是一个异步方法，我会这样做： 
 
 
Task<string> getResultTask = GetResultFromWeb(myUrl); 
// Do whatever I need to do that doesn't need the query result
string result = await getResultTask;
Process(result);
 
 
 但由于不是，我不能等待它 - 它告诉我字符串不是等待的。 所以我的问题是：是否有任何方式异步执行这些IO操作，而不必为每个查询创建一个线程？ 如果可以的话，我想创建尽可能少的线程，而不必阻塞任何线程。 
 
 我发现的一种方式是通过实施APM，遵循Jeffrey Richter的这篇文章 ，然后在我的Begin方法中，我调用ThreadPool.QueueWorkItem（GetResultFromWeb，asyncResult）。 喜欢这个： 
 
 
public class A {
    private void DoQuery(Object ar){
        AsyncResult<string> asyncResult = (AsyncResult<string>) ar;
        string result = GetResultFromWeb();
        asyncResult.SetAsCompleted(result, false);
    }

    public IAsyncResult BeginQuery(AsyncCallback){
        AsyncResult<string> asyncResult = new AsyncResult<string>(callback, this);
        ThreadPool.QueueUserWorkItem(DoQuery, asyncResult);
        return asyncResult;
    }

    public string EndQuery(IAsyncResult ar){
        AsyncResult<string> asyncResult = (AsyncResult<string>)ar;
        return asyncResult.EndInvoke();
    }
}
 
 
 然后，我使用AsyncEnumerator并开始（BeginQuery）几个查询，并在每个查询结束时处理结果（使用yield return / EndQuery）。 这似乎运作良好。 但是，在阅读了太多以至于APM已经过时之后，我想知道如何使用TAP来做到这一点。 另外，这种APM方法有什么问题吗？ 
 
 谢谢！ 

I'm new to asynchronous programming in C# and I'm still confused about a few things. I've read that after .NET 4.5, the APM and EAP are no longer recommended for new development since the TAP is supposed to replace them (source).
 
I think I understood how async/await works and I'd be able to use them for performing IO operations that have async methods. For example, I could write an async method that awaits for an HttpWebClient's GetStringAsync result, since it's declared as an async method. That's great.
 
My question is: what if we have an IO operation that happens in a method that is not declared as async? Like this: suppose I have an API that has a method
 
 
string GetResultFromWeb()
 
 
which queries something from the Web. And I have lots of different queries to do and I must use this method to do so. And then I need to process each query result. I understand that I'd do this if that was an async method:
 
 
Task<string> getResultTask = GetResultFromWeb(myUrl); 
// Do whatever I need to do that doesn't need the query result
string result = await getResultTask;
Process(result);
 
 
But since it's not, I cannot await for it -- it tells me string is not awaitable. So my question is: is there any way of performing these IO operations asynchronously without having to create one thread for each query? If I could, I'd like to create as less threads as possible, without having to block any of the threads.
 
One way I found to do so was by implementing APM, following this article from Jeffrey Richter and then, in my Begin method, I call ThreadPool.QueueWorkItem(GetResultFromWeb, asyncResult). Like this:
 
 
public class A {
    private void DoQuery(Object ar){
        AsyncResult<string> asyncResult = (AsyncResult<string>) ar;
        string result = GetResultFromWeb();
        asyncResult.SetAsCompleted(result, false);
    }

    public IAsyncResult BeginQuery(AsyncCallback){
        AsyncResult<string> asyncResult = new AsyncResult<string>(callback, this);
        ThreadPool.QueueUserWorkItem(DoQuery, asyncResult);
        return asyncResult;
    }

    public string EndQuery(IAsyncResult ar){
        AsyncResult<string> asyncResult = (AsyncResult<string>)ar;
        return asyncResult.EndInvoke();
    }
}
 
 
Then I use an AsyncEnumerator and begin (BeginQuery) several queries and process the results as each one of them finishes (using yield return / EndQuery). This seems to work well. But after reading so much that APM is obsolete, I was wondering how could I do this using TAP. Also, is there any problem with this APM approach? 
 
Thanks! 

原文：https://stackoverflow.com/questions/27790468