c# - 使用 Task 异步并等待同步方法

Question

我对 async/await 概念还很陌生，所以很抱歉问了一些显而易见的问题。

我需要发送电子邮件，新 API 要求我使用 async 并等待。问题是我的很多方法都需要同步调用这个“发送电子邮件”。

因此，我创建了一个同步包装器方法:

private Task SendEmailAsync(string email, string content)
{
   ...
   ...
   ...
}

private void SendEmail(string email, string content)
{
    Task tsk = Task.Factory.StartNew(async () => await SendEmailAsync(email, content));
    try { tsk.Wait(); }
    catch (AggregateException aex)
    {
        throw aex.Flatten();
    }
}

但出于某种原因，tsk.Wait() 不会等待 await SendEmailAsync(...) 完成。所以，我需要添加 ManualResetEvent。像这样

private void SendEmail(string email, string content)
{
    ManualResetEvent mre = new ManualResetEvent(false);
    Task tsk = Task.Factory.StartNew(async () =>
    {
        mre.Reset();
        try { await SendEmailAsync(email, content); }
        finally { mre.Set(); }
    });

    mre.WaitOne();
    try { tsk.Wait(); }
    catch (AggregateException aex)
    {
        throw aex.Flatten();
    }
}

但是 SendEmailAsync(...) 抛出的任何异常都不会被 tsk.Wait() 捕获。我的问题是:

1. 为什么 tsk.Wait() 不等待 await SendEmailAsync(...)
2. 如何捕获 await SendEmailAsync(...) 抛出的异常

谢谢!

Answer 1

您可以使用以下扩展以同步方式运行异步代码。

https://stackoverflow.com/a/5097066/5062791

public static class AsyncHelpers
{
    /// <summary>
    /// Execute's an async Task<T> method which has a void return value synchronously
    /// </summary>
    /// <param name="task">Task<T> method to execute</param>
    public static void RunSync(Func<Task> task)
    {
        var oldContext = SynchronizationContext.Current;
        var synch = new ExclusiveSynchronizationContext();
        SynchronizationContext.SetSynchronizationContext(synch);
        synch.Post(async _ =>
        {
            try
            {
                await task();
            }
            catch (Exception e)
            {
                synch.InnerException = e;
                throw;
            }
            finally
            {
                synch.EndMessageLoop();
            }
        }, null);
        synch.BeginMessageLoop();

        SynchronizationContext.SetSynchronizationContext(oldContext);
    }

    /// <summary>
    /// Execute's an async Task<T> method which has a T return type synchronously
    /// </summary>
    /// <typeparam name="T">Return Type</typeparam>
    /// <param name="task">Task<T> method to execute</param>
    /// <returns></returns>
    public static T RunSync<T>(Func<Task<T>> task)
    {
        var oldContext = SynchronizationContext.Current;
        var synch = new ExclusiveSynchronizationContext();
        SynchronizationContext.SetSynchronizationContext(synch);
        T ret = default(T);
        synch.Post(async _ =>
        {
            try
            {
                ret = await task();
            }
            catch (Exception e)
            {
                synch.InnerException = e;
                throw;
            }
            finally
            {
                synch.EndMessageLoop();
            }
        }, null);
        synch.BeginMessageLoop();
        SynchronizationContext.SetSynchronizationContext(oldContext);
        return ret;
    }

    private class ExclusiveSynchronizationContext : SynchronizationContext
    {
        private bool done;
        public Exception InnerException { get; set; }
        readonly AutoResetEvent workItemsWaiting = new AutoResetEvent(false);
        readonly Queue<Tuple<SendOrPostCallback, object>> items =
            new Queue<Tuple<SendOrPostCallback, object>>();

        public override void Send(SendOrPostCallback d, object state)
        {
            throw new NotSupportedException("We cannot send to our same thread");
        }

        public override void Post(SendOrPostCallback d, object state)
        {
            lock (items)
            {
                items.Enqueue(Tuple.Create(d, state));
            }
            workItemsWaiting.Set();
        }

        public void EndMessageLoop()
        {
            Post(_ => done = true, null);
        }

        public void BeginMessageLoop()
        {
            while (!done)
            {
                Tuple<SendOrPostCallback, object> task = null;
                lock (items)
                {
                    if (items.Count > 0)
                    {
                        task = items.Dequeue();
                    }
                }
                if (task != null)
                {
                    task.Item1(task.Item2);
                    if (InnerException != null) // the method threw an exeption
                    {
                        throw new AggregateException("AsyncHelpers.Run method threw an exception.", InnerException);
                    }
                }
                else
                {
                    workItemsWaiting.WaitOne();
                }
            }
        }

        public override SynchronizationContext CreateCopy()
        {
            return this;
        }
    }
}