在C# 5.0中，为了更方便地异步编程，引入了async和await两个关键字。但是这造成了一定的麻烦，因为在WinForms，WPF，XXXXXXT中，返回的Task只能await，不能Wait()或者Result，否则会导致死锁。

async的返回值只能通过await等待，而await又要求父方法是async的，这是一个循环，这会导致哪里调用了async方法，async传染到哪里。有时候我们必须调用async方法，但是却不想让自己写的方法也成为async方法，怎么办呢？

一个方法是对执行上下文（SynchronizationContext）动手脚，方便起见，这里写了个Helper。注意，由于操作了全局上下文，因此不再是线程安全的。

另一个方法，是使用Task的ContinueWith方法设置回调函数，代替await的功能。这种方法只是避免了async关键字传染，并没有改变异步方法的本质。

第三种方法，是查阅async方法的源代码(必要且可能时，修改它)，确保每一处都调用了ConfigureAwait(false)，也就是不会阻塞主线程，然后对返回的Task使用Wait()或Result进行等待。

现在我们只讨论第一种，使用方法如下：

<code class="lang-c">async Task<int> DelayAsync(int n)
{
    await Task.Delay(n);
    return n;
}
    
void Button1Click(object sender, EventArgs e)
{
    int n = AsyncHelper.AsyncInline.Run(async () => {     
        return await DelayAsync(1000);
    });
    MessageBox.Show(n.ToString());
}</int></code>

AsyncHelper代码如下：

<code class="lang-c">using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
    
namespace AsyncHelper
{
    public static class AsyncInline
    {
        // 使用方法：AsyncInline.Run(async () => { ... });
        public static void Run(Func<task> item)
        {
            var oldContext = SynchronizationContext.Current;
            var synch = new ExclusiveSynchronizationContext();
            SynchronizationContext.SetSynchronizationContext(synch);
            synch.Post(async _ =>
            {
                try
                {
                    await item();
                }
                catch (Exception e)
                {
                    synch.InnerException = e;
                    throw;
                }
                finally
                {
                    synch.EndMessageLoop();
                }
            }, null);
            synch.BeginMessageLoop();
            SynchronizationContext.SetSynchronizationContext(oldContext);
        }
            
        // 使用方法：var retval = AsyncInline.Run(async () => { ... });
        public static T Run<t>(Func<task<t>> item)
        {
            var oldContext = SynchronizationContext.Current;
            var synch = new ExclusiveSynchronizationContext();
            SynchronizationContext.SetSynchronizationContext(synch);
            T ret = default(T);
            synch.Post(async _ =>
            {
                try
                {
                    ret = await
                    item();
                }
                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("AsyncInline.Run method threw an exception.",
                             InnerException);
                        }
                    }
                    else
                    {
                        workItemsWaiting.WaitOne();
                    }
                }
            }
            public override SynchronizationContext CreateCopy()
            {
                return this;
            }
        }
    }
}</sendorpostcallback,></tuple<sendorpostcallback,></tuple<sendorpostcallback,></task<t></t></task></code>