异步task处理

public async Task<Customers> GetCustomers()
{
    return await Service.GetCustomersAsync();
}

public async void GetCustomers()
{
    customerList = await GetCustomers();
}

引用方法  http://stackoverflow.com/questions/5095183/how-would-i-run-an-async-taskt-method-synchronously

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;
        }
    }
}

调用：

customerList = AsyncHelpers.RunSync<List<Customer>>(() => GetCustomers());

也可以使用如下方式：

.Net 4.5 下

// For Task<T>: will block until the task is completed...
var result = task.Result; 

// For Task (not Task<T>):
task2.RunSynchronously();

.Net 4.0 下

var x = (IAsyncResult)task;
task.Start();

x.AsyncWaitHandle.WaitOne();

或:

task.Start();
task.Wait();

还有一个开源项目：https://github.com/tejacques/AsyncBridge/blob/master/src/AsyncBridge/AsyncHelper.cs

using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace AsyncBridge
{
    using EventTask = Tuple<SendOrPostCallback, object>;
    using EventQueue = ConcurrentQueue<Tuple<SendOrPostCallback, object>>;

    /// <summary>
    /// A Helper class to run Asynchronous functions from synchronous ones
    /// </summary>
    public static class AsyncHelper
    {
        /// <summary>
        /// A class to bridge synchronous asynchronous methods
        /// </summary>
        public class AsyncBridge : IDisposable
        {
            private ExclusiveSynchronizationContext CurrentContext;
            private SynchronizationContext OldContext;
            private int TaskCount;

            /// <summary>
            /// Constructs the AsyncBridge by capturing the current
            /// SynchronizationContext and replacing it with a new
            /// ExclusiveSynchronizationContext.
            /// </summary>
            internal AsyncBridge()
            {
                OldContext = SynchronizationContext.Current;
                CurrentContext =
                    new ExclusiveSynchronizationContext(OldContext);
                SynchronizationContext
                    .SetSynchronizationContext(CurrentContext);
            }

            /// <summary>
            /// Execute's an async task with a void return type
            /// from a synchronous context
            /// </summary>
            /// <param name="task">Task to execute</param>
            /// <param name="callback">Optional callback</param>
            public void Run(Task task, Action<Task> callback = null)
            {
                CurrentContext.Post(async _ =>
                {
                    try
                    {
                        Increment();
                        await task;

                        if (null != callback)
                        {
                            callback(task);
                        }
                    }
                    catch (Exception e)
                    {
                        CurrentContext.InnerException = e;
                    }
                    finally
                    {
                        Decrement();
                    }
                }, null);
            }

            /// <summary>
            /// Execute's an async task with a T return type
            /// from a synchronous context
            /// </summary>
            /// <typeparam name="T">The type of the task</typeparam>
            /// <param name="task">Task to execute</param>
            /// <param name="callback">Optional callback</param>
            public void Run<T>(Task<T> task, Action<Task<T>> callback = null)
            {
                if (null != callback)
                {
                    Run((Task)task, (finishedTask) =>
                        callback((Task<T>)finishedTask));
                }
                else
                {
                    Run((Task)task);
                }
            }

            /// <summary>
            /// Execute's an async task with a T return type
            /// from a synchronous context
            /// </summary>
            /// <typeparam name="T">The type of the task</typeparam>
            /// <param name="task">Task to execute</param>
            /// <param name="callback">
            /// The callback function that uses the result of the task
            /// </param>
            public void Run<T>(Task<T> task, Action<T> callback)
            {
                Run(task, (t) => callback(t.Result));
            }

            private void Increment()
            {
                Interlocked.Increment(ref TaskCount);
            }

            private void Decrement()
            {
                Interlocked.Decrement(ref TaskCount);
                if (TaskCount == )
                {
                    CurrentContext.EndMessageLoop();
                }
            }

            /// <summary>
            /// Disposes the object
            /// </summary>
            public void Dispose()
            {
                try
                {
                    CurrentContext.BeginMessageLoop();
                }
                catch (Exception e)
                {
                    throw e;
                }
                finally
                {
                    SynchronizationContext
                        .SetSynchronizationContext(OldContext);
                }
            }
        }

        /// <summary>
        /// Creates a new AsyncBridge. This should always be used in
        /// conjunction with the using statement, to ensure it is disposed
        /// </summary>
        public static AsyncBridge Wait
        {
            get { return new AsyncBridge(); }
        }

        /// <summary>
        /// Runs a task with the "Fire and Forget" pattern using Task.Run,
        /// and unwraps and handles exceptions
        /// </summary>
        /// <param name="task">A function that returns the task to run</param>
        /// <param name="handle">Error handling action, null by default</param>
        public static void FireAndForget(
            Func<Task> task,
            Action<Exception> handle = null)
        {
#if NET_45
            Task.Run(
#elif NET_40
            TaskEx.Run(
#endif
            () =>
            {
                ((Func<Task>)(async () =>
                {
                    try
                    {
                        await task();
                    }
                    catch (Exception e)
                    {
                        if (null != handle)
                        {
                            handle(e);
                        }
                    }
                }))();
            });
        }

        private class ExclusiveSynchronizationContext : SynchronizationContext
        {
            private readonly AutoResetEvent _workItemsWaiting =
                new AutoResetEvent(false);

            private bool _done;
            private EventQueue _items;

            public Exception InnerException { get; set; }

            public ExclusiveSynchronizationContext(SynchronizationContext old)
            {
                ExclusiveSynchronizationContext oldEx =
                    old as ExclusiveSynchronizationContext;

                if (null != oldEx)
                {
                    this._items = oldEx._items;
                }
                else
                {
                    this._items = new EventQueue();
                }
            }

            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)
            {
                _items.Enqueue(Tuple.Create(d, state));
                _workItemsWaiting.Set();
            }

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

            public void BeginMessageLoop()
            {
                while (!_done)
                {
                    EventTask task = null;

                    if (!_items.TryDequeue(out task))
                    {
                        task = null;
                    }

                    if (task != null)
                    {
                        task.Item1(task.Item2);
                        if (InnerException != null) // method threw an exeption
                        {
                            throw new AggregateException(
                                "AsyncBridge.Run method threw an exception.",
                                InnerException);
                        }
                    }
                    else
                    {
                        _workItemsWaiting.WaitOne();
                    }
                }
            }

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