I have a public async void Foo() method that I want to call from synchronous method. So far all I have seen from MSDN documentation is calling async methods via async methods, but my whole program is not built with async methods.
Is this even possible?
Here's one example of calling these methods from an asynchronous method:
Walkthrough: Accessing the Web by Using Async and Await (C# and Visual Basic)
Now I'm looking into calling these async methods from sync methods.
Asynchronous programming does "grow" through the code base. It has been compared to a zombie virus. The best solution is to allow it to grow, but sometimes that's not possible.
I have written a few types in my Nito.AsyncEx library for dealing with a partially-asynchronous code base. There's no solution that works in every situation, though.
Solution A
If you have a simple asynchronous method that doesn't need to synchronize back to its context, then you can use Task.WaitAndUnwrapException:
var task = MyAsyncMethod();
var result = task.WaitAndUnwrapException();
You do not want to use Task.Wait or Task.Result because they wrap exceptions in AggregateException.
This solution is only appropriate if MyAsyncMethod does not synchronize back to its context. In other words, every await in MyAsyncMethod should end with ConfigureAwait(false). This means it can't update any UI elements or access the ASP.NET request context.
Solution B
If MyAsyncMethod does need to synchronize back to its context, then you may be able to use AsyncContext.RunTask to provide a nested context:
var result = AsyncContext.RunTask(MyAsyncMethod).Result;
*Update 4/14/2014: In more recent versions of the library the API is as follows:
var result = AsyncContext.Run(MyAsyncMethod);
(It's OK to use Task.Result in this example because RunTask will propagate Task exceptions).
The reason you may need AsyncContext.RunTask instead of Task.WaitAndUnwrapException is because of a rather subtle deadlock possibility that happens on WinForms/WPF/SL/ASP.NET:
Task.Task.async method uses await without ConfigureAwait.Task cannot complete in this situation because it only completes when the async method is finished; the async method cannot complete because it is attempting to schedule its continuation to the SynchronizationContext, and WinForms/WPF/SL/ASP.NET will not allow the continuation to run because the synchronous method is already running in that context.This is one reason why it's a good idea to use ConfigureAwait(false) within every async method as much as possible.
Solution C
AsyncContext.RunTask won't work in every scenario. For example, if the async method awaits something that requires a UI event to complete, then you'll deadlock even with the nested context. In that case, you could start the async method on the thread pool:
var task = Task.Run(async () => await MyAsyncMethod());
var result = task.WaitAndUnwrapException();
However, this solution requires a MyAsyncMethod that will work in the thread pool context. So it can't update UI elements or access the ASP.NET request context. And in that case, you may as well add ConfigureAwait(false) to its await statements, and use solution A.
Update, 2019-05-01: The current "least-worst practices" are in an MSDN article here.
Adding a solution that finally solved my problem, hopefully saves somebody's time.
Firstly read a couple articles of Stephen Cleary:
From the "two best practices" in "Don't Block on Async Code", the first one didn't work for me and the second one wasn't applicable (basically if I can use await, I do!).
So here is my workaround: wrap the call inside a Task.Run<>(async () => await FunctionAsync()); and hopefully no deadlock anymore.
Here is my code:
public class LogReader
{
ILogger _logger;
public LogReader(ILogger logger)
{
_logger = logger;
}
public LogEntity GetLog()
{
Task<LogEntity> task = Task.Run<LogEntity>(async () => await GetLogAsync());
return task.Result;
}
public async Task<LogEntity> GetLogAsync()
{
var result = await _logger.GetAsync();
// more code here...
return result as LogEntity;
}
}
Microsoft built an AsyncHelper (internal) class to run Async as Sync. The source looks like:
internal static class AsyncHelper
{
private static readonly TaskFactory _myTaskFactory = new
TaskFactory(CancellationToken.None,
TaskCreationOptions.None,
TaskContinuationOptions.None,
TaskScheduler.Default);
public static TResult RunSync<TResult>(Func<Task<TResult>> func)
{
return AsyncHelper._myTaskFactory
.StartNew<Task<TResult>>(func)
.Unwrap<TResult>()
.GetAwaiter()
.GetResult();
}
public static void RunSync(Func<Task> func)
{
AsyncHelper._myTaskFactory
.StartNew<Task>(func)
.Unwrap()
.GetAwaiter()
.GetResult();
}
}
The Microsoft.AspNet.Identity base classes only have Async methods and in order to call them as Sync there are classes with extension methods that look like (example usage):
public static TUser FindById<TUser, TKey>(this UserManager<TUser, TKey> manager, TKey userId) where TUser : class, IUser<TKey> where TKey : IEquatable<TKey>
{
if (manager == null)
{
throw new ArgumentNullException("manager");
}
return AsyncHelper.RunSync<TUser>(() => manager.FindByIdAsync(userId));
}
public static bool IsInRole<TUser, TKey>(this UserManager<TUser, TKey> manager, TKey userId, string role) where TUser : class, IUser<TKey> where TKey : IEquatable<TKey>
{
if (manager == null)
{
throw new ArgumentNullException("manager");
}
return AsyncHelper.RunSync<bool>(() => manager.IsInRoleAsync(userId, role));
}
For those concerned about the licensing terms of code, here is a link to very similar code (just adds support for culture on the thread) that has comments to indicate that it is MIT Licensed by Microsoft. https://github.com/aspnet/AspNetIdentity/blob/master/src/Microsoft.AspNet.Identity.Core/AsyncHelper.cs
async Main is now part of C# 7.2 and can be enabled in the projects advanced build settings.
For C# < 7.2, the correct way is:
static void Main(string[] args)
{
MainAsync().GetAwaiter().GetResult();
}
static async Task MainAsync()
{
/*await stuff here*/
}
You'll see this used in a lot of Microsoft documentation, for example: https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-dotnet-how-to-use-topics-subscriptions
public async Task<string> StartMyTask()
{
await Foo()
// code to execute once foo is done
}
static void Main()
{
var myTask = StartMyTask(); // call your method which will return control once it hits await
// now you can continue executing code here
string result = myTask.Result; // wait for the task to complete to continue
// use result
}
You read the 'await' keyword as "start this long running task, then return control to the calling method". Once the long-running task is done, then it executes the code after it. The code after the await is similar to what used to be CallBack methods. The big difference being the logical flow is not interrupted which makes it much easier to write and read.
I'm not 100% sure, but I believe the technique described in this blog should work in many circumstances:
You can thus use
task.GetAwaiter().GetResult()if you want to directly invoke this propagation logic.
There is, however, a good solution that works in (almost: see comments) every situation: an ad-hoc message pump (SynchronizationContext).
The calling thread will be blocked as expected, while still ensuring that all continuations called from the async function don't deadlock as they'll be marshaled to the ad-hoc SynchronizationContext (message pump) running on the calling thread.
The code of the ad-hoc message pump helper:
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
namespace Microsoft.Threading
{
/// <summary>Provides a pump that supports running asynchronous methods on the current thread.</summary>
public static class AsyncPump
{
/// <summary>Runs the specified asynchronous method.</summary>
/// <param name="asyncMethod">The asynchronous method to execute.</param>
public static void Run(Action asyncMethod)
{
if (asyncMethod == null) throw new ArgumentNullException("asyncMethod");
var prevCtx = SynchronizationContext.Current;
try
{
// Establish the new context
var syncCtx = new SingleThreadSynchronizationContext(true);
SynchronizationContext.SetSynchronizationContext(syncCtx);
// Invoke the function
syncCtx.OperationStarted();
asyncMethod();
syncCtx.OperationCompleted();
// Pump continuations and propagate any exceptions
syncCtx.RunOnCurrentThread();
}
finally { SynchronizationContext.SetSynchronizationContext(prevCtx); }
}
/// <summary>Runs the specified asynchronous method.</summary>
/// <param name="asyncMethod">The asynchronous method to execute.</param>
public static void Run(Func<Task> asyncMethod)
{
if (asyncMethod == null) throw new ArgumentNullException("asyncMethod");
var prevCtx = SynchronizationContext.Current;
try
{
// Establish the new context
var syncCtx = new SingleThreadSynchronizationContext(false);
SynchronizationContext.SetSynchronizationContext(syncCtx);
// Invoke the function and alert the context to when it completes
var t = asyncMethod();
if (t == null) throw new InvalidOperationException("No task provided.");
t.ContinueWith(delegate { syncCtx.Complete(); }, TaskScheduler.Default);
// Pump continuations and propagate any exceptions
syncCtx.RunOnCurrentThread();
t.GetAwaiter().GetResult();
}
finally { SynchronizationContext.SetSynchronizationContext(prevCtx); }
}
/// <summary>Runs the specified asynchronous method.</summary>
/// <param name="asyncMethod">The asynchronous method to execute.</param>
public static T Run<T>(Func<Task<T>> asyncMethod)
{
if (asyncMethod == null) throw new ArgumentNullException("asyncMethod");
var prevCtx = SynchronizationContext.Current;
try
{
// Establish the new context
var syncCtx = new SingleThreadSynchronizationContext(false);
SynchronizationContext.SetSynchronizationContext(syncCtx);
// Invoke the function and alert the context to when it completes
var t = asyncMethod();
if (t == null) throw new InvalidOperationException("No task provided.");
t.ContinueWith(delegate { syncCtx.Complete(); }, TaskScheduler.Default);
// Pump continuations and propagate any exceptions
syncCtx.RunOnCurrentThread();
return t.GetAwaiter().GetResult();
}
finally { SynchronizationContext.SetSynchronizationContext(prevCtx); }
}
/// <summary>Provides a SynchronizationContext that's single-threaded.</summary>
private sealed class SingleThreadSynchronizationContext : SynchronizationContext
{
/// <summary>The queue of work items.</summary>
private readonly BlockingCollection<KeyValuePair<SendOrPostCallback, object>> m_queue =
new BlockingCollection<KeyValuePair<SendOrPostCallback, object>>();
/// <summary>The processing thread.</summary>
private readonly Thread m_thread = Thread.CurrentThread;
/// <summary>The number of outstanding operations.</summary>
private int m_operationCount = 0;
/// <summary>Whether to track operations m_operationCount.</summary>
private readonly bool m_trackOperations;
/// <summary>Initializes the context.</summary>
/// <param name="trackOperations">Whether to track operation count.</param>
internal SingleThreadSynchronizationContext(bool trackOperations)
{
m_trackOperations = trackOperations;
}
/// <summary>Dispatches an asynchronous message to the synchronization context.</summary>
/// <param name="d">The System.Threading.SendOrPostCallback delegate to call.</param>
/// <param name="state">The object passed to the delegate.</param>
public override void Post(SendOrPostCallback d, object state)
{
if (d == null) throw new ArgumentNullException("d");
m_queue.Add(new KeyValuePair<SendOrPostCallback, object>(d, state));
}
/// <summary>Not supported.</summary>
public override void Send(SendOrPostCallback d, object state)
{
throw new NotSupportedException("Synchronously sending is not supported.");
}
/// <summary>Runs an loop to process all queued work items.</summary>
public void RunOnCurrentThread()
{
foreach (var workItem in m_queue.GetConsumingEnumerable())
workItem.Key(workItem.Value);
}
/// <summary>Notifies the context that no more work will arrive.</summary>
public void Complete() { m_queue.CompleteAdding(); }
/// <summary>Invoked when an async operation is started.</summary>
public override void OperationStarted()
{
if (m_trackOperations)
Interlocked.Increment(ref m_operationCount);
}
/// <summary>Invoked when an async operation is completed.</summary>
public override void OperationCompleted()
{
if (m_trackOperations &&
Interlocked.Decrement(ref m_operationCount) == 0)
Complete();
}
}
}
}
Usage:
AsyncPump.Run(() => FooAsync(...));
More detailed description of the async pump is available here.
To anyone paying attention to this question anymore...
If you look in Microsoft.VisualStudio.Services.WebApi there's a class called TaskExtensions. Within that class you'll see the static extension method Task.SyncResult(), which like totally just blocks the thread till the task returns.
Internally it calls task.GetAwaiter().GetResult() which is pretty simple, however it's overloaded to work on any async method that return Task, Task<T> or Task<HttpResponseMessage>... syntactic sugar, baby... daddy's got a sweet tooth.
It looks like ...GetAwaiter().GetResult() is the MS-official way to execute async code in a blocking context. Seems to work very fine for my use case.
You can call any asynchronous method from synchronous code, that is, until you need to await on them, in which case they have to be marked as async too.
As a lot of people are suggesting here, you could call Wait() or Result on the resulting task in your synchronous method, but then you end up with a blocking call in that method, which sort of defeats the purpose of async.
If you really can't make your method async and you don't want to lock up the synchronous method, then you're going to have to use a callback method by passing it as parameter to the ContinueWith() method on task.
Inspired by some of the other answers, I created the following simple helper methods:
public static TResult RunSync<TResult>(Func<Task<TResult>> method)
{
var task = method();
return task.GetAwaiter().GetResult();
}
public static void RunSync(Func<Task> method)
{
var task = method();
task.GetAwaiter().GetResult();
}
They can be called as follows (depending on whether you are returning a value or not):
RunSync(() => Foo());
var result = RunSync(() => FooWithResult());
Note that the signature in the original question public async void Foo() is incorrect. It should be public async Task Foo() as you should return Task not void for async methods that don't return a value (yes, there are some rare exceptions).
After hours of trying different methods, with more or less success, this is what I ended with. It doesn't end in a deadlock while getting result and it also gets and throws the original exception and not the wrapped one.
private ReturnType RunSync()
{
var task = Task.Run(async () => await myMethodAsync(agency));
if (task.IsFaulted && task.Exception != null)
{
throw task.Exception;
}
return task.Result;
}
Here is the simplest solution. I saw it somewhere on the Internet, I didn't remember where, but I have been using it successfully. It will not deadlock the calling thread.
void Synchronous Function()
{
Task.Run(Foo).Wait();
}
string SynchronousFunctionReturnsString()
{
return Task.Run(Foo).Result;
}
string SynchronousFunctionReturnsStringWithParam(int id)
{
return Task.Run(() => Foo(id)).Result;
}
Those windows async methods have a nifty little method called AsTask(). You can use this to have the method return itself as a task so that you can manually call Wait() on it.
For example, on a Windows Phone 8 Silverlight application, you can do the following:
private void DeleteSynchronous(string path)
{
StorageFolder localFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
Task t = localFolder.DeleteAsync(StorageDeleteOption.PermanentDelete).AsTask();
t.Wait();
}
private void FunctionThatNeedsToBeSynchronous()
{
// Do some work here
// ....
// Delete something in storage synchronously
DeleteSynchronous("pathGoesHere");
// Do other work here
// .....
}
Hope this helps!
async void Foo()method does not return aTaskit means a caller cannot know when it completes, it must returnTaskinstead. – Dai