c# - How would I run an async Task method synchronously?

itemprop="text">

I'm learning about async/await, and
ran into a situation where I need to call an async method synchronously. How can I do
that?

Async
method:

public async
            Task GetCustomers()
{

 return await
            Service.GetCustomersAsync();
}

Normal
usage:

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

I've
tried using the following:

Task task =
            GetCustomers();
task.Wait()

Task task =
            GetCustomers();
task.RunSynchronously();


Task
            task = GetCustomers();
while(task.Status !=
            TaskStatus.RanToCompletion)

I
also tried a suggestion from href="http://social.msdn.microsoft.com/Forums/en/async/thread/163ef755-ff7b-4ea5-b226-bbe8ef5f4796"
rel="noreferrer">here, however it doesn't work when the dispatcher is in a
suspended state.

public static
            void WaitWithPumping(this Task task) 
{
 if (task == null) throw new
            ArgumentNullException(“task”);
 var nestedFrame = new
            DispatcherFrame();

 task.ContinueWith(_ => nestedFrame.Continue
            = false);
 Dispatcher.PushFrame(nestedFrame);

            task.Wait();
}

Here
is the exception and stack trace from calling
RunSynchronously:

System.InvalidOperationException

Message: RunSynchronously may not be called on a
task unbound to a delegate.

InnerException: null

Source: mscorlib

StackTrace:

            at System.Threading.Tasks.Task.InternalRunSynchronously(TaskScheduler
            scheduler)
 at System.Threading.Tasks.Task.RunSynchronously()
 at
            MyApplication.CustomControls.Controls.MyCustomControl.CreateAvailablePanelList() in
            C:\Documents and
            Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line
            638
 at
            MyApplication.CustomControls.Controls.MyCustomControl.get_AvailablePanels() in
            C:\Documents and
            Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line
            233
 at
            MyApplication.CustomControls.Controls.MyCustomControl.b__36(DesktopPanel
            panel) in C:\Documents and
            Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line
            597
 at System.Collections.Generic.List`1.ForEach(Action`1 action)

            at
            MyApplication.CustomControls.Controls.MyCustomControl.d__3b.MoveNext()
            in C:\Documents and
            Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line
            625
 at
            System.Runtime.CompilerServices.TaskAwaiter.<>c__DisplayClass7.b__1(Object
            state)
 at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate
            callback, Object args, Int32 numArgs)
 at
            MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method,
            Object args, Int32 numArgs, Delegate catchHandler)

 at
            System.Windows.Threading.DispatcherOperation.InvokeImpl()
 at
            System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object
            state)
 at System.Threading.ExecutionContext.runTryCode(Object
            userData)
 at
            System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode
            code, CleanupCode backoutCode, Object userData)
 at
            System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext,
            ContextCallback callback, Object state)
 at
            System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback
            callback, Object state, Boolean ignoreSyncCtx)
 at
            System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback
            callback, Object state)
 at
            System.Windows.Threading.DispatcherOperation.Invoke()
 at
            System.Windows.Threading.Dispatcher.ProcessQueue()
 at
            System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam,
            IntPtr lParam, Boolean& handled)

 at
            MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam,
            Boolean& handled)
 at
            MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
 at
            System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object
            args, Int32 numArgs)
 at
            MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method,
            Object args, Int32 numArgs, Delegate catchHandler)
 at
            System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan
            timeout, Delegate method, Object args, Int32 numArgs)
 at
            MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr
            lParam)
 at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG&
            msg)
 at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame
            frame)
 at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame
            frame)
 at System.Windows.Threading.Dispatcher.Run()

 at
            System.Windows.Application.RunDispatcher(Object ignore)
 at
            System.Windows.Application.RunInternal(Window window)
 at
            System.Windows.Application.Run(Window window)
 at
            System.Windows.Application.Run()
 at MyApplication.App.Main() in C:\Documents
            and Settings\...\MyApplication\obj\Debug\App.g.cs:line 50
 at
            System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)

            at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity,
            String[] args)
 at
            Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
 at
            System.Threading.ThreadHelper.ThreadStart_Context(Object state)
 at
            System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback
            callback, Object state, Boolean ignoreSyncCtx)

 at
            System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback
            callback, Object state)
 at
            System.Threading.ThreadHelper.ThreadStart()

Answer

Here's a workaround I found that works for
all cases (including suspended dispatchers). It's not my code and I'm still working to
fully understand it, but it does work.

It can be
called using:

customerList =
AsyncHelpers.RunSync>(() =>
GetCustomers());

Code is
from href="http://social.msdn.microsoft.com/Forums/en/async/thread/163ef755-ff7b-4ea5-b226-bbe8ef5f4796"
rel="noreferrer">here

public
            static class AsyncHelpers
{
 /// 

 ///
            Execute's an async Task method which has a void return value
            synchronously
 /// 
 ///             name="task">Task method to execute
 public static
            void RunSync(Func 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);

            }

 /// 

 /// Execute's an async
            Task method which has a T return type synchronously
 ///
            
 /// Return
            Type
 /// Task method to
            execute
 ///
            

 public static T
            RunSync(Func> 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> items =
 new
            Queue>();

 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            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;
 }

            }
}