How would I run an async Task<T> method synchronously?

Question

I am 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<Customers> GetCustomers() {     return await Service.GetCustomersAsync(); } 

Normal usage:

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

I've tried using the following:

Task<Customer> task = GetCustomers(); task.Wait()  Task<Customer> task = GetCustomers(); task.RunSynchronously();  Task<Customer> task = GetCustomers(); while(task.Status != TaskStatus.RanToCompletion) 

I also tried a suggestion from 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.<CreateOpenPanelList>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.<CreateOpenPanelList>d__3b.MoveNext() in C:\Documents and Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line 625    at System.Runtime.CompilerServices.TaskAwaiter.<>c__DisplayClass7.<TrySetContinuationForAwait>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 1

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<List<Customer>>(() => GetCustomers());

Code is from here

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;         }     } }