Timeout an async method implemented with TaskCompletionSource

Question

I have a blackbox object that exposes a method to kick of an async operation, and an event fires when the operation is complete. I have wrapped that into an Task<OpResult> BlackBoxOperationAysnc() method using TaskCompletionSource - that works well.

However, in that async wrapper I'd like to manage completing the async call with a timeout error if the event is not received after a given timeout. Currently I manage it with a timer as:

public Task<OpResult> BlackBoxOperationAysnc() {     var tcs = new TaskCompletionSource<TestResult>();        const int timeoutMs = 20000;     Timer timer = new Timer(_ => tcs.TrySetResult(OpResult.Timeout),                             null, timeoutMs, Timeout.Infinite);      EventHandler<EndOpEventArgs> eventHandler = (sender, args) => {         ...         tcs.TrySetResult(OpResult.BlarBlar);     }     blackBox.EndAsyncOpEvent += eventHandler;     blackBox.StartAsyncOp();     return tcs.Task; } 

Is that the only way to manage a timeout? Is there someway without setting up my own timer - I couldn't see anything timeout built into TaskCompletionSource?

Answer 1

You could use CancellationTokenSource with timeout. Use it together with your TaskCompletionSource like this.

E.g.:

public Task<OpResult> BlackBoxOperationAysnc() {     var tcs = new TaskCompletionSource<TestResult>();      const int timeoutMs = 20000;     var ct = new CancellationTokenSource(timeoutMs);     ct.Token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: false);      EventHandler<EndOpEventArgs> eventHandler = (sender, args) => {         ...         tcs.TrySetResult(OpResult.BlarBlar);     }     blackBox.EndAsyncOpEvent += eventHandler;     blackBox.StartAsyncOp();     return tcs.Task; } 

Updated, here's a complete functional example:

using System; using System.ComponentModel; using System.Threading; using System.Threading.Tasks;  namespace ConsoleApplication {     public class Program     {         // .NET 4.5/C# 5.0: convert EAP pattern into TAP pattern with timeout         public async Task<AsyncCompletedEventArgs> BlackBoxOperationAsync(             object state,             CancellationToken token,             int timeout = Timeout.Infinite)         {             var tcs = new TaskCompletionSource<AsyncCompletedEventArgs>();             using (var cts = CancellationTokenSource.CreateLinkedTokenSource(token))             {                 // prepare the timeout                 if (timeout != Timeout.Infinite)                 {                     cts.CancelAfter(timeout);                 }                  // handle completion                 AsyncCompletedEventHandler handler = (sender, args) =>                 {                     if (args.Cancelled)                         tcs.TrySetCanceled();                     else if (args.Error != null)                         tcs.SetException(args.Error);                     else                         tcs.SetResult(args);                 };                  this.BlackBoxOperationCompleted += handler;                 try                 {                     using (cts.Token.Register(() => tcs.SetCanceled(), useSynchronizationContext: false))                     {                         this.StartBlackBoxOperation(null);                         return await tcs.Task.ConfigureAwait(continueOnCapturedContext: false);                     }                 }                 finally                 {                     this.BlackBoxOperationCompleted -= handler;                 }             }         }          // emulate async operation         AsyncCompletedEventHandler BlackBoxOperationCompleted = delegate { };          void StartBlackBoxOperation(object state)         {             ThreadPool.QueueUserWorkItem(s =>             {                 Thread.Sleep(1000);                 this.BlackBoxOperationCompleted(this, new AsyncCompletedEventArgs(error: null, cancelled: false, userState: state));             }, state);         }          // test         static void Main()         {             try             {                 new Program().BlackBoxOperationAsync(null, CancellationToken.None, 1200).Wait();                 Console.WriteLine("Completed.");                 new Program().BlackBoxOperationAsync(null, CancellationToken.None, 900).Wait();             }             catch (Exception ex)             {                 while (ex is AggregateException)                     ex = ex.InnerException;                 Console.WriteLine(ex.Message);             }             Console.ReadLine();         }     } } 

A .NET 4.0/C# 4.0 vesion can be found here, it takes advantage of the compiler-generated IEnumerator state machine.