Say you have an awaitable object, and you want to await it, but with a timeout. How would you build that?<\/p>\n
What you can do is use a The If the operation produced a result, you’ll have to create a timeout task that completes with the same result type, even if you never actually use that result.<\/p>\n The purpose of the We can wrap this up in a helper:<\/p>\n The usage pattern here is still rather clunky, though.<\/p>\n One common pattern is to call the method, but abandon it and return some fallback value instead (typically This time, our delayed dummy result is no longer a dummy result. If the task times out, then the result of Another way of writing the above would be<\/p>\n which you might choose if only because it give you a rare opportunity to write You could call the function like this:<\/p>\n The value in It might be that the fallback result is expensive to calculate. For example, it As a special case, you might want to raise a This is probably a common enough pattern that we could provide a special helper for it.<\/p>\n Note that in all of this, the task that timed out continues to run to completion. It’s just that we’re not paying attention to it any more. If you want to cancel the abandoned task, you need to hook up a task cancellation source when you create it, assuming that’s even possible.<\/p>\n In the special case where the If you prefer to exit with an exception, then you need to cancel the operation in your timeout handler:<\/p>\n That was a very long discussion, and I haven’t even gotten to the original purpose of writing about task cancellation with timeouts, which is to talk about how to do all of this in C++\/WinRT. I’m tired, so we’ll pick this up next time.<\/p>\nwhen_any<\/code>-like function in combination with a timeout coroutine. For C# this would be something like<\/p>\nawait Task.WhenAny(\r\n DoSomethingAsync(),\r\n Task.Delay(TimeSpan.FromSeconds(1)));\r\n<\/pre>\n
WhenAny<\/code> method completes as soon as any of the passed-in tasks completes. It returns the winner, which you can use to detect whether the operation completed or timed out:<\/p>\nvar somethingTask = DoSomethingAsync();\r\nvar winner = await Task.WhenAny(\r\n somethingTask,\r\n Task.Delay(TimeSpan.FromSeconds(1)));\r\nif (winner == somethingTask)\r\n{\r\n \/\/ hooray it worked\r\n}\r\nelse\r\n{\r\n \/\/ sad, it timed out\r\n}\r\n<\/pre>\nstatic async Task<T>\r\nDelayedDummyResultTask<T>(TimeSpan delay)\r\n{\r\n await Task.Delay(delay);\r\n return default(T);\r\n}\r\n\r\nvar somethingTask = GetSomethingAsync();\r\nvar winner = await Task.WhenAny(\r\n somethingTask,\r\n DelayedDummyResultTask<Something>(TimeSpan.FromSeconds(1)));\r\nif (winner == somethingTask)\r\n{\r\n \/\/ hooray it worked\r\n}\r\nelse\r\n{\r\n \/\/ sad, it timed out\r\n}\r\n<\/pre>\nDelayed\u00adDummy\u00adResult\u00adTask<\/code> is not to produce a result, but rather to provide a delay.<\/p>\nstatic async Task<(bool, Task<T>)>\r\nTaskWithTimeout<T>(\r\n Task<T> task,\r\n TimeSpan timeout)\r\n{\r\n var winner = await Task.WhenAny(\r\n task, DelayedDummyResultTask<T>(timeout));\r\n return (winner == task, winner);\r\n}\r\n\r\nvar (succeeded, task) = await TaskWithTimeout(\r\n GetProgramAsync(), TimeSpan.FromSeconds(1));\r\nif (succeeded) {\r\n UseIt(task.Result);\r\n} else {\r\n \/\/ Timed out\r\n}\r\n<\/pre>\nfalse<\/code> or null<\/code>):<\/p>\nstatic async Task<T>\r\nDelayedResultTask<T>(TimeSpan delay, T result = default(T))\r\n{\r\n await Task.Delay(delay);\r\n return result;\r\n}\r\n\r\nstatic async Task<T>\r\nTaskWithTimeoutAndFallback<T>(\r\n Task<T> task,\r\n TimeSpan timeout,\r\n T fallback = default(T))\r\n{\r\n return (await Task.WhenAny(\r\n task, DelayedResultTask<T>(timeout, fallback))).Result;\r\n}\r\n<\/pre>\nTask.Task\u00adWith\u00adTimeout\u00adAndFallback<\/code>.<\/p>\nstatic async Task<T>\r\nTaskWithTimeoutAndFallback<T>(\r\n Task<T> task,\r\n TimeSpan timeout,\r\n T fallback = default(T))\r\n{\r\n return await await Task.WhenAny(\r\n task, DelayedResultTask<T>(timeout, fallback));\r\n}\r\n<\/pre>\nawait await<\/code>.<\/p>\nvar something = TaskWithTimeoutAndFallback(\r\n GetSomethingAsync(), TimeSpan.FromSeconds(1));\r\n<\/pre>\n
something<\/code> is the result of Get\u00adSomething\u00adAsync()<\/code> or null<\/code>.<\/p>\nGet\u00adSomething\u00adAsync<\/code> times out, maybe you want to query some alternate database to get the fallback value. So maybe we could have a version where the fallback value is generated lazily.<\/p>\nstatic async Task<T>\r\nDelayedResultTask<T>(TimeSpan delay, Func<T> fallbackMaker)\r\n{\r\n await Task.Delay(delay);\r\n return fallbackMaker();\r\n}\r\n\r\nstatic async Task<T>\r\nTaskWithTimeoutAndFallback<T>(\r\n Task<T> task,\r\n TimeSpan timeout,\r\n Func<T> fallbackMaker)\r\n{\r\n return await await Task.WhenAny(\r\n task, DelayedResultTask<T>(timeout, fallbackMaker));\r\n}\r\n\r\nvar something = TaskWithTimeoutAndFallback(\r\n GetSomethingAsync(), TimeSpan.FromSeconds(1),\r\n () => LookupSomethingFromDatabase());\r\n<\/pre>\nTimeout\u00adException<\/code> instead of a fallback value. You could do that by passing a lambda that just throws the Timeout\u00adException<\/code> instead of producing a fallback value.<\/p>\nvar something = TaskWithTimeoutAndFallback(\r\n GetSomethingAsync(), TimeSpan.FromSeconds(1),\r\n () => throw TimeoutException());\r\n<\/pre>\n
static async Task<T>\r\nDelayedTimeoutExceptionTask<T>(TimeSpan delay)\r\n{\r\n await Task.Delay(delay);\r\n throw new TimeoutException();\r\n}\r\n\r\nstatic async Task<T>\r\nTaskWithTimeoutAndException<T>(\r\n Task<T> task,\r\n TimeSpan timeout)\r\n{\r\n return await await Task.WhenAny(\r\n task, DelayedTimeoutExceptionTask<T>(timeout));\r\n}\r\n\r\n\/\/ throws TimeoutException on timeout\r\nvar something = TaskWithTimeoutAndFallback(\r\n GetSomethingAsync(), TimeSpan.FromSeconds(1));\r\n<\/pre>\nTask<\/code> came from a Windows Runtime asynchronous action or operation, you can hook up the cancellation token yourself:<\/p>\nvar source = new CancellationTokenSource();\r\nvar something = TaskWithTimeoutAndFallback(\r\n o.GetSomethingAsync().AsTask(source.token),\r\n TimeSpan.FromSeconds(1));\r\nsource.Cancel();\r\nsource.Dispose();\r\n\r\n\/\/ see what's in the \"something\"\r\n<\/pre>\n
var source = new CancellationTokenSource();\r\ntry {\r\n var something = TaskWithTimeoutAndException(\r\n o.GetSomethingAsync().AsTask(source.token),\r\n TimeSpan.FromSeconds(1));\r\n} catch (TimeoutException) {\r\n source.Cancel();\r\n} finally {\r\n source.Dispose();\r\n}\r\n<\/pre>\n