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A surprise TaskCancelledException
time to read 2 min | 348 words
All of a sudden, my code started getting a lot of TaskCancelledException. It took me a while to figure out what was going on. We can imagine that the code looked like this:
var unwrap = Task.Factory.StartNew(() =>
{
if (DateTime.Now.Month % 2 != 0)
return null;
return Task.Factory.StartNew(() => Console.WriteLine("Test"));
}).Unwrap();
unwrap.Wait();
The key here is that when Unwrap is getting a null task, it will throw a TaskCancelledException, which was utterly confusing to me. It make sense, because if the task is null there isn’t anything that the Unwrap method can do about it. Although I do wish it would throw something like ArgumentNullException with a better error message.
The correct way to write this code is to have:
var unwrap = Task.Factory.StartNew(() =>
{
if (DateTime.Now.Month % 2 != 0)
{
var taskCompletionSource = new TaskCompletionSource<object>();
taskCompletionSource.SetResult(null);
return taskCompletionSource.Task;
}
return Task.Factory.StartNew(() => Console.WriteLine("Test"));
}).Unwrap();
unwrap.Wait();
Although I do wish that there was an easier way to create a completed task.
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