试图了解C＃TaskContinuationOptions.OnlyOnFaulted

Question

I am trying to understand C# TaskContinuationOptions.OnlyOnFaulted and I wrote this simple example. I expect the continuation task to not run because there was no exception in the antecedent task but it runs anyway.

var task = Task.Factory
    .StartNew(() => { Console.WriteLine("Hello world!"); })
    .ContinueWith((x, y) =>
    {
        Console.WriteLine("This should not get printed!");
    }, TaskContinuationOptions.OnlyOnFaulted);

Answer 1

You need to look more closely at what overloads exist for ContinueWith() , and especially what overload you're actually calling.

There is no overload for ContinueWith() that takes only a delegate and a TaskContinuationOptions value specifically. The overload you're calling takes as parameters an Action<Task, object> , and then any object value, which is the "state" value passed to the Action<Task, object> . That state value has no effect whatsoever on what ContinueWith() actually does. It just passes that value to your delegate.

If you want to use the TaskContinuationOptions.OnlyOnFaulted value to actually control the behavior of the ContinueWith() method, you need to use an overload that includes that parameter type specifically.

The easiest way to fix your code would be to simply pass null as the state value for the ContinueWith(Action<Task, object>, object, TaskContinuationOptions) overload:

var task = Task.Factory
    .StartNew(() => { Console.WriteLine("Hello world!"); })
    .ContinueWith((x, y) =>
    {
        Console.WriteLine("This should not get printed!");
    }, null, TaskContinuationOptions.OnlyOnFaulted);

Do note that in this case, your task object will get canceled after the first task finishes rather than executing the continuation (by design, of course).