Continuations

• A continuation task (continuation) is an async task that is invoked by another task (the antecedent) when the antecedent finishes.
• Continuations allow descendant operations to consume the results of an ancestor. In this way, you can chain tasks.
• A continuation is created in the WaitingForActivation state and activated automatically when its antecedent task completes.

Blocking

Continuations, like any Task, do not block the current thread.
To block until a continuation finishes, call Task.Wait.

Do Not Call Task.Start on a Continuation

This will result in an InvalidOperationException.

Creating a Continuation for a Single Antecedent

Use Task.ContinueWith:

public class SimpleExample
{
    public static async Task Main()
    {
        // Declare, assign, and start the antecedent task.
        Task<DayOfWeek> taskA = Task.Run(() => DateTime.Today.DayOfWeek);

        // Execute the continuation when the antecedent finishes.
        var continuation = taskA.ContinueWith(antecedent => Console.WriteLine($"Today is {antecedent.Result}."));
	   await continuation;
    }
}

Passing Data

If the antecedent user delegate is a Task<TResult> and it completed, then the continuation can access the Task<TResult>.Result property of the antecedent. However, if the antecedent was canceled or faulted, this will throw an exception. To avoid this, use TaskContinuationOptions.OnlyOnRanToCompletion:

await taskA.ContinueWith(antecedent => 
{ 
	Console.WriteLine($"Today is {antecedent.Result}.")
},  

Otherwise, guard against the exception.

Creating a Continuation for Multiple Antecedents

Use Task.WhenAll or Task.WhenAny against a collection of Tasks.

TaskContinuationOptions

Task.ContinueWith has an overload that accepts a TaskContinuationOptions instance.

Canceling a Continuation

A continuation is canceled when:

• It receives a cancellation request and throws an OperationCanceledException (like any Task)
• The continuation is passed a CancellationToken whose IsCancellationRequested property is already true.
• The continuation never runs because a condition set by its TaskContinuationOptions was not met.

Exceptions

Exceptions thrown in continuations are not propagated back to the antecedent. Handle them as you would in any other task.