如何在第三个任务中合并两个任务结果？

Question

如何执行System.Threading.Task作为两个或更多其他任务结果的延续？

public Task<FinalResult> RunStepsAsync()
{
    Task<Step1Result> task1 = Task<Step1Result>.Factory.StartNew(Step1);

    // Use the result of step 1 in steps 2A and 2B
    Task<Step2AResult> task2A = task1.ContinueWith(t1 => Step2A(t1.Result));
    Task<Step2BResult> task2B = task1.ContinueWith(t1 => Step2B(t1.Result));

    // Now merge the results of steps 2A and 2B in step 3
    Task<FinalResult> task3 = task2A
        .ContinueWith(
            t2A => task2B.ContinueWith(
                t2B => Step3(t2A.Result, t2B.Result)))
        .Unwrap();
    return task3;
}

这样可行，但是双重ContinueWith似乎效率低下。有没有更好的方法来实现这一点，也许使用TaskCompletionSource？ （我不想使用锁或Task.WaitAll。）

Answer 1

使用TaskFactory.ContinueWhenAll。

class Step1Result {}
class Step2AResult
{
    public Step2AResult(Step1Result result) {}
}
class Step2BResult
{
    public Step2BResult(Step1Result result) {}
}
class FinalResult 
{
    public FinalResult(Step2AResult step2AResult, Step2BResult step2BResult) {}
}

    public Task<FinalResult> RunStepsAsync()
    {
        var task1 = Task<Step1Result>.Factory.StartNew(() => new Step1Result());

        // Use the result of step 1 in steps 2A and 2B
        var task2A = task1.ContinueWith(t1 => new Step2AResult(t1.Result));
        var task2B = task1.ContinueWith(t1 => new Step2BResult(t1.Result));

        // Now merge the results of steps 2A and 2B in step 3
        return Task <FinalResult>
            .Factory
            .ContinueWhenAll(new Task[] { task2A, task2B }, tasks => new FinalResult(task2A.Result, task2B.Result));
    }