c# - 等价于 ContinueWith(delegate, CancellationToken) with await continuation

Question

我有这种情况:

private Task LongRunningTask = /* Something */;

private void DoSomethingMore(Task previousTask) { }

public Task IndependentlyCancelableSuccessorTask(CancellationToken cancellationToken)
{
    return LongRunningTask.ContinueWith(DoSomethingMore, cancellationToken);
}

特别是，我感兴趣的行为在 MSDN's page about Continuation Tasks 中有详细说明在以下条款中:

A continuation goes into the Canceled state in these scenarios:

• [...]
• When the continuation was passed a System.Threading.CancellationToken as an argument and the IsCancellationRequested property of the token is true before the continuation runs. In such a case, the continuation does not start and it transitions to the Canceled state.

上面的代码有效。但是，我正在将尽可能多的延续转换为使用 await 关键字。

是否有使用 await 的等效项允许在等待的任务完成之前取消继续？

Answer 1

下面应该可以做到，尽管它看起来有点笨拙:

private Task LongRunningTask = /* Something */;

private void DoSomethingMore() { }

public async Task IndependentlyCancelableSuccessorTask(
    CancellationToken cancellationToken)
{
    cancellationToken.ThrowIfCancellationRequested();

    var tcs = new TaskCompletionSource<bool>();
    using (cancellationToken.Register(() => tcs.TrySetCanceled()))
        await Task.WhenAny(LongRunningTask, tcs.Task);

    cancellationToken.ThrowIfCancellationRequested();
    DoSomethingMore();
}

[更新] 根据 svick 的建议，这里它被塑造成一个助手，基于 Stephen Toub 的 Implementing Then with Await图案:

public static class TaskExt
{
    /// <summary>
    /// Use: await LongRunningTask.Then(DoSomethingMore, cancellationToken)
    /// </summary>
    public static async Task Then(
        this Task antecedent, Action continuation, CancellationToken token)
    {
        await antecedent.When(token);
        continuation();
    }

    /// <summary>
    /// Use: await LongRunningTask.When(cancellationToken)
    /// </summary>
    public static async Task When(
        this Task antecedent, CancellationToken token)
    {
        token.ThrowIfCancellationRequested();

        var tcs = new TaskCompletionSource<Empty>();
        using (token.Register(() => tcs.TrySetCanceled()))
            await Task.WhenAny(antecedent, tcs.Task);

        token.ThrowIfCancellationRequested();
    }

    struct Empty { };
}

也许第一个 ThrowIfCancellationRequested() 是多余的，但我还没有彻底考虑所有的边缘情况。