Why doesn't the TPL support state-filtered multi-task continuations? [closed]

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The TPL has a number of TaskContinuationOptions values that control what circumstances a task executes under. For example, TaskContinuationOptions.NotOnCanceled prevents a task from running when its parent is canceled.

However, none of these task state filters apply to multi-task continuations. You can't do something like:

TaskFactory f = new TaskFactory();
Task t1 = new Task (() => Thread.Sleep (5000));
Task t2 = new Task (() => Thread.Sleep (4000));
Task t3 = f.ContinueWhenAll (new Task[] { t1, t2 },
                             (tasks) => { ... },
                             TaskContinuationOptions.OnlyOnRanToCompletion);

You end up getting an error that says, "It is invalid to exclude specific continuation kinds for continuations off of multiple tasks."

What I don't understand is why this condition would be excluded from the API. Why wouldn't it be a perfectly valid use case to want a task to run only when all ante开发者_C百科cedents ended in a particular state?

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ContinueWhenAll means "run the continuation when all tasks have completed successfully". Specifying NotOn* or OnlyOn* would be either contrary or superfluous to that definition. See the remarks section of this MSDN article.

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"...to run only when all antecedents ended in a particular state..."

Note your word "all", I'm not MS, but I'll bet it has to do with the fact that another whole TaskContinuationOptions enum would be needed that included All, Any, OnlyOne, AllButOne and so on and so forth.

Furthermore, Eric Lippert is always answering questions like these with "it's expensive and time consuming to add a "simple" feature. Way moreso, than getting the basics right and letting users implement the rest.