How to start a List in parallel?

问题:

I have an object that returns a System.Threading.Tasks.Task:

public class MyClass 
{
    public Task GetTask(object state, CancellationToken cancellationToken)
    {
        return new Task(Execute, state, cancellationToken);
    }

    public void Execute(object context)
    {
        //do stuff
    }
}

Elsewhere I have a List<MyClass>, so I do the following to get a List<Task>:

var myTaskList = myClassList.Select(p => p.GetTask(null, cancellationToken)).ToList();

Now that I have the List<Task>, how can I start them all in parallel? Is there a more concise way to code this?

Thanks!

回答1:

What do you mean by "start them in parallel"? When you run the Task, it executes on another thread, so you probably mean simply:

foreach(var task in myTaskList)
{
    task.Start();
}

But if you have so many of them that you want to move the starting logic to another thread, you can either call the above code in another thread/task (I'm using List<T>.ForEach for shorter code).

Task.Factory.StartNew(() => myTaskList.ForEach(task => task.Start()));

Or you can use TPL's Parallel.ForEach. That would still block the executing thread until all the Tasks are started, but it will execute the start action on an internal threadpool, so for large numbers of items and some free CPU cores/threads, it might speed up the starting considerably.

Parallel.ForEach(myTaskList, task => task.Start());

回答2:

I might be misunderstanding your question, but isn't it just to call Start on every task?

foreach(Task task in myTaskList) 
{
    task.Start();
}

Waiting for all the tasks to finish:

Task.WaitAll(myTaskList.ToArray());