Task.Run（）和await Task.Run（）有什么区别？

Question

Following program is designed using general Task.Run() and using async and await (asynchronous). In both cases a different Thread is taken from Thread pool for new task. So, what is the difference? Asynchronous means it should use the main thread and free it until task completed. But it is also using another thread rather than using main thread.

public class Worker2
{
    public bool IsComplete { get; private set; }
    internal void DoWork()
    {
        this.IsComplete = false;
        Console.WriteLine("Doing Work.");
        Task.Run(new Action(LongOperation));
        Console.WriteLine("Work Completed");
        IsComplete = true;
    }

    private void LongOperation()
    {
            Console.WriteLine("long operation thread thread :" + Thread.CurrentThread.ManagedThreadId);//Thread Id = 7. it is different from main thread id.
            Console.WriteLine("Working!");
            Thread.Sleep(3000);
    }
}

//And asynchronous

public class Worker2
{
    public bool IsComplete { get; private set; }
    internal async void DoWork()
    {
        this.IsComplete = false;
        Console.WriteLine("Doing Work.");       
        await LongOperation();
        Console.WriteLine("Work Completed");
        IsComplete = true;
    }

    private Task LongOperation()
    {

        return Task.Run(() =>
            {
                Console.WriteLine("long operation thread thread :" + Thread.CurrentThread.ManagedThreadId);
                Console.WriteLine("Working!");
                Thread.Sleep(3000);
            });
    }
}

Answer 1

What is the difference between Task.Run() and await Task.Run()?

The first starts a task and then does the work immediately after that task, before the task completes .

The second starts a task and then does different work until the task is completed, at which point it does the work after the task.

Let's make an analogy. Your first program is like doing this:

• Hire someone to mow the lawn.
• Tell your spouse the lawn is mowed.
• Go watch Netflix.

Your second program is:

• Hire someone to mow the lawn.
• Watch Netflix while the lawn is being mowed.
• When the lawn is done being mowed and the movie is over, tell spouse the lawn is mowed.

Clearly those are very different workflows. Both are asynchronous, but only the latter has an asynchronous wait in it. We asynchronously wait to tell the spouse that the lawn is mowed until it actually is mowed .

Answer 2

You misunderstood what asynchronous code means. Whenever you call any function which passes( delegates ) the actual code execution to some other entity(other dedicated thread, thread pool, etc.) and the function immediately returns you have an asynchronous call. An asynchronous call is the antipode to a blocking call.

In C# there are a few method to write an asynchronous code and await / async just make writing simpler. It adds nothing in terms of functional to the language. It is just syntactic sugar to make programs easier to write by providing a way to write asynchronous code in the same style we write blocking code.

That being said, Task.Run(new Action(LongOperation)); is an asynchronous call as well as LongOperation() from your second snippet. The difference between them, is that in the first example, when Task.Run returns the code which immediately follows that line gets executed immediately(provided no thread switch happened). And in the second example the lines which follow await get executed only when LongOperation completed on some thread provided by the thread pool( DoWork immediately returns when it encounters await ).

I recommend you to read more about TPL and async / await . There are plenty blog posts out there about both, not to mention great books covering all these features in details.