What is the difference between Task.Run and Task.Factory.StartNew

Question

I know this question has been asked before, but I didn't get the right answer after googling SO.

I have these lines of code:

Task.Run(() => DoSomething())
    .ContinueWith(t=>Log.Error(t,"Error"), TaskContinuationOptions.OnlyOnFaulted);

Task.Factory.StartNew(() => DoSomething())
    .ContinueWith(t=>Log.Error(t,"Error"),TaskContinuationOptions.OnlyOnFaulted);

After a successful run of DoSomething, Task.Run throws TaskCanceledException while Task.Factory.StartNew works fine. Why?

further reading: Stephen Clearly on why not use Task.Factory.StartNew
MSDN Link

UPDATE 2: Sample Code:

private async void button27_Click(object sender, EventArgs e)
{
    var r = new Random(System.DateTime.Now.Millisecond);

    await Task.Factory.StartNew(
        () => {
            Divide(r.Next(100), r.Next(-1, 10));
            Log.Information("Divide Done!");
        },
        CancellationToken.None,
        TaskCreationOptions.DenyChildAttach,
        TaskScheduler.Default)
    .ContinueWith(
        t => {
            Log.Error(t.Exception,"There is an exception on Divide");
        },
        TaskContinuationOptions.OnlyOnFaulted);
}

private static void Divide(int a, int b)
{
    var c = a/b;
}

Possible duplicate of [Regarding usage of Task.Start() , Task.Run() and Task.Factory.StartNew()](http://stackoverflow.com/questions/29693362/regarding-usage-of-task-start-task-run-and-task-factory-startnew) — smoksnes, Nov 15 '16 at 11:28
No, it's not. It does not answer my question which is why throwing exception occurs. It describes what I already know. Thanks. — Milad, Nov 15 '16 at 11:29
I have rolled back your question, because you completely changed the it, invalidating existing answers. If you have a new question, please post a new question. — svick, Nov 15 '16 at 12:38
@Sepinood - Ok, I retracted the close-vote. Even though I believe the title _What is the difference between Task.Run and Task.Factory.StartNew?_ is answered in the suggested duplicate. And all the current answers more or less states the same thing as the suggested duplicate and quotes the same source. — smoksnes, Nov 15 '16 at 13:36

CharithJ · Accepted Answer · 2016-11-15T11:52:07.587

6

Task.Run is actually implemented in terms of the same logic used for Task.Factory.StartNew, just passing in some default parameters. When you pass an Action to Task.Run:

Task.Run(someAction);

that’s exactly equivalent to:

Task.Factory.StartNew(someAction, 
    CancellationToken.None, TaskCreationOptions.DenyChildAttach, TaskScheduler.Default);

Read more here.

If you pass CancellationToken.None, TaskCreationOptions.DenyChildAttach and TaskScheduler.Default arguments for Task.Factory.StartNew parameters you should see the same result.

edited Nov 15 '16 at 11:52

answered Nov 15 '16 at 11:30

CharithJ

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2

@Sepinood: May be this http://stackoverflow.com/questions/26577519/why-is-taskcanceledexception-thrown-and-does-not-always-breaks-into-the-debugger or http://stackoverflow.com/questions/27196451/when-does-task-runaction-cancellationtoken-throw-taskcanceledexception – CharithJ Nov 15 '16 at 11:38
Thanks, it helped me. I updated the question totally. I think it is related to design of TPL. – Milad Nov 15 '16 at 11:56
3

You won't see the exact same behavior, there is one important diffrence. `Task.Run(` has overloads that take in a `Func` or `Func>` as the paremeter. This allows you to pass in async functions in to `Task.Run(` if you try to do a async function with `StartNew(` you must call `.Unwrap()` on the returned task to get the same behavior. – Scott Chamberlain Nov 18 '16 at 22:39
@ScottChamberlain thanks for the brilliant set of details for good clarity – Mrinal Kamboj Nov 24 '16 at 07:38

What is the difference between Task.Run and Task.Factory.StartNew

1 Answers1