What is the difference between Task Run and Task Factory StartNew

Question

I have Method     private static void Method         Console WriteLine  Method   started         for  var i   0  i  lt  20  i                  Console WriteLine  Method   Counter       i           Thread Sleep 500              Console WriteLine  Method   finished        And I want to start this method in a new Task  I can start new task like this  var task   Task Factory StartNew new Action Method      or this  var task   Task Run new Action Method      But is there any difference between Task Run   and Task Factory StartNew    Both of them are using ThreadPool and start Method   immediately after creating instance of the Task  When we should use first variant and when second

User · Answer

The Task Run got introduced in newer  NET framework version and it is recommended        Starting with the  NET Framework 4 5  the Task Run method is the   recommended way to launch a compute-bound task  Use the StartNew   method only when you require fine-grained control for a long-running    compute-bound task    The Task Factory StartNew has more options  the Task Run is a shorthand      The Run method provides a set of overloads that make it easy to start   a task by using default values  It is a lightweight alternative to the   StartNew overloads    And by shorthand I mean a technical shortcut    public static Task Run Action action        return Task InternalStartNew null  action  null  default CancellationToken   TaskScheduler Default          TaskCreationOptions DenyChildAttach  InternalTaskOptions None  ref stackMark

User · Answer

According to this post by Stephen Cleary  Task Factory StartNew   is dangerous      I see a lot of code on blogs and in SO questions that use Task Factory StartNew to spin up work on a background thread  Stephen Toub has an excellent blog article that explains why Task Run is better than Task Factory StartNew  but I think a lot of people just haven   t read it  or don   t understand it   So  I   ve taken the same arguments  added some more forceful language  and we   ll see how this goes       StartNew does offer many more options than Task Run  but it is quite dangerous  as we   ll see  You should prefer Task Run over Task Factory StartNew in async code    Here are the actual reasons         Does not understand async delegates  This is actually the same as   point 1 in the reasons why you would want to use StartNew  The problem   is that when you pass an async delegate to StartNew  it   s natural to   assume that the returned task represents that delegate  However  since   StartNew does not understand async delegates  what that task actually   represents is just the beginning of that delegate  This is one of the   first pitfalls that coders encounter when using StartNew in async   code     Confusing default scheduler  OK  trick question time  in the   code below  what thread does the method    A    run on       Task Factory StartNew A    private static void A            Well  you know it   s a trick question  eh  If you answered    a thread   pool thread     I   m sorry  but that   s not correct     A    will run on   whatever TaskScheduler is currently executing    So that means it could potentially run on the UI thread if an operation completes and it marshals back to the UI thread due to a continuation as Stephen Cleary explains more fully in his post   In my case  I was trying to run tasks in the background when loading a datagrid for a view while also displaying a busy animation   The busy animation didn t display when using Task Factory StartNew   but the animation displayed properly when I switched to Task Run     For details  please see https   blog stephencleary com 2013 08 startnew-is-dangerous html

User · Answer

Apart from the similarities i e  Task Run   being a shorthand for Task Factory StartNew     there is a minute difference between their behaviour in case of sync and async delegates  Suppose there are following two methods  public async Task lt int gt  GetIntAsync         return Task FromResult 1      public int GetInt         return 1     Now consider the following code  var sync1   Task Run      gt  GetInt     var sync2   Task Factory StartNew      gt  GetInt      Here both sync1 and sync2 are of type Task lt int gt  However  difference comes in case of async methods  var async1   Task Run      gt  GetIntAsync     var async2   Task Factory StartNew      gt  GetIntAsync      In this scenario  async1 is of type Task lt int gt   however async2 is of type Task lt Task lt int gt  gt

User · Answer

See this blog article that describes the difference  Basically doing   Task Run A    Is the same as doing   Task Factory StartNew A  CancellationToken None  TaskCreationOptions DenyChildAttach  TaskScheduler Default

User · Answer

The second method  Task Run  has been introduced in a later version of the  NET framework  in  NET 4 5   However  the first method  Task Factory StartNew  gives you the opportunity to define a lot of useful things about the thread you want to create  while Task Run doesn t provide this  For instance  lets say that you want to create a long running task thread  If a thread of the thread pool is going to be used for this task  then this could be considered an abuse of the thread pool  One thing you could do in order to avoid this would be to run the task in a separate thread  A newly created thread that would be dedicated to this task and would be destroyed once your task would have been completed  You cannot achieve this with the Task Run  while you can do so with the Task Factory StartNew  like below  Task Factory StartNew      TaskCreationOptions LongRunning    As it is stated here   So  in the  NET Framework 4 5 Developer Preview  we   ve introduced the new Task Run method   This in no way obsoletes Task Factory StartNew  but rather should simply be thought of as a quick way to use Task Factory StartNew without needing to specify a bunch of parameters  It   s a shortcut   In fact  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

User · Answer

In my application which calls two services  I compared both Task Run and Task Factory StartNew   I found that in my case both of them work fine   However  the second one is faster

User · Answer

People already mentioned that  Task Run A     Is equivalent to  Task Factory StartNew A  CancellationToken None  TaskCreationOptions DenyChildAttach  TaskScheduler Default     But no one mentioned that  Task Factory StartNew A     Is equivalent to   Task Factory StartNew A  CancellationToken None  TaskCreationOptions None  TaskScheduler Current     As you can see two parameters are different for Task Run and Task Factory StartNew    TaskCreationOptions - Task Run uses TaskCreationOptions DenyChildAttach which means that children tasks can not be attached to the parent  consider this   var parentTask   Task Run      gt        var childTask   new Task      gt                Thread Sleep 10000           Console WriteLine  Child task finished            TaskCreationOptions AttachedToParent       childTask Start         Console WriteLine  Parent task finished          parentTask Wait    Console WriteLine  Main thread finished       When we invoke parentTask Wait    childTask will not be awaited  even though we specified TaskCreationOptions AttachedToParent for it  this is because TaskCreationOptions DenyChildAttach forbids children to attach to it  If you run the same code with Task Factory StartNew instead of Task Run  parentTask Wait   will wait for childTask because Task Factory StartNew uses TaskCreationOptions None TaskScheduler - Task Run uses TaskScheduler Default which means that the default task scheduler  the one that runs tasks on Thread Pool  will always be used to run tasks  Task Factory StartNew on the other hand uses TaskScheduler Current which means scheduler of the current thread  it might be TaskScheduler Default but not always  In fact when developing Winforms or WPF applications it is required to update UI from the current thread  to do this people use TaskScheduler FromCurrentSynchronizationContext   task scheduler  if you unintentionally create another long running task inside task that used TaskScheduler FromCurrentSynchronizationContext   scheduler the UI will be frozen  A more detailed explanation of this can be found here   So generally if you are not using nested children task and always want your tasks to be executed on Thread Pool it is better to use Task Run  unless you have some more complex scenarios

[c#] What is the difference between Task.Run() and Task.Factory.StartNew()

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