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Task.Run vs Task.Factory.StartNew

| Wednesday, January 30, 2013
In .NET 4, Task.Factory.StartNew was the primary method for scheduling a new task.  Many overloads provided for a highly configurable mechanism, enabling setting options, passing in arbitrary state, enabling cancellation, and even controlling scheduling behaviors.  The flip side of all of this power is complexity.  You need to know when to use which overload, what scheduler to provide, and the like. And “Task.Factory.StartNew” doesn’t exactly roll off the tongue, at least not quickly enough for something that’s used in such primary scenarios as easily offloading work to background processing threads.

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:


that’s exactly equivalent to:

    CancellationToken.None, TaskCreationOptions.DenyChildAttach, TaskScheduler.Default);

In this way, Task.Run can and should be used for the most common cases of simply offloading some work to be processed on the ThreadPool (what TaskScheduler.Default targets).  That doesn’t mean Task.Factory.StartNew will never again be used; far from it.  Task.Factory.StartNew still has many important (albeit more advanced) uses.  You get to control TaskCreationOptions for how the task behaves.  You get to control the scheduler for where the task should be queued to and run.  You get to use overloads that accept object state, which for performance-sensitive code paths can be used to avoid closures and the corresponding allocations.  For the simple cases, though, Task.Run is your friend.

Task.Run provides eight overloads, to support all combinations of the following:

  • Task vs Task<TResult>
  • Cancelable vs non-cancelable
  • Synchronous vs asynchronous delegate

The first two bullets should be self-explanatory.  For the first bullet, there are overloads that return Task (for operations that don’t have a result) and there are overloads that return Task<TResult> (for operations that have a result of type TResult).  There are also overloads that accept a CancellationToken, which enables the Task Parallel Library (TPL) to transition the task to a Canceled state if cancellation is requested prior to the task beginning its execution.

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