Task.FromResult() vs. Task.Run()

Question

I've come across quite a few situations lately where async methods execute synchronously, but return a Task anyway, so they can be awaited, e.g.

public virtual Task CreateAsync(TUser user) {     ThrowIfDisposed();     if (user == null) throw new ArgumentNullException("user");     Context.Save(user);     Context.Flush();     return Task.FromResult(0); } 

Surely it is better to dispatch the probably-long-running operation to a thread and return the still active task, to genuinely be awaited:

public virtual Task CreateAsync(TUser user) {     ThrowIfDisposed();     if (user == null) throw new ArgumentNullException("user");     return Task.Run(() =>     {         Context.Save(user);         Context.Flush();     }); } 

I some suspicion, though, that just spinning off TPL threads isn't the safest practice. Any commentary on these two different patterns?

Answer 1

If your method is synchronous you shouldn't return a Task to begin with. Just create a traditional synchronous method.

If for some reason that's not possible (for example, you implement some async interface) returning a completed task using Task.FromResult or even better in this case Task.CompletedTask (added in .NET 4.6) is much better than using Task.Run in the implementation:

public virtual Task CreateAsync(TUser user) {     // ...     return Task.CompletedTask; } 

If the consumer of your API cares strongly about the Task-returning method not running synchronously they can use Task.Run themselves to make sure.

You should keep in mind that async methods may have a considerable synchronous part (the part before the first await) even if they do eventually continue asynchronously. You can't assume async methods return a Task immediately anyway.

Answer 2

Task.FromResult doesn't actually creates or runs a task but it just wraps the returned result in a task object. I personally used it in Unit Tests where I need to simulate the Async methods and Of course I wouldn't want to run actual tasks in Unit tests.

Besides Task.Run will actually create a task and run a task on TaskScheduler. It is not recommended to use Task.Run when you're doing Async programming. Rather use await on tasks. See few do's and don't of Tasks by Stephen Cleary.