When correctly use Task Run and when just async-await

Question

I would like to ask you on your opinion about the correct architecture when to use Task Run  I am experiencing laggy UI in our WPF  NET 4 5 application  with Caliburn Micro framework    Basically I am doing  very simplified code snippets    public class PageViewModel   IHandle lt SomeMessage gt               public async void Handle SomeMessage message             ShowLoadingAnimation              Makes UI very laggy  but still not dead       await this contentLoader LoadContentAsync           HideLoadingAnimation            public class ContentLoader       public async Task LoadContentAsync                 await DoCpuBoundWorkAsync            await DoIoBoundWorkAsync            await DoCpuBoundWorkAsync                I am not really sure what all I can consider as CPU bound as slowing down the UI         await DoSomeOtherWorkAsync              From the articles videos I read saw  I know that await async is not necessarily running on a background thread and to start work in the background you need to wrap it with await Task Run async      gt         Using async await does not block the UI  but still it is running on the UI thread  so it is making it laggy   Where is the best place to put Task Run   Should I just   Wrap the outer call because this is less threading work for  NET   or should I wrap only CPU-bound methods internally running with Task Run as this makes it reusable for other places  I am not sure here if starting work on background threads deep in core is a good idea    Ad  1   the first solution would be like this   public async void Handle SomeMessage message        ShowLoadingAnimation        await Task Run async      gt  await this contentLoader LoadContentAsync         HideLoadingAnimation          Other methods do not use Task Run as everything regardless    if I O or CPU bound would now run in the background    Ad  2   the second solution would be like this   public async Task DoCpuBoundWorkAsync         await Task Run      gt               Do lot of work here            public async Task DoSomeOtherWorkAsync           I am not sure how to handle this methods -        probably need to test one by one  if it is slowing down UI

User · Accepted Answer

Note the guidelines for performing work on a UI thread, collected on my blog:

Don't block the UI thread for more than 50ms at a time.
You can schedule ~100 continuations on the UI thread per second; 1000 is too much.

There are two techniques you should use:

1) Use ConfigureAwait(false) when you can.

E.g., await MyAsync().ConfigureAwait(false); instead of await MyAsync();.

ConfigureAwait(false) tells the await that you do not need to resume on the current context (in this case, "on the current context" means "on the UI thread"). However, for the rest of that async method (after the ConfigureAwait), you cannot do anything that assumes you're in the current context (e.g., update UI elements).

For more information, see my MSDN article Best Practices in Asynchronous Programming.

2) Use Task.Run to call CPU-bound methods.

You should use Task.Run, but not within any code you want to be reusable (i.e., library code). So you use Task.Run to call the method, not as part of the implementation of the method.

So purely CPU-bound work would look like this:

// Documentation: This method is CPU-bound.
void DoWork();

Which you would call using Task.Run:

await Task.Run(() => DoWork());

Methods that are a mixture of CPU-bound and I/O-bound should have an Async signature with documentation pointing out their CPU-bound nature:

// Documentation: This method is CPU-bound.
Task DoWorkAsync();

Which you would also call using Task.Run (since it is partially CPU-bound):

await Task.Run(() => DoWorkAsync());

User · Answer

One issue with your ContentLoader is that internally it operates sequentially   A better pattern is to parallelize the work and then sychronize at the end  so we get  public class PageViewModel   IHandle lt SomeMessage gt               public async void Handle SomeMessage message             ShowLoadingAnimation              makes UI very laggy  but still not dead       await this contentLoader LoadContentAsync            HideLoadingAnimation               public class ContentLoader        public async Task LoadContentAsync                 var tasks   new List lt Task gt             tasks Add DoCpuBoundWorkAsync             tasks Add DoIoBoundWorkAsync             tasks Add DoCpuBoundWorkAsync             tasks Add DoSomeOtherWorkAsync              await Task WhenAll tasks  ConfigureAwait false             Obviously  this doesn t work if any of the tasks require data from other earlier tasks  but should give you better overall throughput for most scenarios

[c#] When correctly use Task.Run and when just async-await

Examples related to c#

Examples related to asynchronous

Examples related to task

Examples related to async-await