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 · 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<SomeMessage>
{
   ...

   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<Task>();
        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.

User · 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      gt  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      gt  DoWorkAsync

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

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