If my interface must return Task what is the best way to have a no-operation implementation

Question

In the code below  due to the interface  the class LazyBar must return a task from its method  and for argument s sake can t be changed   If LazyBars implementation is unusual in that it happens to run quickly and synchronously - what is the best way to return a No-Operation task from the method  I have gone with Task Delay 0  below  however I would like to know if this has any performance side-effects if the function is called a lot  for argument s sake  say hundreds of times a second    Does this syntactic sugar un-wind to something big  Does it start clogging up my application s thread pool  Is the compiler cleaver enough to deal with Delay 0  differently  Would return Task Run      gt        be any different   Is there a better way  using System Threading Tasks   namespace MyAsyncTest       internal interface IFooFace               Task WillBeLongRunningAsyncInTheMajorityOfImplementations                    lt summary gt          An implementation  that unlike most cases  will not have a long-running         operation in  WillBeLongRunningAsyncInTheMajorityOfImplementations           lt  summary gt      internal class LazyBar   IFooFace                region IFooFace Members          public Task WillBeLongRunningAsyncInTheMajorityOfImplementations                            First  do something really quick             var x   1                  Can t return  null  here  Does  Task Delay 0   have any performance considerations                 Is it a real no-op  or if I call this a lot  will it adversely affect the                underlying thread-pool  Better way              return Task Delay 0                   Any different                 return Task Run      gt                        If my task returned something  I would do                 return Task FromResult lt int gt  12345                       endregion            internal class Program               private static void Main string   args                        Test                       private static async void Test                         IFooFace foo   FactoryCreate                await foo WillBeLongRunningAsyncInTheMajorityOfImplementations                return                     private static IFooFace FactoryCreate                         return new LazyBar

User · Answer

Recently encountered this and kept getting warnings/errors about the method being void.

We're in the business of placating the compiler and this clears it up:

    public async Task MyVoidAsyncMethod()
    {
        await Task.CompletedTask;
    }

This brings together the best of all the advice here so far. No return statement is necessary unless you're actually doing something in the method.

User · Answer

When you must return specified type   Task FromResult lt MyClass gt  null

User · Answer

To add to Reed Copsey s answer about using Task FromResult  you can improve performance even more if you cache the already completed task since all instances of completed tasks are the same   public static class TaskExtensions       public static readonly Task CompletedTask   Task FromResult false       With TaskExtensions CompletedTask you can use the same instance throughout the entire app domain     The latest version of the  Net Framework  v4 6  adds just that with the Task CompletedTask static property  Task completedTask   Task CompletedTask

User · Answer

Task Delay 0  as in the accepted answer was a good approach  as it is a cached copy of a completed Task   As of 4 6 there s now Task CompletedTask which is more explicit in its purpose  but not only does Task Delay 0  still return a single cached instance  it returns the same single cached instance as does Task CompletedTask   The cached nature of neither is guaranteed to remain constant  but as implementation-dependent optimisations that are only implementation-dependent as optimisations  that is  they d still work correctly if the implementation changed to something that was still valid  the use of Task Delay 0  was better than the accepted answer

User · Answer

If you are using generics  all answer will give us compile error  You can use return default T    Sample below to explain further   public async Task lt T gt  GetItemAsync lt T gt  string id                                try                                       var response   await this  container ReadItemAsync lt T gt  id  new PartitionKey id                        return response Resource                                    catch  CosmosException ex  when  ex StatusCode    System Net HttpStatusCode NotFound                                         return default T

User · Answer

return Task CompletedTask     this will make the compiler happy

User · Answer

return await Task FromResult new MyClass

User · Answer

Today  I would recommend using Task CompletedTask to accomplish this   Pre  net 4 6  Using Task FromResult 0  or Task FromResult lt object gt  null  will incur less overhead than creating a Task with a no-op expression   When creating a Task with a result pre-determined  there is no scheduling overhead involved

User · Answer

I prefer the Task completedTask   Task CompletedTask  solution of  Net 4 6  but another approach is to mark the method async and return void       public async Task WillBeLongRunningAsyncInTheMajorityOfImplementations                 You ll get a warning  CS1998 - Async function without await expression   but this is safe to ignore in this context

[c#] If my interface must return Task what is the best way to have a no-operation implementation?

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