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

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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(() => { }); be any different?

Is there a better way?

using System.Threading.Tasks;

namespace MyAsyncTest
{
    internal interface IFooFace
    {
        Task WillBeLongRunningAsyncInTheMajorityOfImplementations();
    }

    /// <summary>
    /// An implementation, that unlike most cases, will not have a long-running
    /// operation in 'WillBeLongRunningAsyncInTheMajorityOfImplementations'
    /// </summary>
    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(() => { });

            // If my task returned something, I would do:
            // return Task.FromResult<int>(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();
        }
    }
}

Today, I would recommend using Task.CompletedTask to accomplish this.


Pre .net 4.6:

Using Task.FromResult(0) or Task.FromResult<object>(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.

Async return types (C#), Learn about the return types that async methods can have in C# with Task, for an async method that performs an operation but returns no value. The object returned by the GetAwaiter method must implement the System. You can better understand how await retrieves the result from a Task<T> by� c# - If my interface must return Task what is the best way to have a no-operation implementation? Today, I would recommend using Task.CompletedTask to accomplish this. Pre .net 4.6:


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;

Asynchronous programming, If you have any I/O-bound needs (such as requesting data from a network, The core of async programming is the Task and Task<T> objects, which model await an operation that returns a Task or Task<T> inside of an async method. Additionally, you should always measure the execution of your code. If my interface must return Task what is the best way to have a no-operation implementation? Vis Team Juni 23, 2019 In the code below, due to the interface, the class LazyBar must return a task from it's method (and for arguments sake can't be changed).


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.

C#: Non-async "Task/Task<T>" methods should not return null, Non-async "Task/Task<T>" methods should not return null. Analyze your code. Bug. Critical� Task.Delay(0) as in the question 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.


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.

Object Interfaces - Manual, All methods in the interface must be implemented within a class; failure to do so will result return $template; provides a good way to make sure that a particular object contains particular methods. An INTERFACE is not specifically provided for abstraction. I want to write a class that accesses the data in my database. Async methods that don't contain a return statement or that contain a return statement that doesn't return an operand usually have a return type of Task. Such methods return void if they run synchronously. If you use a Task return type for an async method, a calling method can use an await operator to suspend the caller's completion until the


return Task.CompletedTask; // this will make the compiler happy

JRas programming interfaces for logging (deprecated), The JRas framework described in this task and its sub-tasks is deprecated. However, you It is your responsibility to restrict access to the user-written extensions appropriately. User-written handlers must implement the RASIHandler interface. You can implement these methods as you want, including as no operations. If my interface must return Task what is the best way to have a no-operation implementation? 1421. Why not inherit from List<T>? 40.


Event handlers naturally return void, so async methods return void so that you can have an asynchronous event handler. However, some semantics of an async void method are subtly different than the semantics of an async Task or async Task<T> method. Async void methods have different error-handling semantics.


An operation must have at most one argument, and must not have any overloads or generic type arguments. An operation meant to be called from an orchestration using an interface must return Task or Task<T>. Arguments and return values must be serializable values or objects. What can operations do?


You can use .NET tasks (the Task class and generic Task<TResult> class) to implement your asynchronous method. You must return a task that represents an ongoing operation, such as a task that is returned from an asynchronous method written in C# or Visual Basic, or a task that is returned from the Task.Run method.