How Return Void in Async Method in Web API

Question

I am calling a method in Web API(C#) from repository. The method is repository is not returning anything. It is Void. whay should I return in my API method as a async method cannot have a Void return type.

This is my Async method in API:

    [HttpPost]
    [Route("AddApp")]
    public async Task<?> AddApp([FromBody]Application app)
    {        
        loansRepository.InsertApplication(app);
    }

and this is EntityFrame work Insert in repository(I can cot change this by the way)

     public void InsertApplication(Application app)
    {

        this.loansContext.Application.Add(app);
    }

Sorry I made changes to the question, I am not sure what should I have for ? in the Task

@ Philippe Paré sorry made changes to the Code, I dont have any return type it is void. — Alma
– Alma, Commented May 4, 2016 at 23:34
If the Web API method/action will not involve any async work (e.g. IO), you shouldn't really make it async — Yacoub Massad
– Yacoub Massad, Commented May 4, 2016 at 23:36
Yes @YacoubMassad is right, but if you need it to happen on another thread (if you don't want the code to block there), you can call Task.Run(()=>//Your cp-bound work here, happening on another thread)); — Philippe Paré
– Philippe Paré, Commented May 4, 2016 at 23:40

Gabor · Accepted Answer · 2016-05-04 23:34:38Z

14

If you do not want to return anything then the return type should be Task.

[HttpPost]
[Route("AddApp")]
public async Task AddApp([FromBody]Application app)
{
    // When you mark a method with "async" keyword then:
    // - you should use the "await" keyword as well; otherwise, compiler warning occurs
    // - the real return type will be:
    // -- "void" in case of "Task"
    // -- "T" in case of "Task<T>"
    await loansRepository.InsertApplication(app);
}

public Task InsertApplication(Application app)
{
    this.loansContext.Application.Add(app);

    // Without "async" keyword you should return a Task instance.
    // You can use this below if no Task is created inside this method.
    return Task.FromResult(0);
}

answered May 4, 2016 at 23:34

Gabor

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1 Comment

Davy Over a year ago

You can also return Task.CompletedTask instead of FromResult(0)

Stephen Cleary · Accepted Answer · 2016-05-05 01:08:59Z

8

Since you "cannot" change the Entity Framework repository, you shouldn't make your action method asynchronous, and you should just return void:

[HttpPost]
[Route("AddApp")]
public void AddApp([FromBody]Application app)
{        
    loansRepository.InsertApplication(app);
}

answered May 5, 2016 at 1:08

Stephen Cleary

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3 Comments

Sam Rueby Over a year ago

Stephen is the authority on all things async - I would go with his answer.

Faith Wins Over a year ago

How would you then call this method from a C# client?

Stephen Cleary Over a year ago

@FaithWins: You'd post to /AddApp with the serialized Application as the request body. If you want more information, I recommend asking your own question.

suulisin · Accepted Answer · 2016-05-05 10:53:37Z

-5

The compiler will warn "Return statement missing". Please modify the code to:

 [HttpPost]
 [Route("AddApp")]
 public void AddApp([FromBody]Application app)
 {  
     //add configureAwait(false) as it would give better performance.

   loansRepository.InsertApplication(app)     
 }

edited May 5, 2016 at 10:53

answered May 4, 2016 at 23:29

suulisin

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6 Comments

Leon Cullens Over a year ago

This is not an answer to his question, but a comment. Please don't use the answers section for this.

Leon Cullens Over a year ago

And your remark about ConfigureAwait is completely off. Please read a bit about what it actually does and why you would use it.

Aron Over a year ago

The usage of async/await add additional unneccessary overhead for no reason. Consider directly returning to Task.Run result.

Soma Yarlagadda Over a year ago

My remarks about ConfigureAwait are very valid. Please read about it here link. Also I have first hand information from Microsoft about it.

Soma Yarlagadda Over a year ago

Stephen Cleary's is the best approach since the repository call is synchronous. But, the question was about how to return void from the async method, I had to wrap it in Task.Run. Task.Run is not a recommended approach in asp.net applications as you are using a thread from the thread pool.

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