The return type of an async method must be void, Task or Task<T>

Question

I have the following code here:

public async Dictionary<string, float> GetLikelihoodsAsync(List<string> inputs)
{
    HttpClient client = new HttpClient(); 

    string uri = GetUri();
    string body = GetRequestBody(inputs);
    byte[] requestData = Encoding.UTF8.GetBytes(body);

    Dictionary<string, float> result = await GetResponseAsync(requestData, client, uri)
        .ContinueWith(responseTask => ParseResponseAsync(responseTask.Result))
        .ContinueWith(task => task.Result.Result);

    return result;
}

with

async Task<HttpResponseMessage> GetResponseAsync(byte[] requestData, HttpClient client, string uri) {...}

async Task<Dictionary<string, float>> ParseResponseAsync(HttpResponseMessage response) {...}

Basically after GetResponseAsync completes I want to take the results and feed it to ParseResponseAsync and get a task with the results of it.

Currently, it gives an compiler error saying

The return type of an async method must be void, Task or Task

What is the best way to achieve this goal and get rid of this error? Other (better solutions) are welcomed and some explanations of why do task.Result.Result in the last ContinueWith are also welcomed.

Solution 1

Change the return type to Task<Dictionary<string, float>>:

public async Task<Dictionary<string, float>> GetLikelihoodsAsync(List<string> inputs)

you can also replace your usage of ContinueWith to use await:

var response = await GetResponseAsync(requestData, client, uri);
var result = await ParseResponseAsync(response);
return result;

Solution 2

As stated in the error:

The return type of an async method must be void, Task or Task<T>

In your case that would be Task<T>, or specifically Task<Dictionary<string, float>>. So, your method needs to be declared as this:

public async Task<Dictionary<string, float>> GetLikelihoodsAsync(List<string> inputs)

Note that you don't actually need to return a Task<T>, you only need to return T. You can read more about it here.

Comments

[Tamas Ionut]

And the two awaits have the same semantic as the ContinueWith?

[Servy]

@TamasIonut They propagate exceptions properly, rather than just swallowing them. Other than that they're the same.

[Tamas Ionut]

@Servy So why then having the extra ContinueWith API?

[Lee]

@TamasIonut - The Task classes were added in .net 4, which coincided with the release C#4. async/await was added later in C#5, and effectively provides a way for the compiler to write the continuations for you.

[Servy]

@TamasIonut 1. Not all problems can be solved with await. There will be times, albeit rarely, where it's needed 2. It predates await by a full language version 3. await simply refactors the code into calls to ContinueWith; it's necessary for it to be there as that's what allows it to work.

[Tamas Ionut]

@Servy By those rare times, you mean situations where better cancellation or context support is needed?

[Servy]

@TamasIonut I've never come across a situation where I wanted cancellation or context behavior that couldn't be more easily implemented using async, but I suppose it's possible for someone to be in such a situation. Typically I see it used to add trivial continuations where extracting the code out into an async method just isn't worth it, even though it would work.

[Tamas Ionut]

Thanks for the answer but the previous one is a bit more complete, since it provides a better way for ContinueWith API.