What is the use for Task.FromResult<TResult> in C#

Question

In C# and TPL (Task Parallel Library), the Task class represents an ongoing work that produces a value of type T.

I'd like to know what is the need for the Task.FromResult method ?

That is: In a scenario where you already have the produced value at hand, what is the need to wrap it back into a Task?

The only thing that comes to mind is that it's used as some adapter for other methods accepting a Task instance.

Solution 1

There are two common use cases I've found:

1. When you're implementing an interface that allows asynchronous callers, but your implementation is synchronous.
2. When you're stubbing/mocking asynchronous code for testing.

Solution 2

One example would be a method that makes use of a cache. If the result is already computed, you can return a completed task with the value (using Task.FromResult). If it is not, then you go ahead and return a task representing ongoing work.

Cache Example: Cache Example using Task.FromResult for Pre-computed values

Solution 3

Use it when you want to create an awaitable method without using the async keyword. I found this example:

public class TextResult : IHttpActionResult
{
    string _value;
    HttpRequestMessage _request;

    public TextResult(string value, HttpRequestMessage request)
    {
        _value = value;
        _request = request;
    }
    public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
    {
        var response = new HttpResponseMessage()
        {
            Content = new StringContent(_value),
            RequestMessage = _request
        };
        return Task.FromResult(response);
    }
}

Here you are creating your own implementation of the IHttpActionResult interface to be used in a Web Api Action. The ExecuteAsync method is expected to be asynchronous but you don't have to use the async keyword to make it asynchronous and awaitable. Since you already have the result and don't need to await anything it's better to use Task.FromResult.

Solution 4

From MSDN:

This method is useful when you perform an asynchronous operation that returns a Task object, and the result of that Task object is already computed.

http://msdn.microsoft.com/en-us/library/hh228607.aspx

Solution 5

Use the Task.FromResult when you want to have a asynchronous operation but sometimes the result is in hand synchronously. You can find a good sample here http://msdn.microsoft.com/en-us/library/hh228607.aspx.

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Comments

[Izikon]

is this helps you? msdn.microsoft.com/en-us/library/hh228607.aspx

[Alex Edelstein]

To some extent I agree with that, but the creation of dense, useful, consolidated, discsussion-oriented pages like this is a huge benefit. I almost always learn more from a good, dense stackoverflow page than from googling and doing research across multiple places, so in this case, I'm really glad he posted this.

[gmail user]

I think Google brings me to SO and SO ask me to go to Google. It is a circular reference :)

[Paulo Morgado]

And completed tasks, such as the ones returned from Task.FromResult, can be cached.

[Ben Voigt]

@Paulo: keeping the entire Task object in memory seems far more wasteful than caching just the result.

[Paulo Morgado]

Expect "value tasks" are already cached. I don't recall exactly which ones, but I think Task.FromResult(0), Task.FromResult(1), Task.FromResult(false) and Task.FromResult(true) are cached. You're not supposed to cache a task for a network access but one from result is perfectly fine. Would you prefer to create one each time you need to return the value?

[Nelson Rothermel]

A good case for #1 is a web service. You could have a synchronous service method that returns Task.FromResult and a client that awaits asynchronously for the network I/O. This way you can share the same interface between client/server using ChannelFactory.

[John Henckel]

For instance the ChallengeAsync method. WTF were the designers at MS thinking? There is absolutely no reason for this method to return a Task. And all the sample code from MS simply has FromResult(0). Hopefully the compiler is smart enough to optimize this away, and doesn't actually spawn a new thread and then kill it right away!

[Stephen Cleary]

@JohnHenckel: OWIN is designed from the ground up to be async-friendly. Interfaces and base classes often use async signatures because it just allows (not forces) the implementation to be async. So it's similar to IEnumerable<T> deriving from IDisposable - it allows the enumerable to have disposable resources, not forces it to. Neither FromResult, async, nor await will spawn threads.

[John Henckel]

@StephenCleary hmhm, thanks for explaining that. I had assumed that await would spawn, but I tried it and I see it doesn't. Only Task.Run does. Therefore, x = await Task.FromResult(0); is equivalent to saying x = 0; that's confusing, but good to know!

[ToolmakerSteve]

@PauloMorgado I don't think you've responded to Ben's point. What is the benefit to caching the Task rather than simply caching the TResult that the task returned?

[ToolmakerSteve]

... and to answer my own question, the benefit of a cache of Tasks is that some of them can be completed tasks, and others can be ones that have not yet finished. Callers don't have to care: they do an asynchronous call, and if it is already completed, they get an answer immediately when they await, if not, they get it later. Without these cached tasks, would either (a) require two different mechanisms, one sync and one async - cumbersome for caller, or (b) have to dynamically create a Task, every time a caller asks for an answer that is already available (if we had only cached a TResult).

[Brain2000]

I get it now. The wording of the answer was slightly confusing. The Task itself doesn't have a built in "caching" mechanism. But if you wrote a caching mechanism for ... say .... downloading files, Task<File> GetFileAync( ), you could instantly return a file that is already in cache by using Task.FromResult(cachedFile), and the await would run synchronously, saving time by not having the thread switch.

[Brain2000]

@JohnHenckel I think the first thing await does is check the status of IsCompleted, and if it is true, it continues down the same thread synchronously. Task.FromResult( ) creates a task that has the IsCompleted already set to true for the purpose of optimization.

[OlegI]

1 - but why would you do this? In awaitable method you still can call both sync and async methods? Can you explain? From.Result will allow to call it as async, but in reality it will run synchronously and will block the thread. So it will simply mislead the caller

[Stephen Cleary]

@OlegI: For I/O operations, the best solution is to implement it asynchronously, but sometimes you don't have that choice. Also, sometimes you can implement it synchronously (e.g., a cached result, falling back to an asynchronous implementation if the value isn't cached). More generally, a Task-returning method means "may be asynchronous". So sometimes methods are given an asynchronous signature knowing full well that some implementations will be synchronous (e.g., NetworkStream should be async, but MemoryStream should be sync).

[OlegI]

@StephenCleary understand. Also, I think, when you create abstraction you want it to be awaitable, even though if a concrete implementation of that abstraction doesn't run anything asynchronous YET, but probably when you will change concrete abstraction implementation it WILL call something awaitable. So declaring abstraction with Task as return type will make your concrete implementation flexible enough to run both sync and async operations without changing abstraction signature. I feel like this is one of the main purposes of Task.FromResult if I am correct...

[Stephen Cleary]

@OlegI: Yes. This is one use case for Task.FromResult; another is for unit testing (stub implementations of asynchronous interfaces).

[Rodrigo Reis]

Task.FromResult is not for this type of use, it's recomended to use with an async operation, i.e.: when your async method caches something, on calling this method and the cache meeets your needs, you can use Task.FromResult, that means you is returning something from "Result" of an previously async operation.

[Rodrigo Reis]

in your good sample, the result is not in hand synchronously, the operation is all async, the Task.FromResult is used to get a previously cached async result.

[nicholas]

Not sure why SO sorting puts this answer at the top of the list, but it is incorrect. There is no implicit parallel processing by calling await Task.FromResult(...) (see this question for an explanation). Rather it runs synchronously. If you wish to convert a long-running synchronous method such that it can be called in parallel with another, you should use await Task.Run(() => LongRunningTask()); instead.