Task return value, without Task<T> (async/await pattern)

Question

I would like to write the following:

public string GetSomeValue()
{
    //directly return the value of the Method 'DoSomeHeavyWork'...
    var t = DoSomeHeavyWork();
    return t.Result;
}

public Task<string> DoSomeHeavyWork()
{
    return Task.Run(() => {
        // do some long working progress and return a string
        return "Hello World!";
    });
}

As you can see to return the result from the DoSomeHeavyWork() I have used the Task.Result property, which works okay, but according to researches this will block the Thread.

I would like to use the async/await pattern for this but cant seem to find how to do this. If I did the same with async/await with my current knowledge I will always end up with this:

public async Task<string> GetSomeValue()
{
    //directly return the value of the Method 'DoSomeHeavyWork'...
    var t = DoSomeHeavyWork();
    return await t;
}

public Task<string> DoSomeHeavyWork()
{
    return Task.Run(() => {
        // do some long working progress and return a string
        return "Hello World!";
    });
}

This solution doesnt quite fit my needs because I want to return ONLY the string and not a Task<string>, how can this be achieved with async/await?

Why can't you return Task<string> because when you await it it returns a string. Isn't that what you want? — Enigmativity
– Enigmativity, Commented Apr 22, 2016 at 4:21

Stephen Cleary · Accepted Answer · 2014-05-16 15:28:10Z

11

You can't.

The entire point of async is to run code asynchronously. So the code returns a promise of a future string value, represented in .NET as Task<string>.

Think about it this way: if your code calls public string GetSomeValue(), then by the time the method returns, you already have a string. This is synchronous, by definition.

In your example, you have "heavy" work which I interpret to mean "CPU-bound". In that case, just do the work synchronously:

public string DoSomeHeavyWork()
{
  // do some long working progress and return a string
  return "Hello World!";
}

In general, APIs should not "lie"; if they are synchronous, then they should have a synchronous (non-Task-returning) signature.

Edit: As per your comment, the "heavy" work is a WCF call, which is I/O-bound, not CPU-bound.

In that case, the work is naturally asynchronous. Use asynchronous WCF methods (not Task.Run), and allow the asynchrony to grow through your codebase:

public async Task<string> GetSomeValueAsync()
{
  //directly return the value of the Method 'DoSomeHeavyWork'...
  var t = DoSomeHeavyWorkAsync();
  return await t;
}

public async Task<string> DoSomeHeavyWorkAsync()
{
  // Call asynchronous WCF client.
  await ...;
  return "Hello World!";
}

edited May 16, 2014 at 15:28

answered May 16, 2014 at 15:21

Stephen Cleary

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7 Comments

Rand Random Over a year ago

How would you solve my problem, with the Task.Result property or do you know of anything better? (Would be happy about code snippets if you know a better alternative)

Rand Random Over a year ago

In my real live example I am calling a WCF request to a client, and wanted to have a method thats running async to not block the client while server is calling the method (both Server/Client are WPF) should I than return the Task object instead of the resolved string/object?

Stephen Cleary Over a year ago

I've edited my answer with my preferred solution: just do the work directly. And I've edited it again since the "heavy" work is a WCF request.

Rand Random Over a year ago

So its no issue if the WCF client implements the following interface: [OperationContract()] Task<string> GetAccountValue(string accNr, string propertyName); instead of a simple string/object?

Stephen Cleary Over a year ago

Right; anything Task-returning is asynchronous, and if you consume it asynchronously (via await), it all works.

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