CODEWITHSUNDEEP
c#.nettask-parallel-libraryasync-await
Null Reference
Null Reference

Reputation: 11340

Getting return value from Task.Run

I have the following code:

public static async Task<string> Start(IProgress<ProcessTaskAsyncExProgress> progress)
{
    const int total = 10;
    for (var i = 0; i <= total; i++)
    {
        await Task.Run(() => RunLongTask(i.ToString(CultureInfo.InvariantCulture)));
        if (progress != null)
        {
            var args = new ProcessTaskAsyncExProgress
            {
                ProgressPercentage = (int)(i / (double)total * 100.0),
                Text = "processing " + i
            };
            progress.Report(args);
        }
    }
    return "Done";
}

private static string RunLongTask(string taskName)
{
    Task.Delay(300);
    return taskName + "Completed!";
}

How do I get back the string value of RunLongTask from this line: await Task.Run(() => RunLongTask(i.ToString(CultureInfo.InvariantCulture)));?

I've tried:

var val = await Task.Run(() => RunLongTask(i.ToString(CultureInfo.InvariantCulture))).Result;

But I get an error saying "string is not awaitable".

Upvotes: 81

Views: 153136

Answers (2)

Darren
Darren

Reputation: 9479

This is not a direct answer to old question, but for others searching:

"Normally" you shouldn't do this, but sometimes you need to match a library API so you can use a wrapper function like below:

private async Task<string> WrapSomeMethod(string someParam)
{
    //adding .ConfigureAwait(false) may NOT be what you want but google it.
    return await Task.Run(() => SomeObj.SomeMethodAsync(someParam)).ConfigureAwait(false);
}

And then call that instead with .Result like below:

string blah = WrapSomeMethod(someParam).Result;

Upvotes: 23

Haris Hasan
Haris Hasan

Reputation: 30097

Remove the Result from the end. When you await you will get the Result back from the await-able method.

var val = await Task.Run(() => RunLongTask(i.ToString(CultureInfo.InvariantCulture)));

Upvotes: 126

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