Do not call Task.WhenAll with a single argument

[terrajobst]

There's no reason to call WhenAll with a single Task; just use the Task.

Category: Performance

[jeffhandley]

Estimates:

• Analyzer: Small
• Fixer: Medium

[carlossanlop]

If the API review folks agree, we can group this proposal with #33807.

If they prefer a separate rule ID, then please approve separately.

Here is a usage example:

using System.Threading.Tasks;

namespace MyNamespace
{
    class Sample
    {
        public static void Main()
        {
            Task task = ...;

            // Instead of this
            await Task.WhenAll(task);
            // Suggest this
            await task;

            // Instead of this
            await Task.WhenAll(GetTask());
            // Suggest this
            await GetTask();

            // Analyzer should trigger, but should not offer a fix because whenResult is used somewhere else
            Task whenResult = Task.WhenAll(task);
            DoSomethingWithTask(whenResult);
        }

        static Task GetTask()
        {
            return ...;
        }

        static void DoSomethingWithTask(Task task)
        {
            ...;
        }
    }
}

• This analyzer should not affect any of the other WhenAll overloads, because they take a Task[]. Only this overload, which takes a params Task[] task would be affected.

• This analyzer should also have a fixer, except for the 3rd case indicated in the usage example.

• Suggested severity: Info.

• Suggested milestone if approved: 6.0 (seems simple to address).

• Edit: Removed the incorrect Wait() usages and added await as described in the video.

[terrajobst]

Video

• Makes sense, but we shouldn't use Wait():

async Task M(Task task)
{
    // Before
    await Task.WhenAll(task);

    // After
    await task;
}

[carlossanlop]

Thanks for the correction, and thanks for approving.

[carlossanlop]

This is also yours, @ryzngard, per your request in #33807. Thanks for your help! Let us know if you have any questions.

[jeffhandley]

@ryzngard Here's a getting started guide as well. Included in there is our typical "Definition of Done" for creating .NET Libraries analyzers.

[ryzngard]

@jeffhandley @terrajobst checked into https://github.com/dotnet/roslyn-analyzers/tree/release/6.0.1xx-preview4

[stephentoub]

This was implemented as CA1842/1843. Thanks!