是否有必要检查 `ValueTask.IsCompleted` 的性能？

Question

案例 1：

bool result = await DoAsync();

案例 2：

ValueTask<bool> task = DoAsync();
bool result = task.IsCompleted ? task.Result : await task;

上面有case 1和case 2，
谁能说 case 2 对性能（cpu、内存等）更好？
或者，task.IsCompleted 只是重复和多余的吗？

Answer 1

情况 1 没问题。如果你根本不使用 await，你会使用 task.IsCompeted。

Answer 2

要真正确定，您需要使用合适的基准进行衡量。但是，我不会期望这会产生任何重大影响，因为 await 已经确实如此 - 尽管通过 GetAwaiter() （不分配）。它实际上会使真正的异步场景 变得更糟 。

类似于此优化的东西在库代码中很常见，但通常用于完全避免状态机结果是很多时候可能是同步的；例如：

var pending = DoAsync(); // note: not awaited; this is a [Value]Task[<T>]
return pending.IsCompletedSuccessfully
    ? new ValueTask<Foo>(PostProcess(pending.Result))
    : Awaited(pending);

static async ValueTask<Foo> Awaited(ValueTask<Bar> pending)
    => PostProcess(await pending.ConfigureAwait(false));

这里的关键点是原始方法在这种情况下是而不是async，所以我们只在真正的异步中支付任何async开销路径（或在失败情况下，标准化错误堆栈）。

这里有一个 sharplab.io link 显示了此优化的进展情况；在右侧，您可以看到未使用 async 的优化版本，结果为：

    public ValueTask<Foo> ExampleAsync()
    {
        ValueTask<Bar> pending = DoAsync();
        if (!pending.IsCompletedSuccessfully)
        {
            return <ExampleAsync>g__Awaited|0_0(pending);
        }
        return new ValueTask<Foo>(PostProcess(pending.Result));
    }

Awaited方法，不过是全部：

    [AsyncStateMachine(typeof(<<ExampleAsync>g__Awaited|0_0>d))]
    [CompilerGenerated]
    internal static ValueTask<Foo> <ExampleAsync>g__Awaited|0_0(ValueTask<Bar> pending)
    {
        <<ExampleAsync>g__Awaited|0_0>d stateMachine = default(<<ExampleAsync>g__Awaited|0_0>d);
        stateMachine.pending = pending;
        stateMachine.<>t__builder = AsyncValueTaskMethodBuilder<Foo>.Create();
        stateMachine.<>1__state = -1;
        stateMachine.<>t__builder.Start(ref stateMachine);
        return stateMachine.<>t__builder.Task;
    }
    [StructLayout(LayoutKind.Auto)]
    [CompilerGenerated]
    private struct <<ExampleAsync>g__Awaited|0_0>d : IAsyncStateMachine
    {
        public int <>1__state;

        public AsyncValueTaskMethodBuilder<Foo> <>t__builder;

        public ValueTask<Bar> pending;

        private ConfiguredValueTaskAwaitable<Bar>.ConfiguredValueTaskAwaiter <>u__1;

        private void MoveNext()
        {
            int num = <>1__state;
            Foo result;
            try
            {
                ConfiguredValueTaskAwaitable<Bar>.ConfiguredValueTaskAwaiter awaiter;
                if (num != 0)
                {
                    awaiter = pending.ConfigureAwait(false).GetAwaiter();
                    if (!awaiter.IsCompleted)
                    {
                        num = (<>1__state = 0);
                        <>u__1 = awaiter;
                        <>t__builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
                        return;
                    }
                }
                else
                {
                    awaiter = <>u__1;
                    <>u__1 = default(ConfiguredValueTaskAwaitable<Bar>.ConfiguredValueTaskAwaiter);
                    num = (<>1__state = -1);
                }
                result = PostProcess(awaiter.GetResult());
            }
            catch (Exception exception)
            {
                <>1__state = -2;
                <>t__builder.SetException(exception);
                return;
            }
            <>1__state = -2;
            <>t__builder.SetResult(result);
        }

        void IAsyncStateMachine.MoveNext()
        {
            //ILSpy generated this explicit interface implementation from .override directive in MoveNext
            this.MoveNext();
        }

        [DebuggerHidden]
        private void SetStateMachine(IAsyncStateMachine stateMachine)
        {
            <>t__builder.SetStateMachine(stateMachine);
        }

        void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine)
        {
            //ILSpy generated this explicit interface implementation from .override directive in SetStateMachine
            this.SetStateMachine(stateMachine);
        }
    }

是否有必要检查 `ValueTask.IsCompleted` 的性能？

Is it necessary to check `ValueTask.IsCompleted` for performance?

c#

task

async-await

.net-core

valuetask