使用 Task.Run 时避免捕获当前执行上下文

Question

我想创建/运行任务而不捕获当前执行上下文。这可能吗？

考虑以下示例：

private static readonly AsyncLocal<object> AsyncLocal = new AsyncLocal<object>();

[TestMethod]
public void TaskRunCapturesContext()
{
    AsyncLocal.Value = 1;
    var task = Task.Run(static async () =>
        {
            await Task.Delay(1).ConfigureAwait(false);
            Console.WriteLine(AsyncLocal.Value); // Prints "1", expected is "" <-- Problem HERE
            AsyncLocal.Value = 2;
            Console.WriteLine(AsyncLocal.Value); // Prints "2", expected is "2"
        });

    Task.WaitAll(task);
    Console.WriteLine(AsyncLocal.Value); // Prints "1", expected is "1"
}

是否可以使第一个 Console.WriteLine(AsyncLocal.Value); 打印 "" 而不是 "1" （因为 AsyncLocal.Value 为 null/未初始化）？

我还尝试了更明确的 Task.Factory 方法的所有重载，但我找不到解决方案。从源代码中还可以看出，Task.Run(Action) 转换为对 Task.InternalStartNew(null, action, null, ..) 的调用，该调用显式传递 null 作为 state 参数，所以我没有看到更明确的方法。

提前致谢

Answer 1

感谢 @canton7 的评论，答案相当简单：您可以使用 ExecutionContext.SuppressFlow。

更正了上面的示例：

private static readonly AsyncLocal<object> AsyncLocal = new AsyncLocal<object>();
[TestMethod]
public void TaskRunNoLongerCapturesContext()
{
    AsyncLocal.Value = 1;
    using (ExecutionContext.SuppressFlow()) // <-- THIS
    {
        var task = Task.Run(static async () =>
            {
                await Task.Delay(1).ConfigureAwait(false);
                Console.WriteLine(AsyncLocal.Value); // Prints "", expected is "" <-- Hurrah
                AsyncLocal.Value = 2;
                Console.WriteLine(AsyncLocal.Value); // Prints "2", expected is "2"
            });

        Task.WaitAll(task);
    }

    Console.WriteLine(AsyncLocal.Value); // Prints "1", expected is "1"
}