When async-await change thread?

问题

When I call test1() method then ManagedThreadId will be changed after Delay(1).
When I call test2() method (instead of test1()) then ManagedThreadId will stay same.

In test2() method time required to finish method is even longer then in test1()

    [Route("api/[controller]")]
    [HttpGet]
    public async Task<IActionResult> Get()
    {
        await MainAsync();
        return new ObjectResult(null);
    }

    static async Task MainAsync()
    {
        Console.WriteLine("Main Async: " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString());
        await test1();
        //await test2();
        // . . .more code
    }

    private static async Task test1()
    {
        Console.WriteLine("thisIsAsyncStart: " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString());
        await Task.Delay(1);
        Console.WriteLine("thisIsAsyncEnd: " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString());
    }

    private static async Task test2()
    {
        Console.WriteLine("thisIsAsyncStart: " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString());
        System.Threading.Thread.Sleep(5000);
        await Task.FromResult(0);
        Console.WriteLine("thisIsAsyncEnd: " + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString());
    }

回答1:

test1 awaits Task.Delay(1), which isn't going to be completed at the time it goes to await it, meaning the rest of test1 needs to be scheduled as a continuation.

For test2 you're awaiting Task.FromResult, which will always return an already completed Task. When you await an already completed task the method can just keep running on the current thread, without needing to schedule a continuation.

来源：https://stackoverflow.com/questions/43498365/when-async-await-change-thread

标签

async-await