当我们不等待异步方法时会发生什么

Question

I have a .NET CORE 2 backend. In one of my controller endpoints, I'm creating invitations to be sent out via email. This seems to be a huge bottleneck on the endpoint and after thinking about it, I don't really need to wait for these invitations. If the email fails to send out, I can't really do anything about it anyway.

If I don't do await sendFn() would it essentially be a fire and forget method? I was reading on another stackoverflow thread that I'd have to do sendFn().ContinueWith(t => throw(t)) to be able to catch the exception since it'll be in another thread.

I have similar mailing functions around the code base. They each do slightly different things, but is there a service fn I can do to wrap these to make them fire and forget? I think some places I can just not use await (if that works), but some things alter the database context so if I don't await them I can potentially run into a case where something is accessing the same db context.

[HttpPost]
public async Task<IActionResult> CreateEvent([FromBody] Event val)
{
    _ctx.Event.Add(val);
    await _ctx.SaveChangesAsync();

    await SendInvitations(val); // fn in question

    return Ok();
}

public async Task SendInvitation(Event event)
{
   forEach (var person in event.people)
   {
     await _ctx.Invitation.Add(person); // This shouldn't happen while another iteration or some other async code elsewhere is using the db ctx.
     _ctx.SaveChangesAsync();
     await _mailService.SendMail(person.email,"you have been invited"); // don't really need to await this.

   }
}

I'm posting to my server with data about an event. After I create and save the event to the database, I go and create invitations for each person. These invitations are also database items. I then send out an email. I'm mostly worried that if I drop the await, then when I'm creating invitations, it may conflict with db context elsewhere or the next iteration.

Answer 1

To get your code to compile and run I had to make these changes:

public async Task<IActionResult> CreateEvent(Event val)
{
    _ctx.Event.Add(val);
    await _ctx.SaveChangesAsync();
    await SendInvitation(val);
    return Ok();
}

public async Task SendInvitation(Event @event)
{
    foreach (var person in @event.people)
    {
        await _ctx.Invitation.Add(person);
        await _ctx.SaveChangesAsync();
        await _mailService.SendMail(person.email, "you have been invited");
    }
}

I also had to write this harness code:

public OK Ok() => new OK();

public class Event
{
    public List<Person> people = new List<Person>();
}

public class Person
{
    public string email;
}

public interface IActionResult { }

public class OK : IActionResult { }

public class Invitation
{
    public Task Add(Person person) => Task.Run(() => { });
}

public static class _ctx
{
    public static List<Event> Event = new List<Event>();
    public static Invitation Invitation = new Invitation();
    public static Task SaveChangesAsync() { return Task.Run(() => { }); }
}

public static class _mailService
{
    public static Task SendMail(string email, string message) { return Task.Run(() => { }); }
}

Then I updated SendInvitation like this:

public async Task SendInvitation(Event @event)
{
    Thread.Sleep(2000);
    foreach (var person in @event.people)
    {
        await _ctx.Invitation.Add(person);
        await _ctx.SaveChangesAsync();
        await _mailService.SendMail(person.email, "you have been invited");
    }
    Console.WriteLine("Done `SendInvitation`.");
}

Now, I can run it all like so:

var e = new Event();
e.people.Add(new Person() { email = "foo@bar.com" });
CreateEvent(e).ContinueWith(t => Console.WriteLine("Done `CreateEvent`."));
Console.WriteLine("Done `Main`.");

That outputs:

Done `Main`.

Then 2 seconds later:

Done `SendInvitation`.
Done `CreateEvent`.

If I simply change CreateEvent to this:

public async Task<IActionResult> CreateEvent(Event val)
{
    _ctx.Event.Add(val);
    await _ctx.SaveChangesAsync();
    Task.Run(() => SendInvitation(val));
    return Ok();
}

Then I get this output:

Done `Main`.
Done `CreateEvent`.

Then 2 seconds later:

Done `SendInvitation`.

That seems to be what you want.

Answer 2

The short answer is that you have no guarantees that that the execution of that code will complete.

That's why ASP.NET Core has infrastructure for background work: Implementing background tasks in .NET Core 2.x webapps or microservices with IHostedService and the BackgroundService class