C#•4mo ago

✅ Double-Async

Is Method1 double async here?

public static async Task<string> Method1()
    {
        var client = new HttpClient();
        Console.WriteLine("in ");
        var content = await client.GetStringAsync("https://microsoft.com");
        return content;
    }

    public static Task<string> Method2()
    {
        var client = new HttpClient();
        return client.GetStringAsync("https://microsoft.com");
    }

public static async Task<string> Method1()
    {
        var client = new HttpClient();
        Console.WriteLine("in ");
        var content = await client.GetStringAsync("https://microsoft.com");
        return content;
    }

    public static Task<string> Method2()
    {
        var client = new HttpClient();
        return client.GetStringAsync("https://microsoft.com");
    }

29 Replies

Jason_BjornOP•4mo ago

double because the method itself is async and then async again because I am using await

333fred•4mo ago

No idea what "double async" means to you The method is async because you declared it so. That's it

Jason_BjornOP•4mo ago

What if a method that is declared async calls another method that is also async? @333fred

333fred•4mo ago

Still not sure what double async means to you

Jason_BjornOP•4mo ago

What I just described would be it

333fred•4mo ago

But that doesn't mean anything to me You declared the method async, therefore it's async That's it, end of story

Jason_BjornOP•4mo ago

ok, thanks

333fred•4mo ago

What impact were you imagining that double async would have?

Jason_BjornOP•4mo ago

The outer one would be basically useless since calling a function does not need to be async-ed

333fred•4mo ago

But what impact do you imagine that would have?

Jason_BjornOP•4mo ago

none, really But I guess 2 state machines would be created

333fred•4mo ago

Every async method gets one, and only one, state machine It may have more states when you await, but it's still only one machine

Jason_BjornOP•4mo ago

method 2 seems better anyway since it avoids it all together

C# 1.0
Program.Method1 ()
   <Method1>d__0 stateMachine = new <Method1>d__0 ();
   stateMachine.<>t__builder = AsyncTaskMethodBuilder<string>.Create ();
   stateMachine.<>1__state = -1;
   stateMachine.<>t__builder.Start (ref stateMachine);
   return stateMachine.<>t__builder.Task;
Program.Method2 ()
   HttpClient client = new HttpClient ();
   return client.GetStringAsync ("https://microsoft.com");
<Method1>d__0.MoveNext ()
   int num = <>1__state;
   string result;
   try
   {
        TaskAwaiter<string> awaiter;
        if (num != 0)
        {
             <client>5__1 = new HttpClient ();
             Console.WriteLine ("in ");
             awaiter = <client>5__1.GetStringAsync ("https://microsoft.com").GetAwaiter ();
             if (!awaiter.IsCompleted)
             {
                  num = (<>1__state = 0);
                  <>u__1 = awaiter;
                  <Method1>d__0 stateMachine = this;
                  <>t__builder.AwaitUnsafeOnCompleted (ref awaiter, ref stateMachine);
                  return;
             }
        }
        else
        {
             awaiter = <>u__1;
             <>u__1 = default(TaskAwaiter<string>);
             num = (<>1__state = -1);
        }
        <>s__3 = awaiter.GetResult ();
        <content>5__2 = <>s__3;
        <>s__3 = null;
        result = <content>5__2;
   }
   catch (Exception exception)
   {
        <>1__state = -2;
        <client>5__1 = null;
        <content>5__2 = null;
        <>t__builder.SetException (exception);
        return;
   }
   <>1__state = -2;
   <client>5__1 = null;
   <content>5__2 = null;
   <>t__builder.SetResult (result);
Press Shift+Alt+R to open in ILSpy

C# 1.0
Program.Method1 ()
   <Method1>d__0 stateMachine = new <Method1>d__0 ();
   stateMachine.<>t__builder = AsyncTaskMethodBuilder<string>.Create ();
   stateMachine.<>1__state = -1;
   stateMachine.<>t__builder.Start (ref stateMachine);
   return stateMachine.<>t__builder.Task;
Program.Method2 ()
   HttpClient client = new HttpClient ();
   return client.GetStringAsync ("https://microsoft.com");
<Method1>d__0.MoveNext ()
   int num = <>1__state;
   string result;
   try
   {
        TaskAwaiter<string> awaiter;
        if (num != 0)
        {
             <client>5__1 = new HttpClient ();
             Console.WriteLine ("in ");
             awaiter = <client>5__1.GetStringAsync ("https://microsoft.com").GetAwaiter ();
             if (!awaiter.IsCompleted)
             {
                  num = (<>1__state = 0);
                  <>u__1 = awaiter;
                  <Method1>d__0 stateMachine = this;
                  <>t__builder.AwaitUnsafeOnCompleted (ref awaiter, ref stateMachine);
                  return;
             }
        }
        else
        {
             awaiter = <>u__1;
             <>u__1 = default(TaskAwaiter<string>);
             num = (<>1__state = -1);
        }
        <>s__3 = awaiter.GetResult ();
        <content>5__2 = <>s__3;
        <>s__3 = null;
        result = <content>5__2;
   }
   catch (Exception exception)
   {
        <>1__state = -2;
        <client>5__1 = null;
        <content>5__2 = null;
        <>t__builder.SetException (exception);
        return;
   }
   <>1__state = -2;
   <client>5__1 = null;
   <content>5__2 = null;
   <>t__builder.SetResult (result);
Press Shift+Alt+R to open in ILSpy

333fred•4mo ago

Well, Method2 isn't marked async So it's just a normal method

Jason_BjornOP•4mo ago

yeah, but it seems to behave the same while needing less

333fred•4mo ago

Exception behavior is different

Jason_BjornOP•4mo ago

Which do you favor?

333fred•4mo ago

Method2 will not appear in a stack trace $itdepends

MODiX•4mo ago

https://cdn.discordapp.com/attachments/569261465463160900/826859856719380500/1niu5wN.png

Jason_BjornOP•4mo ago

-_-

333fred•4mo ago

The main question is: what do you want the exception behavior to be? Do you want it to appear in a stack trace? If so, prefer async If you don't care, then prefer non-async

Jason_BjornOP•4mo ago

If I don't use the async method will the exception be lost or what? Will I not be able to find the source of it?

333fred•4mo ago

Depends on the application type and the global sync context, but an unawaited Task could either do nothing much or crash your app An exception from a task-returning method is (usually) observed at the await point My plane is about to take off, so that's probably it for me responding for a while

Angius•4mo ago

The general rule of thumb is $rulesofasync

MODiX•4mo ago

TLDR on async/await: * every .net API that is suffixed with Async (eg: .Read() and .ReadAsync()) => use the Async version * if the API name ends with Async => await it * if the API returns a Task => await it * if you have to await in a method: * that method must have the async keyword (you could even suffix your function name with Async, up to you) * if it was returning T (eg:string) => it should now return Task<T> (eg: Task<string>) * APIs to ban and associated fix:

var r = t.Result;                   /* ===> */ var r = await t;
t.Wait();                           /* ===> */ await t;
var r = t.GetAwaiter().GetResult(); /* ===> */ var r = await t;
Task.WaitAll(t1, t2);               /* ===> */ await Task.WhenAll(t1, t2);

var r = t.Result;                   /* ===> */ var r = await t;
t.Wait();                           /* ===> */ await t;
var r = t.GetAwaiter().GetResult(); /* ===> */ var r = await t;
Task.WaitAll(t1, t2);               /* ===> */ await Task.WhenAll(t1, t2);

Angius•4mo ago

While you can return a Task from a method without it being async and without it awaiting anything, in most cases it's not a good idea if only because it disappears from the stack trace

Jason_BjornOP•4mo ago

thank you, have a good flight why does it dissapear from the stacktrace, I thought Tasks are supposed to be bascially like writing blocking code

Angius•4mo ago

The error is raised where the task is awaited, should that task cause an error

Task<int> Foo()
{
    return GetNumberAsync();
}

async Task<int> Bar()
{
    return await GetNumberAsync();
}

async Task CallFoo()
{
    var i = await Foo(); // error happens in `CallFoo`
}

async Task CallBar()
{
    var i = await Bar(); // error happens in `Bar`
}

Task<int> Foo()
{
    return GetNumberAsync();
}

async Task<int> Bar()
{
    return await GetNumberAsync();
}

async Task CallFoo()
{
    var i = await Foo(); // error happens in `CallFoo`
}

async Task CallBar()
{
    var i = await Bar(); // error happens in `Bar`
}

Jason_BjornOP•4mo ago

good little demo, helps a lot. Thank you

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✅ Double-Async