NDC Sydney 2019 conference in Sydney, AU - 2019/10/15 Talk: War stories from .NET team by Karel Zikmund https://sessionize.com/s/karel-zikmund/async-demystified/24175 https://www.youtube.com/watch?v=TgUYcZV-foM

1. Async demystified
NDC Sydney 2019
Karel Zikmund – @ziki_cz

2. Agenda
• History of async patterns in .NET
• History and evolution of
• Task
• async-await

3. APM pattern:
Asynchronous Programming Model
.NET Framework 1.0/1.1 … 2002-2003
interface IAsyncResult
{
bool IsCompleted;
bool IsCompletedSynchronously;
object AsyncState;
WaitHandle AsyncWaitHandle;
}
Across BCL:
IAsyncResult BeginFoo(..., AsyncCallback callback, object state);
void EndFoo(IAsyncResult iar);

4. APM pattern
IAsyncResult BeginFoo(..., AsyncCallback callback, object state);
void EndFoo(IAsyncResult iar);
Synchronous call:
Foo();
Achieving the same:
EndFoo(BeginFoo(..., null, null));
Taking advantage of the asynchronous nature:
BeginFoo(..., iar => {
T val = EndFoo(iar);
// do stuff ...
});

5. APM – Example
Copy stream to stream:
int bytesRead;
while ((bytesRead = input.Read(buffer)) != 0) {
output.Write(buffer, 0 /* offset */, bytesRead);
}

6. APM – Nesting problem
BeginRead(..., iar => {
int bytesRead = EndRead(iar);
input.BeginWrite(..., iar2 => {
int bytesWritten2 = EndWrite(iar2);
BeginRead(..., iar3 => {
int bytesRead3 = EndRead(iar3);
BeginWrite(..., iar4 => {
// ... again and again
});
});
});
});

7. APM – IsCompletedSynchronously
IAsyncResult r = BeginRead(..., iar => {
if (!iar.IsCompletedSynchronously) {
// ... asynchronous path as shown earlier
}
});
if (r.IsCompletedSynchronously) {
// ... Synchronous path
}
• Even worse in loop
• Overall very complicated
• Queueing on ThreadPool much simpler

8. EAP:
Event-based Asynchronous Pattern
• .NET Framework 2.0
obj.Completed += (sender, eventArgs) => {
// ... my event handler
}
obj.SendPacket(); // returns void
• Did not solve multiple-calls problem, or loops
• Introduced ExecutionContext

9. Task
• .NET Framework 4.0
• MSR project – parallel computing
• Divide & conquer efficiently (e.g. QuickSort)
• Shaped Task – similar to today
• Task – represents general work (compute, I/O bound, etc.)
= promise / future / other terminology
• Task / Task<T> – operation (with optional result T) contains:
1. T … in the case of Task<T>
2. State related to synchronization
3. State related to callback

10. Task / TaskCompletionSource
• Task
• Here's a callback, invoke it when you're done, or right now if you've already
completed
• I want to block here, until your work is done
• Cannot be completed by user directly
• TaskCompletionSource … wrapper for Task
• Holds Task internally and operates on it via internal methods
• Methods:
• SetResult
• SetException
• SetCancelled

11. Task – Consumption
Task<T> t;
Either:
t.Wait(); // Blocks until Task is completed
Or:
t.ContinueWith(callback); // Will be executed after Task is completed
Even multiple times:
t.ContinueWith(callback2);
t.ContinueWith(callback3);
ContinueWith:
• Does not guarantee order of executions
• Always asynchronous (queued to ThreadPool/scheduler in general)

12. Task.Run
We complicated things 
Task<T> Task.Run(delegate d)
• Adds field to Task with ‘d’
• Queues work to ThreadPool
• Thread grabs it, executes it, marks task completed

13. Task.Run implementation
Task<T> Run(Func<T> f) {
var tcs = new TaskCompletionSource<T>();
ThreadPool.QueueUserWorkItem(() => {
try {
T result = f();
tcs.SetResult(result);
} catch (ex) {
tcs.SetException(ex);
}
});
return tcs.Task;
}

14. async-await
.NET Framework 4.5 / C# 5
Example of asynchronous code:
Task<int> GetDataAsync();
Task PutDataAsync(int i);
Code:
Task<int> t = GetDataAsync();
t.ContinueWith(a => {
var t2 = PutDataAsync(a.Result);
t2.ContinueWith(b => Console.WriteLine("done"));
});

15. async-await
Task<int> t = GetDataAsync();
t.ContinueWith(a => {
var t2 = PutDataAsync(a.Result);
t2.ContinueWith(b => Console.WriteLine("done"));
});
C# 5 with async-await helps us:
Task<int> t = GetDataAsync();
int aResult = await t;
Task t2 = PutDataAsync(aResult);
await t2;
Console.WriteLine("done");

16. Awaiter pattern
int aResult = await t;
Translated to:
var $awaiter1 = t.GetAwaiter();
if (! $awaiter1.IsCompleted) { // returns bool
// ... (complicated) ...
}
int aResult = $awaiter1.GetResult(); // returns void or T
// If exception, it will throw it

17. Awaiter pattern – details
void MoveNext() {
if (__state == 0) goto label0;
if (__state == 1) goto label1;
if (__state == 42) goto label42;
if (! $awaiter1.IsCompleted) {
__state = 42;
$awaiter1.OnCompleted(MoveNext);
return;
}
label42:
int aResult = $awaiter1.GetResult();
}

18. State Machine
string x = Console.ReadLine();
int aResult = await t;
Console.WriteLine("done" + x);
State machine:
struct MethodFooStateMachine {
void MoveNext() { ... }
local1; // would be ‘x’ in example above
local2;
params;
_$awaiter1;
__state;
}

19. State Machine – Example
public async Task Foo(int timeout) {
await Task.Delay(timeout);
}
Compiler generates:
public Task Foo(int timeout) {
FooStateMachine sm = default;
sm._timeout = timeout;
sm._state = 0;
sm.MoveNext();
return ???;
}
struct FooStateMachine {
int _timeout; // param
// locals would be here too
void MoveNext() { ... }
int __state;
TaskAwaiter _$awaiter1;
}

20. State Machine – Example
public Task Foo(int timeout) {
FooStateMachine sm = default;
sm._tcs = new TaskCompletionSource();
sm._timeout = timeout;
sm._state = 0;
sm.MoveNext();
return sm._tcs.Task;
}
Builder pattern (can return struct):
AsyncValueTaskMethodBuilder.Create();
_tcs.Task -> _builder.Task;
struct FooStateMachine {
int _timeout; // param
// locals would be here too
void MoveNext() {
// ...
_tcs.SetResult(...);
}
int _state;
TaskAwaiter _$awaiter1;
TaskCompletionSource _tcs;
}

21. State Machine – Perf improvements
What about Task allocation?
• Builder can reuse known tasks
• Task.CompletedTask (without value)
• boolean – True/False
• int … <-1,8>
• LastCompleted (e.g. on MemoryStream)
• Does not work on SslStream (alternates headers and body)
• Size: 64B (no value) / 72B (with value)
• Azure workloads OK (GC will collect)
• Hot-path: up to 5%-10% via more GCs

22. ValueTask
• .NET Core 2.0
• Also as nuget package down-level
struct ValueTask<T> {
T;
Task<T>;
}
• Only one of them: T+null or default+Task<T>
• NET Core 2.1
ValueTask<int> Stream.ReadAsync(Memory<byte>, ...)

23. ValueTask – Can we improve more?
• What about the 1% asynchronous case?
• .NET Core 2.1
struct ValueTask<T> {
T;
Task<T>;
IValueTaskSource<T>;
}
struct ValueTask {
Task;
IValueTaskSource;
}

24. Summary
• APM pattern = Asynchronous Programming Model
• .NET Framework 1.0/1.1 (2002-2003)
• IAsyncResult, BeginFoo/EndFoo
• Limited nesting / loops
• EAP = Event-based Asynchronous Pattern (.NET Framework 2.0)
• Events – similar problems as APM
• Task (.NET Framework 4.0)
• Wait / ContinueWith
• TaskCompletionSource (for control/writing)
• async-await (.NET Framework 4.5 / C# 5)
• Awaiter pattern, state machine
• ValueTask (.NET Core 2.0)
• Don’t use unless you are on hot-path
• Hyper-optimizations possible, stay away, it is dangerous! 
@ziki_cz