It's all about the state...
What lies beneath async/await in C#?
Dariusz Pawlukiewicz
16.12.2017 dotNetConf PL
About me
- foreverframe.net
- devreview.pl
- github.com/GooRiOn
- @d_pawlukiewicz
Agenda
1. async/await in a nutshell
2. Compilation in .NET
3. Reveal the mystery...
4. Demo
5. Task.Result
6. Summary
async/await in a nutshell
STATE 1
STATE 2
Compilation in .NET
C#
VB
F#
C# compiler
VB compiler
F# compiler
IL
Metadata
JIT
01010101
Why is that important?
async/await does not exist on the IL level
... it's just a syntactic sugar
What lies beneath async/await?
STATE MACHINE
Async state machine
- struct generated by C# compiler
- one for each async method
- number of states depends on number of await operators
- kicked off by AsyncMethodBuilder
- uses other mechnisms to schedule tasks (TimerQueue, MoveNextRunner etc.)
DEMO
Summary
- async/await is syntactic sugar
- compiler generates state machine instead
- states are created by await operators
- SM uses if/switch + goto to move across the states
- Async methods captures context by default
- ... therefore Task.Result might be evil
Thanks!
@d_pawlukiewicz
foreverframe.net