The document discusses async-await in C# and how it works under the hood. It begins with questions about what happens when await is used. It then explains that async-await is used to compose asynchronous code using task continuations generated by the compiler. It discusses how await marks a continuation and depends on the context to determine threading. Windows I/O is used as an example of asynchronous operations without threads. The document concludes with recommendations to avoid deadlocks by not blocking on async code and to only use async-void for event handlers.

1. ASYNC, AWAIT, OH… WAIT!
YOU’D BETTER WATCH YOUR STEP
December 2016 – Alt.Net Paris
@tpierrain (use case driven)

2. “USE CASE DRIVEN”, BUT ALSO…
• Reactive Programmer (> 10 years)
• eXtreme Programmer (> 10 years)
• Programmer (> 18 years)
@tpierrain
NFluent Value (DDD)

4. LET’S START WITH A QUESTION…

5. LET’S START WITH A QUESTION…
Q:What happens line 24, when await occurs?

6. OUPS! LET’S ADD SOME CONTEXT

7. LET’S START WITH A QUESTION…
Q:What happens line 24, when await occurs?

8. LET’S START WITH A QUESTION…
Q:What happens line 24, when await occurs?

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32. WAS ITYOUR FIRST ANSWER?
Replay?

33. GUESS WHAT…
SAME CODE IN ASP.NET OR UI CONTEXT
WILL DEADLOCK!

34. OH… WAIT! AGENDA
1. Why
2. What
3. How
4. Pitfalls & Recommendations

35. CHAPTER 1: WHY

36. LIKE ME, YOU DON’T LIKE WASTE?

37. A SIMPLE RULE TO AVOID WASTE OF CPU

38. ASYNC-AWAIT INTENTION IS...
…TO EASILYTRANSFORM
SYNCHRONOUS CODE  ASYNCHRONOUS CODE
(WITHOUT HURTINGYOUR EXISTING CODE STRUCTURE)

39. WITHOUT HURTINGYOUR EXISTING CODE
STRUCTURE

40. WAIT A MINUTE… ASYNCHRONOUS,
CONCURRENT, PARALLEL?

41. SYNCHRONOUS
• PERFORM something here
and now.
• The caller thread will be
blocked until it’s done
ASYNCHRONOUS
• INITIATE something here and
now (off-loading).
• The caller thread is released
immediately
• Free for something else
• No waste of CPU resource ;-)

42. SYNCHRONOUS
• PERFORM
THIS IS ABOUT INVOCATION!
(NOT ABOUT HOWTHE GODDAMNTHING IS EXECUTED)
ASYNCHRONOUS
• INITIATE

43. WHAT ABOUT EXECUTION

44. CONCURRENCY
• Multiple “threads” of execution
• Independent logical segments

45. CONCURRENCY PARALLELISM
CONCURRENCY
+
SIMULTANEOUS EXECUTION
• Multiple “threads” of execution
• Independent logical segments

46. CONCURRENCY
THIS IS ABOUT EXECUTION!
(OFTHE GODDAMNTHING ;-)
PARALLELISM
CONCURRENCY
+
SIMULTANEOUS EXECUTION
• Multiple “threads” of execution
• Independent logical segments

47. CHAPTER 2: WHAT

48. ASYNC-AWAIT IS NOT ABOUT GOING ASYNC

49. ASYNC-AWAIT IS ABOUT COMPOSINGTHE ASYNC

50. COMPOSINGTHE ASYNC
WITH TASK CONTINUATIONS

51. CHAPTER 3: HOW

52. (PREREQUISITE - TPL)

53. TASK PARALLEL LIBRARY REMINDER (TPL)
• 3 ways to instantiate and run aTASK:
• var task =Task.Run(lambda);
• var task = newTask(lambda).Start();
• Task.Factory.StartNew(lambda);

54. TASK PARALLEL LIBRARY REMINDER (TPL)
• 3 ways to instantiate and run aTASK:
• var task =Task.Run(lambda);
• var task = newTask(lambda).Start();
• Task.Factory.StartNew(lambda);

55. TASK PARALLEL LIBRARY REMINDER (TPL)
• 3 ways to instantiate and run aTASK:
• var task =Task.Run(lambda);
• var task = newTask(lambda).Start();
• Task.Factory.StartNew(lambda);
• task.Wait(), task.Result & task.Exception ( all blocking ;-(

56. TASK PARALLEL LIBRARY REMINDER (TPL)
• CONTINUATION
• ATask that will be achieve once a previousTask has finished
• var continuationTask = previousTask.ContinueWith(lambda);
PreviousTask
ContinuationTask

57. ASYNC-AWAIT

58. ASYNC-AWAIT

59. ASYNC
GENERATES STATE MACHINE

60. AWAIT
MARKS A CONTINUATION

61. ASYNC

62. ASYNC GENERATES STATE MACHINE

63. AT COMPILETIME
ASYNC GENERATES STATE MACHINE

64. AT COMPILETIME
ASYNC
GENERATES STATE MACHINE

65. AT COMPILETIME
ASYNC
GENERATES STATE MACHINE

66. AWAIT

67. AWAIT
MARKS A CONTINUATION

68. AWAIT
RETURNSTOTHE CALLER WITH A CONTINUATIONTASK

69. WHO’S DOING THE CONTINUATION?
WELL… IT DEPENDS ;-)

70. IT DEPENDS ON THE AWAITER CONTEXT
The following code:

71. IT DEPENDS ON THE AWAITER CONTEXT
The following code:
Is equivalent to:
WinForms,
WPF,
ASP.NET…

72. ASYNC-AWAIT
GENERATES STATE MACHINE
MARKS A CONTINUATION

73. THREAD(S) OR NOTHREAD?

74. WINDOWS I/O: UNDER THE HOOD

75. WINDOWS I/O: UNDER THE HOOD

76. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode

77. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
UI thread
THREAD

78. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
BCL.WriteAsync()
THREAD

79. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
P/Invoke
handle
THREAD

80. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Create
I/O Request
Packet (IRP)
(asynchronous)
THREAD

81. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
ReturnTask
(await)
THREAD

82. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Release UI
thread
THREAD

83. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
NOTHREAD

84. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Interrupt!
NOTHREAD

85. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Interrupt
Service
Routine (ISR)
NOTHREAD

86. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Queue
Deferred
Procedure Call
(DPC).
NOTHREAD

87. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Mark IRP
as completed
NOTHREAD

88. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Create
Asynchronous
Procedure Call
(APC)
to notify the UI
process (handle)
NOTHREAD

89. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Execute
APC via I/O
Completion port
(IOCP) thread
THREAD

90. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Execute
APC via I/O
Completion port
(IOCP) thread
THREAD

91. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Post the
continuation for
UI thread
execution
THREAD

92. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode

93. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Execute
continuation
(UI thread)
THREAD

94. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Execute
continuation
(UI thread)
THREAD

95. WINDOWS I/O: UNDER THE HOOD
User mode
Kernel mode
Replay?

96. CHAPTER 4: PITFALLS & RECOS

97. #1: DEADLOCK

98. INITIAL QUESTION, BUT IN A GUI CONTEXT
1

99. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

100. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

101. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

102. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

103. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

104. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

105. 21
INITIAL QUESTION, BUT IN A GUI CONTEXT

106. 21
INITIAL QUESTION, BUT IN A GUI CONTEXT

107. 2
1
INITIAL QUESTION, BUT IN A GUI CONTEXT

108. 1
2
INITIAL QUESTION, BUT IN A GUI CONTEXT

109. 1
2
INITIAL QUESTION, BUT IN A GUI CONTEXT

110. 1
2
INITIAL QUESTION, BUT IN A GUI CONTEXT

111. 1
INITIAL QUESTION, BUT IN A GUI CONTEXT

112. 1
2
INITIAL QUESTION, BUT IN A GUI CONTEXT

113. 1
x
INITIAL QUESTION, BUT IN A GUI CONTEXT
The GUI case

114. 1
x
INITIAL QUESTION, BUT IN A GUI CONTEXT

116. DEADLOCKS

117. THE DUPDOB PRINCIPLE
“QUI DIT LOCKS…
DIT DEADLOCKS”
“WHOEVER LOCKS…
EVENTUALLY DEADLOCKS”
@cyrdup

118. DEADLOCK MEME

119. DEADLOCK MEME

120. DEADLOCK MEME

121. DO NOT BLOCKTO AVOID DEADLOCK

122. DO NOT BLOCK TO AVOID DEADLOCK
« AWAIT » DON’T BLOCKTHE UITHREAD

123. YOU CODE A LIBRARY?
SYNCHRONIZATION CONTEXT IS NOTYOUR DECISION!
AVOID DEADLOCKS IMPROVE PERFORMANCE
USE CONFIGUREAWAIT(FALSE) EVERYWHERE!

124. YOU CODE A LIBRARY?
USE CONFIGUREAWAIT(FALSE) EVERYWHERE!

125. #2: ENTER THE ASYNC VOID

126. THE ASYNC VOID CASE
• Async void is a “fire-and-forget” mechanism...
• The caller is unable to know when an async void has finished
• The caller is unable to catch exceptions thrown from an async void
• (instead they get posted to the UI message-loop)

127. THE ASYNC VOID CASE
• Async void is a “fire-and-forget” mechanism...
• The caller is unable to know when an async void has finished
• The caller is unable to catch exceptions thrown from an async void
• (instead they get posted to the UI message-loop)

128. TRY-CATCH YOUR EVENT HANDLERS!

129. MS GUIDELINES
• Use async void methods only for top-level event handlers (and their like)
• Use asyncTask-returning methods everywhere else
• Try-Catch your Async event handlers!

130. MS EVEN SAID:

131. #3: USE IT WISELY

132. ONLY FOR I/O!

133. WRAP-UP

134. WRAP-UP
1. Never block! Unless you want to deadlock
• Locks, Wait without timeout,Task.Result…
• Use top-level await when coding UI or Web
• Use ConfigureAwait(false) everywhere within your libraries
2. Never create « async void » methods
• And try catch all such existing event handlers
3. Only for I/Os
DON’T USE ASYNC-AWAIT
UNLESSYOU UNDERSTAND HOW ITWORKS

135. THANKS!

136. APPENDIX

137. DON’T SYSTEMATIZE ASYNC-AWAIT?
NOTHING INTHE CONTINUATION?
NO NEED FOR AWAIT! (UNLESS FOR ‘USING’)
StateMachine
StateMachine
StateMachine
StateMachine
StateMachine

138. ASYNC METHOD GC IMPACT
• 3 allocations for every Async method
• Heap-allocated state machine
• With a field for every local variable in your method
• Completion delegate
• Task

139. TROUBLESHOOTING?
Namespace
public type nested type static method

140. FEW REFS
• Bart De Smet deep dive: https://channel9.msdn.com/Events/TechDays/Techdays-2014-the-
Netherlands/Async-programming-deep-dive
• Filip Ekberg at Oredev 2016: https://vimeo.com/191077931
• Async-Await Best practices: https://msdn.microsoft.com/en-us/magazine/jj991977.aspx
• Compiler error: http://stackoverflow.com/questions/12115168/why-does-this-async-await-
code-generate-not-all-code-paths-return-a-value
• Task.Run etiquette: http://blog.stephencleary.com/2013/11/taskrun-etiquette-examples-
dont-use.html
• There is no thread: http://blog.stephencleary.com/2013/11/there-is-no-thread.html
• Does usingTasks (TPL) library make an application multithreaded? :
http://stackoverflow.com/questions/23833255/does-using-tasks-tpl-library-make-an-
application-multithreaded
• Eliding Async-Await : http://blog.stephencleary.com/2016/12/eliding-async-await.html