C# Async

Question 1

Asynchronous vs Parallel Programming

Answer 1

• these are not the same thing
• they can be used in conjunction
• you can fire off async job that runs pieces in parallel
• metaphor of async and parallel - boiling eggs

Question 2

What is AJAX?

Answer 2

• Asynchronous JavaScript and XML

Question 3

What is the TPL?

Answer 3

• Task Parallel Library
• introduced in .NET 4.0
• in System.Threading.Tasks
• introduced the Task.Run functionality

Question 4

What is Task.Run?

Answer 4

• takes an Action (no return) or a Func (return)

- always returns a Task so you can use Continuation

Question 5

What does an Anonymous Function look like?

Answer 5

() => { do stuff here }

Question 6

Task.Run and UI thread?

Answer 6

• Task.Run operates on a different thread so UI interactions must be properly handled
• depends on the platform
• ex. UWP uses Dispatcher.Invoke() which can interact with UI thread

Question 7

How about Task.Result?

Answer 7

• returns result of the Task
• this WILL block the UI thread until the Task completes
• this can be a cause of deadlocking if UI thread is locked but a Task tries to interact with UI thread

Question 8

What is Task.IsFaulted?

Answer 8

• this will tell you if a Task has completed due to unhandled exception

Question 9

What is Task.ConfigureAwait?

Answer 9

• ConfigureAwait(true) is the default and will attempt to marshal the thread back to the original calling context
• using ConfigureAwait(false) can save resources by staying on the current thread

Question 10

How about async/await?

Answer 10

• introduced in .NET 4.5
• add async keywork to methods
• marking method as async doesn’t mean everything runs async
• using await before a task is like scheduling Continuation
• using await the Continuation runs on calling context
• async operations fire up a state machine (expensive) to keep track of async ops

Question 11

What about async method returns?

Answer 11

• async methods should always return a Task or Task

- void is unacceptable except on event handlers

Question 12

How about Task.WhenAll()?

Answer 12

• use this to await more than 1 task completing

Question 13

What about the State Machine?

Answer 13

• every time you use async keyword a State Machine is created
• you can drill into this using something like .NET Reflector
• complex and expensive object
• using a lot of awaits introduces complexity into the state machine

Question 14

What is a deadlock example?

Answer 14

• use Task.Wait() which locks UI thread
• the Task fired is trying to access the UI thread, which is now locked
• this equates to a deadlock
• Task.Wait() and Task.Result() both will block UI thread
• if necessary some things must be wrapped in a new task