1. C # 5.0
New Features
By S. M. Ali Raza Naqvi
Senior Software Engineer
2. Agenda
Basic introduction to Async and Await keywords
Tracing attributes:
CallerFilePathAttribute, CallerLineNumberAttribute
and CallerMemberNameAttribute (if we got time)
3. Async & Await
A keyword use to create async methods
Async method must contains ‘Await’ keyword
Async methods invoke synchronously
Await is the point where method suspend and resume
asynchronously
Purpose of Await keyword is to wait for the task to
complete
4. Async & Await (Cont.)
Only methods that return void, Task or Task<Tresult>
can mark as Async
Methods which can’t mark async: Main, method
implementing synchronized attribute or security
Await can’t be use
in methods that are not mark Async
Inside Properties
Inside lock, catch or finally block
6. Async & Await - Behind the scene
Compiler uses state machine to track the current
state of the flow
State machine holds the parameters, local variables
and the current state of the method
When encounter await, It stores state machine object
as variable and resume it when awaiter gets the
response
7. Async & Await - Ease
Readable and maintainable code
Allow you to write asynchronous code in synchronous
fashion
Exception handling using try..catch block
Exceptions with much more details
Allow you to use “using” block
8. Async & Await - Moreover
You can create customize awaitable types using
IAwaitable interface
You can cancel an existing ongoing task using
CancellationTokenSource object
You can deal with multiple task all at once and can use
await for all of them at a time
10. Tracing attributes:
CallerFilePathAttribute, CallerLineNumberAttribute
and CallerMemberNameAttribute
You can obtain information about the caller to a
method
To help in tracing
CallerFilePath – return the caller’s file location
CallerLineNumber – return the line number from
where that method called
CallerMemberName – return the name of the method
that calls that method
if you have a synchronous method that returns void and all you do to change it is mark it as “async”, invocations of that method will still run synchronouslySimilarly, if you have a synchronous method that returns some TResult, and all you do is mark it as “async” and change the return type to be “Task<TResult>”, invocations of that method will still run synchronously.if “await” isn’t used at all in a method’s code, there will only be one piece, and since that piece will start running synchronouslyIAwaiter an interface that contains callback methods
Because Task has GetAwaiter Method
For example: If you have a single Await keyword in your aysnc method. Then the compiler will create the state machine with 2 methods. One will contains the code from the first line of the method till Await line and the other method will contain the rest of the logic. Then the compiler executes the first method first and then move to main method from where it calls that async method and then when the awaited task (which was fired in the first method) returns the response then it calls the second method
- http://weblogs.asp.net/dixin/archive/2012/11/08/understanding-c-async-await-2-awaitable-awaiter-pattern.aspxhttp://enumeratethis.com/2010/11/03/c-5-0-rx-schedulers/InitializeCancellationTokeSouce global object under async method and then call its Cancel method when you want to cancel the on going taskAwait Task.Whenall(<list of tasks>)
