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If we were to instead invoke g from within a TPL task and f waited on it, we would need to do something special for exception handling The call f makes to wait will now result in an Agg rega t e E x c e pt io n if an exception were thrown We'd write this as follows
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T a s k StartNew ) = > g ( Wa it ( ) j } c a t c h ( Aggregat e E x c e ption ae ) { ae Handle ( e = > { FooExcept ion fe = e a s F oo E x c e ption j if ( fe ! = n U l l )
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Parents and Children By default, tasks created from within other tasks will form parenti child trees A task B that is created within another task A will become A's child (and similarly A becomes B's parent) The P a r e n t property retrieves this information at runtime and comes in handy for debugging There is no equivalent property to fetch the list of running children For example, this code snippet illustrates this particular situation
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We say that such tasks are structured because TPL enforces the hierar chy This means that TPL will not consider a parent finished until all of its outstanding children have also finished It's as if a parent always implicitly waits on its children before completing (This also means that when you wait on a parent of a structured task tree, you're also implicitly waiting on all of its children) This snippet illustrates a simplistic implementation of this idea
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Things are more complicated than this due to unhandled exceptions (as we' ll see soon), but as a mental model, this isn' t too far from reality Struc tured tasks are useful because having a well defined scope where concur rency begins and ends, as mentioned in 1 , Introduction, can help reduce the occurrence of hazards such as race conditions This approach also guarantees that exceptions from children are always propagated up the ancestor hierarchy such that a thread that waits on the topmost task will see them all As the exceptions make their way up the hierarchy, the aggrega tion can become deep This is an example of why Agg rega t e E x c e pt i on ' 5 F l atte n method can be very useful That said, unstructured concurrency is sometimes necessary, and TPL provides this capability In this model, children are permitted to survive their parent task Unstructured tasks are opt in instead of being the default: pass the Det a c h e d option at task creation time
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T a s k t a s kA = Ta s k Sta rtNew ( delegate
T a s k t a s k B = Ta s k S t a rtNew( , Ta s kC reationOption s De t a c hed ) ; I I a s sert ( t a s k B P a r e nt ! = Ta s k C u rrent ) ;
In this example, task A will not automatically wait for B to finish, and B's Pa r e nt property will return n u l l as though it were created in a situation where there was no active task
Cancellation TPL offers first class cancellation through the C a n c e l and C a n c e lAndWa i t functions When called on a task, the runtime first checks to see if it has begun running If not, the task will never run: it is effectively removed from the scheduler 's queue, and its state immediately transitions to the final C a n c e l ed state Otherwise, the task's I s C a n c e l l a t i o n R e q u e st e d flag is set to t r ue The point of this flag is to enable cooperative cancellation if a task begins running and is then asked to cancel itself, as we saw in 1 3, Data and Task Parallelism If a task is canceled, any calls to Wa i t will awaken with an Agg regat e E x c e p t i o n containing a single T a s kC a n c e l e d E x c e pt i o n This i s a basic exception class that also offers a Ta s k property to indicate which particular task was canceled Another useful aspect to using structured parallelism is that cancella tion requests may be automatically flowed through a hierarchy of tasks By default, this does not occur, but by specifying the R e s p e c t P a r e n t C a n c e l l a t i o n flag at task creation time, a child task will inherit its parent cancellation flag (Note that detached tasks do not flow the cancellation flag, no matter whether the option is specified or not) This feature is opt in because any task that can be canceled must be treated specially: all W a i t call sites must be hardened to be correct in the face of unexpected cancellation exceptions For systems that need cancellation (most notably GUI driven applications), the ability to flow cancellation this way can be a great feature