Can someone please explain the result of this small snippet

Question

Sebouh Aguehian · Answer

```fun main val job = GlobalScope launch Dispatchers Main println This is executed before toBeCancelled toBeCancelled println This is executed after toBeCancelled suspend fun toBeCancelled withContext EmptyCoroutineContext coroutineContext Job cancel println This is executed before the delay delay 2000 println This is executed after the delay ``` ```This is executed before toBeCancelled This is executed before the delay ``` How come This is executed after toBeCancelled is not printed My understanding is that `withContext` would create a child job and calling `coroutineContext Job cancel ` should only cancel that child job How come it is impacting the parent

streetsofboston · Answer

Because calling `withContext ` throws a `CancellationException` This bubbles up to the caller

streetsofboston · Answer

Try this and you can see the exception being thrown ``` suspend fun toBeCancelled try withContext EmptyCoroutineContext coroutineContext Job cancel println This is executed before the delay delay 2000 println This is executed after the delay catch e Throwable println withContext threw exception $e throw e ```

streetsofboston · Answer

Removing the `throw e` from the `catch` clause will eat that cancellation exception and This is executed after toBeCancelled will be printed out note Don t eat forget to rethrow CancellationExceptions It can break the Structured Concurrency

Sebouh Aguehian · Answer

Hmm Is it a special case with `withContext` The following doesn t cancel the parent ```fun main val job = GlobalScope launch Dispatchers Main println This is executed before toBeCancelled launch coroutineContext Job cancel println This is executed before the delay delay 2000 println This is executed after the delay join println This is executed after toBeCancelled ```

Sebouh Aguehian · Answer

Is the parent child relationship different here

streetsofboston · Answer

Because `launch join ` is not like `withContex`

streetsofboston · Answer

`withContext` is more like `async await ` It returns the value of its lambda

Sebouh Aguehian · Answer

I see

Sebouh Aguehian · Answer

So when a result is expected a cancellation bubbles up then

streetsofboston · Answer

A call to `launch` will never throw an exception A call to `join` won t either A `launch` will cause the CoroutineScope s CoroutineExceptionHandler to be called a call to `async` will never throw an exception either but a call to `await` will throw an exception if there was one thrown by the lambda of `async`

streetsofboston · Answer

`withContext` like `async await ` re throws any exception thrown in its lambda And that exception can be a CancellationException caused by a call to `cancel` somewhere

streetsofboston · Answer

You may be interested in the article I wrote here slightly smiling face <https medium com the kotlin chronicle coroutine exceptions 3378f51a7d33>

Sebouh Aguehian · Answer

But in case of a non cancellation exception within `launch` that bubble up right

Sebouh Aguehian · Answer

Just tested it indeed seems to

Sebouh Aguehian · Answer

Ok that s clearer in my head now Thanks a lot I ll check your article

streetsofboston · Answer

I won t bubble up outside of the launch An exception from a `launch` will be handled by the CoroutineScope s CoroutineExceptionHandler CEH If that exception is a CancellationException nothing really will happen except for the CoroutineScope getting cancelled unless a SupervisorScope is used If it is not a CancelationException depending whether you have a CEH installed or not and the implementation of your CEH and the implementation of your app s uncaught exception handler your app may crash

Sebouh Aguehian · Answer

In the following case ```fun main val job = GlobalScope launch Dispatchers Main println This is executed before toBeCancelled try launch throw Exception shiiiii join catch e Exception println e println This is executed after toBeCancelled ``` I m getting ```This is executed before toBeCancelled JobCancellationException StandaloneCoroutine is cancelling caused by Exception shiiiii job=StandaloneCoroutine Cancelling 11 This is executed after toBeCancelled Exception shiiiii ``` Seems like I m able to catch it outside the launch

Pablichjenkov · Answer

I think in your initial code `coroutineContext Job cancel ` inside `withContext` will return the parents job `withContext` switches Context but I don t think it creates a new child Job

Sebouh Aguehian · Answer

I printed the jobs and they were different

Pablichjenkov · Answer

humm that defeats my statement Never mind