c# - Preserve exception when continuing Task<T> -

i've got task<t>:

task<a> sometask = ... 

this task can result in being successful, faulted or cancelled.

i want transform result when task successful, , preserve outcome if not.

this seems difficult when sometask throws exception.

what i've tried:

task<b> resulttask = startmytask().continuewith<b>(     t => foo(t.result),     taskcontinuationoptions.onlyonrantocompletion); 

this results in resulttask being cancelled if sometask faults. want fault.

task<b> resulttask = startmytask().continuewith<b>(     t => foo(t.result)); 

this breaks visual studio debugger because .result throws exception. if press f5, resulttask faults expected, smells.

is there way let resulttask have same outcome sometask if sometask faults?

---

essentially i'm trying express tasks:

int f() {     throw new someexception(); }  string g(int x) {     return x.tostring(); }  try {     string result = g(f()); } catch (someexception e) {     ... } 

task continuations independent. can used build want, they're not designed scenario.

the first question ask is: can relationship viewed parent/child relationship (e.g., foo parent of startmytask)? if makes sense, may able take advantage of state propogation child parent.

if treating foo "parent" , startmytask "child" doesn't work design-wise, there few other options. continuations bit low-level need (remember, they're "run task when other task completes").

it sounds may trying more "pipeline". currently, rx more suitable kind of thing.

task-based pipelines aren't here yet. parallelextensionsextras library has task-based pipeline class, , async ctp has tpl dataflow library, both of these under-developed @ present. (e.g., pipeline insists on running each stage of pipeline in separate thread, , dataflow has no mechanism automatically propogating exceptions or completion).

so, if can't use rx, write own "pipelinetransform" extension method task , use explicit tcs handle 3 completion situations correctly.