c# - How do I throw an Exception from inside a Parallel.ForEach loop? -

i have parallel.foreach loop downloads files so:

try {      var paralleloptions = new paralleloptions();      paralleloptions.maxdegreeofparallelism = 8;      int failedfiles = 0;      parallel.foreach(filestodownload, paralleloptions, tile =>      {          bool downloaded = downloadtile(file);          if (downloaded)          {               //downloaded :)          }          else          {                failedfiles++;                if (failedfiles > 10)                {                    throw new exception("10 files failed download. failing download");                }           }            paralleloptions.cancellationtoken.throwifcancellationrequested();       }); } catch (exception ex) {     throw; //this throws main method cancels program } 

and wondering correct way throwing exception inside parallel.foreach method? in instance think going see exception thrown 8 times first exception thrown.

what correct way throw exceptions in parallel.foreach loops?

first, better use interlocked.increment(ref failedfiles) instead of failedfiles++. otherwise can happen have 10-15 failures, end counter value 7-8, because of lack of cache synchronization , effect of compiler/jitter optimizations. loop of program might throw more exceptions, @ end aggregated single aggregateexception, , outter catch() receive single exception instance. if not want more exceptions in aggregateexception, can use == instead of >

if (interlocked.increment(ref failedfiles) == 10) 

when exception thrown inside loop, parallel.foreach prevents other iterations start, waits running iterations finish, aggregates exceptions caught, packs aggregateexception , throws single instance. means in case single exception prevent further downloads.