The two code examples here show what happens when exceptions are thrown within tasks that are not handled within the task. In each case the task that has the error throws an exception.
In the first example, only one task throws an exception. Although from the output you can see that more tasks were expected to be launched the framework no longer schedules tasks to be started once an exception is thrown. Any existing tasks are continued to completion.
In the second example, all the tasks will throw exceptions. This is just to show that the aggregate exception is bringing back all the exceptions from the various tasks that were running.
Code Example 1 : Some bad
class Program
{
static void Main(string[] args)
{
List<HotelRoomAvailability> hotelList = GetHotels();
Console.WriteLine("These are the hotels to process");
foreach(var hotel in hotelList)
Console.WriteLine(hotel.HotelCode);
Console.WriteLine(new string('=',79));
try
{
Parallel.ForEach(hotelList, item => PopulateDetails(item));
}
catch (AggregateException aggex)
{
Console.WriteLine(aggex.Message);
foreach(Exception ex in aggex.InnerExceptions)
Console.WriteLine(ex.Message);
}
Console.WriteLine("Program finished");
Console.ReadLine();
}
private static void PopulateDetails(HotelRoomAvailability hotel)
{
Console.WriteLine("Populating details of {0}", hotel.HotelCode);
hotel.Name = HotelRespository.GetHotelName(hotel.HotelCode);
hotel.Rates = AvailabilityRespository.GetRateInformation(
hotel.HotelCode, hotel.StayDate, hotel.NumberOfNights);
}
private static List<HotelRoomAvailability> GetHotels()
{
List<HotelRoomAvailability> result = new List<HotelRoomAvailability>
{
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONSOHO"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONLHRT4"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONLHRT5" // Not valid
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONWATERL" // Not valid
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONLHR123"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONCOVGDN"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONCTYAIR"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONLEISQR"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONPADDIN" // Not Valid
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONHIGHOL"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONKINGSX"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LONEUSTON"
}
};
return result;
}
}
Output
The following is output first
These are the hotels to process LONSOHO LONLHRT4 LONLHRT5 LONWATERL LONLHR123 LONCOVGDN LONCTYAIR LONLEISQR LONPADDIN LONHIGHOL LONKINGSX LONEUSTON ======================================================== Populating details of LONSOHO Populating details of LONWATERL Populating details of LONCTYAIR Populating details of LONHIGHOL Populating details of LONLHRT4
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Then an exception is thrown
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And the final output looks like this:
These are the hotels to process LONSOHO LONLHRT4 LONLHRT5 LONWATERL LONLHR123 LONCOVGDN LONCTYAIR LONLEISQR LONPADDIN LONHIGHOL LONKINGSX LONEUSTON ======================================================== Populating details of LONSOHO Populating details of LONWATERL Populating details of LONCTYAIR Populating details of LONHIGHOL Populating details of LONLHRT4 One or more errors occurred. The hotel code 'LONWATERL' does not match a known hotel Program finished
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Code Example 2 : All bad
This example replaces the GetHotels method, above, with a method that creates a list of entirely non-existant hotels:
class Program
{
static void Main(string[] args)
{
List<HotelRoomAvailability> hotelList = GetHotels();
Console.WriteLine("These are the hotels to process");
foreach(var hotel in hotelList)
Console.WriteLine(hotel.HotelCode);
Console.WriteLine(new string('=',79));
try
{
Parallel.ForEach(hotelList, item => PopulateDetails(item));
}
catch (AggregateException aggex)
{
Console.WriteLine(aggex.Message);
foreach(Exception ex in aggex.InnerExceptions)
Console.WriteLine(ex.Message);
}
Console.WriteLine("Program finished");
Console.ReadLine();
}
private static void PopulateDetails(HotelRoomAvailability hotel)
{
Console.WriteLine("Populating details of {0}", hotel.HotelCode);
hotel.Name = HotelRespository.GetHotelName(hotel.HotelCode);
hotel.Rates = AvailabilityRespository.GetRateInformation(
hotel.HotelCode, hotel.StayDate, hotel.NumberOfNights);
}
private static List<HotelRoomAvailability> GetHotels()
{
List<HotelRoomAvailability> result = new List<HotelRoomAvailability>
{
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "BRISTOL"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "BIRMINGHAM"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "MANCHESTER" // Not valid
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LIVERPOOL" // Not valid
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "CARLISLE"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "CAMBRIDGE"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "OXFORD"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "READING"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "LEEDS" // Not Valid
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "NEWCASTLE"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "EDINBURGH"
},
new HotelRoomAvailability
{
StayDate = new DateTime(2011, 7, 1),
NumberOfNights = 3,
HotelCode = "GLASGOW"
}
};
return result;
}
}
Output
These are the hotels to process BRISTOL BIRMINGHAM MANCHESTER LIVERPOOL CARLISLE CAMBRIDGE OXFORD READING LEEDS NEWCASTLE EDINBURGH GLASGOW ============================================================ Populating details of BRISTOL Populating details of LIVERPOOL Populating details of OXFORD Populating details of NEWCASTLE Populating details of BIRMINGHAM One or more errors occurred. The hotel code 'LIVERPOOL' does not match a known hotel The hotel code 'OXFORD' does not match a known hotel The hotel code 'NEWCASTLE' does not match a known hotel The hotel code 'BIRMINGHAM' does not match a known hotel The hotel code 'BRISTOL' does not match a known hotel Program finished
More Information
- I wrote more on throwing exceptions inside tasks back in February.
- Also, the full code for this example is available for download.
Filed under: Parallelisation Talk Examples, Parallelization Talk Examples, Talk Examples Tagged: .NET, AggregateException, C#, parallelisation, parallelization Image may be NSFW.
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