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
Then an exception is thrown
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
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 got my feedback from my DDD Scotland 2011 talk on Parallelisation. I was actually pleasantly surprised. I guess I was being a little too self critical and the talk went over a lot better than I thought it had.
The Blog of Colin Mackay 