Parallelism on a ADO.NET call

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I have a project in which a stored procedure is Called on a SQL Server 2014.
I fill a DataTable object with the result of the stored procedure.
This is call is done in a base class and after the fill it is running a Parallel.ForEach() on the DataTable. In that virtual proces (each entity overrides the method in base to fill it's entity classes) the data get's set to the entities in a List<Entity>. When the Parallel.ForEach() is finished I call a virtual method which the entity also overrides This marks the end of the dataloading and the LIST<> is set in cache. However I sometimes see that some of Entities in the List<> are NULL. How is that possible? What am I missing to have all the rows set to their proper values?

What I have tried:

I have tried Parallel.ForEach with and without options. One of the options set the simultanious load maximum of 1 record (in fact row based processing).
When I use a regular foreach I do not see this behaviour.

Posted 21-Aug-17 22:29pm

Herman<T>.Instance

Updated 21-Aug-17 22:41pm

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OriginalGriff · Accepted Answer · 2017-08-21T22:41:00

If you look at the List<T> documentation - List<t> Class, MSDN[^]
You will find this at the bottom:

Quote:
It is safe to perform multiple read operations on a List<t>, but issues can occur if the collection is modified while it’s being read. To ensure thread safety, lock the collection during a read or write operation. To enable a collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization. For collections with built-in synchronization, see the classes in the System.Collections.Concurrent namespace. For an inherently thread–safe alternative, see the ImmutableList<t> class.

If you do not implement locking inside the Parallel.ForEach, you will get unpredictable behaviour.