c# - Parallel.ForEach used with NHibernate resulting in SQL Server locks

Question

Firstly, I am not much of an expert in multi-threading and parallel programming.

I am trying to optimize the performance of a legacy application (.Net 4, NHibernate 2.1).

**So far, upgrading NHibernate is not a priority, but is in the pipeline.

Over time, performance has become a nightmare with the growth of data. One item I have seen is a Parallel.ForEach statement that calls a method that fetches and updates a complex entity(with multiple relationships - propeties & collections).

The piece of code has the following form (simplified for clarity):

void SomeMethod(ICollection<TheClass> itemsToProcess)
{
   Parallel.ForEach(itemsToProcess, item => ProcessItem(item);
}

TheClass ProcessItem(TheClass i)
{
   var temp = NHibernateRepository.SomeFetchMethod(i);
   var result = NHibernateRepository.Update(temp);
   return result;
}

SQL Server intermittently reports database lock errors with the following error:

Transaction (Process ID 20) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction

I suspect it is due to some race condition happening leading up to a deadlock, even if ISessions are separate.

The ICollection<TheClass> can have up to 1000 items and each with properties and sub-collections that are processed, generating many SELECT and UPDATE statements (confirmed using 'NHibernate Profiler')

Is there a better way to handle this in a parallel way, or shall I refactor the code to a traditional loop?

I do know that I can alternatively implement my code using:

1. A foreach loop in the same ISession context
2. With a Stateless Session
3. With Environment.BatchSize set to a reasonable value

OR

1. Using SQL BulkCopy

I have also read quite a bit of good info about SQL Server deadlocks and Parallel.ForEach being an easy pitfall:

1. SQL Transaction was deadlocked
2. Using SQL Bulk Copy as an alternative
3. Potential Pitfalls in Data and Task Parallelism
4. Multi threading C# application with SQL Server database calls