NHibernate sessions are not thread safe, and attempting to use them in multiple threads requires careful synchronization. It is generally better to consider a session only useful within the thread that created it.

There are valid scenarios for cross thread session usage (background loading with careful sync, multi request spanning session), but the invalid scenarios are far more common.

Let us take a look at the following code:

public class CrossThreadSession : IScenario
{
	public void Execute(ISessionFactory factory)
	{
		using(var session = factory.OpenSession())
		{
			var @event = new ManualResetEvent(false);

			ThreadPool.QueueUserWorkItem(state =>
			{
				using(var tx = session.BeginTransaction())
				{
					session.Get<Blog>(1);

					tx.Commit();
				}

				@event.Set();
			});

			@event.WaitOne();
		}
	}
}

If you’ll try to execute it with NH Prof listening, you are going to see this:

NH Prof is properly detecting and warning that we are using a single session in multiple threads.

I did mention that there are valid scenarios for that, so you can either just tell NHibernate to ignore this specific alert by clicking the link, or disable this alert completely if you are doing cross thread session usage extensively and you know it to be safe:

Nice enough feature, for about 30 minutes of work. I love working on NH Prof code base.

And just for fun, here is the test for this:

public class DetectCrossThreadSessionUsage : IntegrationTestBase
{
	[Fact]
	public void CanDetectCrossThreadSessionUsage()
	{
		ExecuteScenarioInDifferentAppDomain<CrossThreadSession>();

		var first = model.Sessions.First();

		Assert.Equal(1, first.Statistics.NumberOfStatements);
		Assert.Equal(2, first.Statistics.NumberOfTransactionsStatements);
		foreach (var statement in first.StatementsModel.Statements)
		{
			Assert.True(
				statement.Alerts.Any(alert => alert.HelpTopic == "CrossThreadSessionUsage"));
		}
	}
}

And for additional fun, here is the code to implement the feature:

public class CrossThreadSessionUsage : AbstractStatementProcessor
{
	public override void AfterAttachingToSession(SessionInformation session, TransactionStatement transaction)
	{
		AfterAttachingToSession(session, (SqlStatement)transaction);
	}

	public override void AfterAttachingToSession(SessionInformation sessionInformation, SqlStatement statement)
	{
		if (sessionInformation.SessionId.IdentifierId.ThreadId ==
			statement.SessionId.IdentifierId.ThreadId)
			return;
		statement.AcceptAlert(AlertInformation.CrossThreadSessionUsage());
	}
}

I did have to change some of the innards so NH Prof will track both the session id and the thread id, but that was quite easy to do and the only thing that actually took any time at all.