So far I´ve described my own .NET Software Transactional Memory´s (NSTM) API for managing transactions. It´s close to what you are used to from relational databases, I´d say. But still, it´s my own API and it stands beside what .NET already provides in terms of transactions. With System.Transactions there is already a general way to work transactionally across different resources like database and message queue, so it would be nice if NSTM was another such resource.
Juval Löwy  has described how this can be accomplished for in-memory data structures. However, making a data structure transactional like he did with the .NET collections does not make it threadsafe. .NET transactions - although attached to a thread - are not designed help make multithreading easier. In addition they cannot be nested truely. Also the data duplication in Juval´s collections is very coarse grained so it will become pretty slow pretty quickly once they are growing larger. Nevertheless the article is worth reading and provides very helpful insights in how .NET transactions work. It helped me a great deal making NSTM compatible with System.Transactions.
From the point of view of System.Transactions a NSTM transaction is a resource. Its state - the transaction log (txlog) - is changed during a .NET transaction and either discarded at the end if the .NET transaction is rolled back, or committed which means the transaction log is applied to the transactional objects (txo).
Recognition of .NET transactions is not switched on automatically, though. To check, if a .NET transaction is running and whether a NSTM transaction is already enlisted with it, is somewhat costly. NSTM needs to walk up the stack of active transactions for this. But you can switch on System.Transaction integration easily with the flag NstmMemory.SystemTransactionMode:
- EnlistOnAccess: NSTM checks on each access to a txo whether a .NET transaction is running. If there is no NSTM enlisted with the .NET transaction it begins a new NSTM transaction, enlists it, and will commit it/roll it back, when the .NET transaction ends.
- EnlistOnBeginTransaction: NSTM checks only for a .NET transaction when a NSTM transaction is explicity started using NstmMemory.BeginTransaction(). If a .NET transaction is running, the new NSTM transaction is enlisted with it. (To be more precise, NSTM even creates two nested transactions: the outer transaction is enlisted with the .NET transaction and the inner transaction is passed back to the application. That way the application can commit/rollback the inner transaction as usual and still get the final vote on the changes automatically from the out transaction coupled to the .NET transaction.)
- Ignore: NSTM does not care if a .NET transaction is already running. All NSTM transactions are independent of System.Transactions.
Here some sample code showing how to use NSTM with System.Transactions:
1 NstmMemory.SystemTransactionMode = NstmSystemTransactionMode.EnlistOnAccess;
3 INstmObject<int> o = NstmMemory.CreateObject<int>(1);
4 using (TransactionScope tx = new TransactionScope())
11 NstmMemory.SystemTransactionMode = NstmSystemTransactionMode.EnlistOnBeginTransaction;
12 using (TransactionScope tx = new TransactionScope())
14 using (INstmTransaction txNSTM = NstmMemory.BeginTransaction())
In line 1 .NET transaction recogniction is switched on. From then on each read/write access to a txo like o checks, if a .NET transaction is running and if so creates an implicit new NSTM transaction. That way the change to o get committed when the .NET transaction is finished in line 7. Please note, this option - although most convenient - is quite expensive since it requires transactional objects to check upon each and every call whether to create a NSTM transaction or not.
This is why the second example is more economic: It just checks for a .NET transaction in line 14. The price to pay for this option is an explicit NSTM transaction, i.e. a little less convenient programming model.
In my previous posting I said, System.Transactions integration was the final piece missing for your understanding of NSTM. But that´s not true anymore. Since then I stumbled across a very cool tool and have spiced up NSTM a bit. Stay tuned for automatic transactions like with COM+ and truely transparent transactionality for your classes.
 Juval Löwy, Can´t Commit: Volatile Resource Managers in .NET Bring Transactions to the Common Type, http://msdn.microsoft.com/msdnmag/issues/05/12/transactions/default.aspx