For the last few months, I've had a very small base class that abstracts out the NHibernate configuration, session creation, etc. It works very well, but is very limited in what it can do. basically, every method in my actual repository implementation would have to open a new session, execute a criteria and close the session. A typical implementation would look like this:

public ICollection<Invoice> GetAll()

{

    ICollection<Invoice> invoices = null;

    try

    {

        OpenSession();

        invoices = Session.CreateCriteria(typeof(Invoice)).List<Invoice>(); 

    }

    finally

    {

        CloseSession();

    }

    return invoices;

}

That certainly is easy and keeps the code fairly clean, removes a lot of duplication, etc.

However, there is a significant limitation - I can't have any code re-use for multi-query scenarios, without duplicating code. In other words, if I want to load that list of invoices and then load some other collection from another repository, I have to use two different Sessions across two different repository implementations. This really becomes an issue when dealing with transactions - I want my entire change set to pass or fail in a single transaction. In my current abstraction, this can't be done.

Fortunately, NHibernate has the solution to my dilemma built right in - all I need to do is create my criteria objects without a session, and then I can execute any / all of them from any session that I want.

public ICollection<Invoice> GetAll()

{

    ICollection<Invoice> invoices = null;

    try

    {

        DetachedCriteria criteria = DetachedCriteria.For<Invoice>();

        OpenSession();

        invoices = criteria.GetExecutableCriteria(Session).List<Invoice>(); 

    }

    finally

    {

        CloseSession();

    }

    return invoices;

}

I don't have a complete abstraction of the separate execution, yet. However, a very basic implementation could look like this (idea stolen from Ray Houston):

protected void Do(Action unitOfWork)

{

    try

    {

        OpenSession();

        unitOfWork();

    }

    finally

    {

        CloseSession();

    }

}

public ICollection<Invoice> GetAll()

{

    ICollection<Invoice> invoices = null;

    DetachedCriteria criteria = DetachedCriteria.For<Invoice>();

    Do(() =>{

        invoices = criteria.GetExecutableCriteria(Session).List<Invoice>();

    });

    return invoices;

}

This simple abstraction provides a lot of benefit for us.

• Eliminates duplicate code (not calling OpenSession / Close Session from all repository methods)
• Removes ugly Try / Catch blocks from Repository methods
• Allows multiple Criteria to be executed from a single Session / Transaction

And most importantly - this gives us the ability to create a better abstraction of NHibernate, to support business level transactions, not just repository level transactions. Obviously, this simple example is not going to provide business level transactions. It does get us down the path, though.