Question

I would like to transform this piece of code to be used with QueryOver

       public IList<T> ListByCriteria( ICriteria criteria, int maxResult )
    {
        IList<T> ret = new List<T>();
        using (ITransaction tx = m_session.BeginTransaction(IsolationLevel.ReadCommitted))
        {
            try
            {
                ret = criteria
                    .SetMaxResults( maxResult )
                    .List<T>();

                tx.Commit();
            }
            finally
            {
                if (tx.IsActive)
                    tx.Rollback();
            }
        }
        return ret;
    }

to something like

    public IList<T> ListByQueryOver( Expression<Func<bool>> expression )
    {
        IList<T> ret = new List<T>();
        using (ITransaction tx = m_session.BeginTransaction(IsolationLevel.ReadCommitted))
        {
            try
            {
                ret = m_session.QueryOver<T>().Where( expression )
                    .List<T>();

                tx.Commit();
            }
            finally
            {
                if (tx.IsActive)
                    tx.Rollback();
            }
        }
        return ret;            
    }

but it doesn’t compile.
The error message states: “T must be a reference type in order to use it as parameter ‘T'” on QueryOver

Isn’t it possible to make this call generic?

What’s wrong?

Thank you, Stefano

Answer 1

Try:

public IList<T> ListByQueryOver( Expression<Func<T, bool>> expression ) where T : class, new()

The where clause restricts T to reference types, and new requires a parameter-less constructor on the class. The QueryOver Where method requires an `Expression>’ parameter.