What are some good ways to implement a data access layer in SoapBox Core and SoapBox Core Add-ins?

SoapBox Core is, by definition, an MVVM framework. In fact, it doesn't ship with any implementation of a Model, so it's really a View-ViewModel framework. (In the case of SoapBox Snap, the Data Access Layer is provided by SoapBox Protocol). Your persistence layer is usually either encapsulated within your Model (if you use the ActiveRecord pattern) or is a service only known to your Model (if you use the Repository or Unit of Work pattern).

Given that SoapBox Core is Model-Agnostic, I'm tempted to say that a DAL doesn't belong there. That doesn't mean there couldn't be a separate SoapBox.Model framework, but I really think you should be able to completely mix and match SoapBox Core with any Model.

To answer your question in the title, from experience, I suggest combining the following:

• POCO Model objects
• NHibernate
• Unit of Work pattern (wrapper around NHibernate Session)
• Generic Repository pattern
• LINQ for Specification pattern

Your repository looks like this:

public interface IRepository<T>
{
    void Save(T entity);
    void Delete(T entity);
    T Get(object key);
    IQueryable<T> AllEntities();
}

Your model layer logic can look like this:

using(var uow = UnitOfWork.Begin())
{
    var widgetRepository = uow.GetRepository<Widget>();
    var activeWidgets = (from w in widgetRepository.AllEntities().ThatAreActive
                         select w)
    ...
}