Why IQueryOver has 2 type arguments?

Question

Why IQueryOver has 2 type arguments:

QueryOver<TRoot,TSubType> is an API for retrieving entities by composing
NHibernate.Criterion.Expression objects expressed using Lambda expression
syntax.

Answer 1

The answer is: to support fluent syntax, and have access to root settings all the time.

not so fluent

So we can use this definition:

IQueryOver<Contact,Contact> rootQuery = session  // here we have Contact query
    .QueryOver<Contact>();                          
IQueryOver<Contact, Employee> emplQueryOver = rootQuery
    .JoinQueryOver<Employee>(c => c.Parnter);       // here we work with its Parnter
IQueryOver<Contact, Employee> creatorQueryOver = emplQueryOver
    .JoinQueryOver<Employee>(e => e.Creator);       // here we have creator of Parnter

So having this, we can do some filtering:

emplQueryOver.Where(...
creatorQueryOver.Where(...

and then go back to root query and set

rootQuery.OrderBy(c => c.ID).Desc
    .ThenBy(c => c.LastName)
      .Asc
    .SelectList(...
    .Take(50);

fluent

But because of the interface IQueryOver<TRoot, TSubType> beeing returned all the way down, we can do that without the middle variables... we can do it in one fluent run

var list = session  // here we have Contact query
    .QueryOver<Contact>()
       .Where(... //filter contact
    // Join Parnter
    .JoinQueryOver<Employee>(c => c.Parnter)
        .Where(... //filter Partner
    // Join More
    .JoinQueryOver<Employee>(e => e.Creator)

    // While still having the Root in the scope we can do
    .SelectList(list => list....
    .OrderBy(c => c.ID)
      .Desc
    .ThenBy(c => c.Code)
      .Asc
    .Take(50)
    .Skip(50)
    .List<Contact>(); // the Root