What is the best practice for readonly lists in NHibernate

Domain Model I am working on has root aggregate and child entities. Something like the following code:

class Order
{
   IList<OrderLine> Lines {get;set;}
}

class OrderLine
{
}

Now I want my Order to control lines. Something like that:

class Order
{
   OrderLine[] Lines {get;}

   void AddLine(OrderLine line);
}

At this time we are using the following pattern:

class Order
{
   private IList<OrderLine> lines = new List<OrderLine>();
   OrderLine[] Lines {get {return this.lines;}}

   void AddLine(OrderLine line)
   {
      this.orders.Add(line);
   {
}

NHibernate is mapped directly to the lines field.

Now questions...

• What do you practice in such situations?
• Does anyone use methods: public IEnumerable GetLines()
• What are you using as return type for property? May be ReadOnlyCollection or IEnumerable;
• May be this is not the best place to ask? Suggest please.

Update: Seems IEnumerable wins, however solution still is not perfect...