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You use IEnumerable when you want to loop through the items in a collection. IList is when you want to add, remove, and access the list contents out of order.
IList doesn't support further filtering. IEnumerable exists in System. Collections Namespace. IEnumerable is a forward only collection, it can't move backward and between the items.
IList<T> is essentially an ICollection<T> with random order-based access. In this case you should decide whether or not your results require list semantics such as order based indexing (then use IList<T> ) or whether you just need to return an unordered "bag" of results (then use ICollection<T> ).
Results. IList<T> uses 40 Bytes more than List<T> .
IEnumerable<T> represents a series of items that you can iterate over (using foreach, for example), whereas IList<T> is a collection that you can add to or remove from.
Typically you'll want to be able to modify an Order by adding or removing OrderLines to it, so you probably want Order.Lines to be an IList<OrderLine>.
Having said that, there are some framework design decisions you should make. For example, should it be possible to add the same instance of OrderLine to two different orders? Probably not. So given that you'll want to be able to validate whether an OrderLine should be added to the order, you may indeed want to surface the Lines property as only an IEnumerable<OrderLine>, and provide Add(OrderLine) and Remove(OrderLine) methods which can handle that validation.
Most of the time I end up going with IList over IEnumerable because IEnumerable doesn't have the Count method and you can't access the collection through an index (although if you are using LINQ you can get around this with extension methods).
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