I see this a lot in tutorials, with navigation properties as
Is this a mandatory requirement for Entity Framework? Can I use
What's the main purpose of using
ICollection instead of
IEnumerable or even
Usually what you choose will depend on which methods you need access to. In general -
IEnumerable<> (MSDN: http://msdn.microsoft.com/en-us/library/system.collections.ienumerable.aspx) for a list of objects that only needs to be iterated through,
ICollection<> (MSDN: http://msdn.microsoft.com/en-us/library/92t2ye13.aspx) for a list of objects that needs to be iterated through and modified,
List<> for a list of objects that needs to be iterated through, modified, sorted, etc (See here for a full list: http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx).
From a more specific standpoint, lazy loading comes in to play with choosing the type. By default, navigation properties in Entity Framework come with change tracking and are proxies. In order for the dynamic proxy to be created as a navigation property, the virtual type must implement
A navigation property that represents the "many" end of a relationship must return a type that implements ICollection, where T is the type of the object at the other end of the relationship. -Requirements for Creating POCO ProxiesMSDN
ICollection<T> is used because the
IEnumerable<T> interface provides no way of adding items, removing items, or otherwise modifying the collection.