Why use ICollection and not IEnumerable or List<T> on many-many/one-many relationships?

Question

I see this a lot in tutorials, with navigation properties as ICollection<T>.

Is this a mandatory requirement for Entity Framework? Can I use IEnumerable?

What's the main purpose of using ICollection instead of IEnumerable or even List<T>?

Answer 1

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 ICollection.

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 Proxies^MSDN

More information on Defining and Managing Relationships^MSDN

Answer 2

ICollection<T> is used because the IEnumerable<T> interface provides no way of adding items, removing items, or otherwise modifying the collection.