Returning 'IList' vs 'ICollection' vs 'Collection'
Generally you should return a type that is as general as possible, i.e. one that knows just enough of the returned data that the consumer needs to use. That way you have greater freedom to change the implementation of the API, without breaking the code that is using it.
Consider also the IEnumerable<T> interface as return type. If the result is only going to be iterated, the consumer doesn't need more than that.
ICollection<T> is an interface that exposes collection semantics such as Add(), Remove(), and Count.
Collection<T> is a concrete implementation of the ICollection<T> interface.
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>).
The main difference between the IList<T> and ICollection<T> interfaces is that IList<T> allows you to access elements via an index. IList<T> describes array-like types. Elements in an ICollection<T> can only be accessed through enumeration. Both allow the insertion and deletion of elements.
If you only need to enumerate a collection, then IEnumerable<T> is to be preferred. It has two advantages over the others:
It disallows changes to the collection (but not to the elements, if they are of reference type).
It allows the largest possible variety of sources, including enumerations that are generated algorithmically and are not collections at all.
Allows lazy evaluation and can be queried with LINQ.
Collection<T> is a base class that is mainly useful to implementers of collections. If you expose it in interfaces (APIs), many useful collections not deriving from it will be excluded.
One disadvantage of IList<T> is that arrays implement it but do not allow you to add or remove items (i.e. you cannot change the array length). An exception will be thrown if you call IList<T>.Add(item) on an array. The situation is somewhat defused as IList<T> has a Boolean property IsReadOnly that you can check before attempting to do so. But in my eyes, this is still a design flaw in the library. Therefore, I use List<T> directly, when the possibility to add or remove items is required.
Which one should I choose? Let's consider just List<T> and IEnumerable<T> as examples for specialized / generalized types:
- Method input parameter
IEnumerable<T>greatest flexibility for the caller. Restrictive for the implementer, read-only.List<T>Restrictive for the caller. Gives flexibility to the implementer, can manipulate the collection.
- Method ouput parameter or return value
IEnumerable<T>Restrictive for the caller, read-only. Greatest flexibility for the implementer. Allows to return about any collection or to implement an iterator (yield return).List<T>Greatest flexibility for the caller, can manipulate the returned collection. Restrictive for the implementer.
Well, at this point you may be disappointed because I don't give you a simple answer. A statement like "always use this for input and that for output" would not be constructive. The reality is that it depends on use case. A method like void AddMissingEntries(TColl collection) will have to provide a collection type having an Add method or may even require a HashSet<T> for efficiency. A method void PrintItems(TColl collection) can happily live with an IEnumerable<T>.