Best Type to set as return type for methods that return a collection?

Question

Which is the best type to us for returning collections?

Should I use IList<T>, IEnumerable<T>, IQueryable<T>, something else? Which is best and why?

I'm trying to decide which I should use typically, both in the interface and the implementation of a few classes that I'm writing.

edit Let me nail this down a little further, I am using LINQ to SQL to return data over a WCF Service. It feels like that may make a change in the best type to use?

Answer 1

The Framework Design Guidelines state:

Use Collection<T> or a subclass of Collection<T> for properties or return values representing read/write collections.

public Collection<Session> Sessions { get; }

Use ReadOnlyCollection<T>, a subclass of ReadOnlyCollection<T>, or in rare cases IEnumerable<T> for properties or return values representing read-only collections.

public ReadOnlyCollection<Session> Sessions { get; }

In general, prefer ReadOnlyCollection<T>.

Regarding LINQ, the guidelines, which were created for .NET 3.5, are clear but not (imo) entirely convincing in the justification:

The review body made an explicit decision that LINQ should not change this guideline ["Do not return IEnumerator<T>, except as the return type of a GetEnumerator method"]. Your callers can end up with a clumsy object model if they choose not to use LINQ or a language that does not support it.