Question

Usually my methods are as the following:

public List<int> Method1(int input) {     var output = new List<int>();     //add some items to output     return output; } 

But FxCop advises another IList implementation instead of List, but I can’t remember which. The alternatives include returning it as an IList, ICollection or IEnumerable for more flexibility or a whole different way as the code below.:

public int[] Method2(int input) {     var output = new List<int>();     //add some items to output     return output.ToArray(); } 

Of all alternatives, all provided and all possibilities, which is considered the best practice?

Answer 1

‘Framework Design Guidelines’ (2nd ed) in §8.3.1 has quite a lot to say about collections as return values, summary:

• DO NOT provide settable collection properties.
• DO use Collection<T> or a subclass of Collection<T> for properties or return values representing read/write collections.
• DO use ReadOnlyCollection<T>, a subclass of ReadOnlyCollection<T>, or in rare cases IEnumerable<T> for properties or return values representing read-only collections.

(and more, but these three capture the core).

The first of those above: don’t return reference to internal collection unless you want the user to be able to change it (and then likely you should have a custom type so you have a degree of control).

I would return IList<T> and ensure I do not define the actual type I am returning, unless I was returning an iterator (when I would use IEnumerable<T>).