Question

What is more efficient way to make methods return IList<T> or IEnumerable<T>?

IEnumerable<T> it is immutable collection but IList<T> mutable and contain a lot of useful methods and properties.

To cast IList<T> to IEnumerable<T> it is just reference copy:

IList<T> l = new List<T>();
IEnumerable<T> e = l;

To cast IEnumerable<T> to List<T> we need to iterate each element or to call ToList() method:

IEnumerable<T>.ToList(); 

or may pass IEnumerable<T> to List<T> constructor which doing the same iteration somewhere within its constructor.

List<T> l = new List<T>(e);

Which cases you think is more efficient? Which you prefer more in your practice?

Answer 1

As far as efficiency is concerned both are interfaces so the efficiency will depend on the actual concrete class you are returning. As a rule of thumb you should always return the type that’s highest in the object hierarchy that works for the consumers of this method. In your case IEnumerable<T>. If the consumers of the class need to add elements, access elements by index, remove elements you are better off using an IList<T>.

So as always in programming: it depends 🙂