c# generic, covering both arrays and lists?

Question

Here's a very handy extension, which works for an array of anything:

public static T AnyOne<T>(this T[] ra) where T:class {     int k = ra.Length;     int r = Random.Range(0,k);     return ra[r]; } 

Unfortunately it does not work for a List<> of anything. Here's the same extension that works for any List<>

public static T AnyOne<T>(this List<T> listy) where T:class {     int k = listy.Count;     int r = Random.Range(0,k);     return listy[r]; } 

In fact, is there a way to generalise generics covering both arrays and List<>s in one go? Or is it know to be not possible?

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Could the answer even (gasp) encompass Collections?

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PS, I apologize for not explicitly mentioning this is in the Unity3D milieu. For example "Random.Range" is just a Unity call (which does the obvious), and "AnyOne" is a completely typical extension or call in game programming.

Obviously, the question of course applies in any c# milieu.

Answer 1

In fact the most appropriate common interface between T[] and List<T> for your case is IReadOnlyList<T>

public static T AnyOne<T>(this IReadOnlyList<T> list) where T:class {     int k = list.Count;     int r = Random.Range(0,k);     return list[r]; } 

As mentioned in another answer, IList<T> also works, but the good practice requires you to request from the caller the minimum functionality needed by the method, which in this case is Count property and read only indexer.

IEnumerable<T> also works, but it allows the caller to pass a non collection iterator where Count and ElementAt extension methods could be highly inefficient - like Enumerable.Range(0, 1000000), database query etc.

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2020, quick for Unity3D programmers: of course, nowadays modern versions of .Net are available in Unity!

Answer 2

T[] and List<T> actually both implement IList<T>, which provides enumeration, a Count property and an indexer.

public static T AnyOne<T>(this IList<T> ra)  {     int k = ra.Count;     int r = Random.Range(0,k);     return ra[r]; } 

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Historical note: in past decades, this was the correct and only solution for Unity3D specifically, as in the Olden Days modern .Net was not available in Unity.