Expose IList or ReadOnlyCollection for Read-Only Property?

Question

I realize that since .NET 4.5 that the ReadOnlyCollection<T> class implements the new interface IReadOnlyCollection<T> but I am constrained to an older version of the library which doesn't have this.

I would like to know which of the better is considered to be best practice:

public sealed class OptionA {

    /// <summary>
    /// Gets a read-only list of numbers.
    /// </summary>
    public ReadOnlyCollection<int> SomeNumbers {
        get { return new ReadOnlyCollection<int>(_someNumbers); }
    }

    ...

}

public sealed class OptionB {

    /// <summary>
    /// Gets a read-only list of numbers.
    /// </summary>
    public IList<int> SomeNumbers {
        get { return new ReadOnlyCollection<int>(_someNumbers); }
    }

    ...

}

Best practice under what circumstances? It depends on which one you want. — Ant P, Oct 28 '14 at 18:44
There isn't much point in suggesting to the client programmer that the Add() method is going to work. — Hans Passant, Oct 28 '14 at 18:46
@BartoszKP Since `IList` provides random access and `IEnumerable` does not. — Lea Hayes, Oct 28 '14 at 20:12
@HansPassant I agree, that is certainly a nice trait that the mutable methods are hidden. — Lea Hayes, Oct 28 '14 at 20:15
@AntP Where possible I like to expose interfaces rather than a concrete implementation. In the case of this wrapper class I was unsure as to how people typically expose the read-only collections with it in their libraries. — Lea Hayes, Oct 28 '14 at 20:16
@BartoszKP Thanks! I wasn't aware of that implementation detail in Linq. That is great to know. Post this as the answer and I will accept :) — Lea Hayes, Oct 28 '14 at 20:28

score 3 · Accepted Answer · answered Oct 28 '14 at 20:35

You can also use IEnumerable<T> - it will make your code as loosely coupled as possible in this case. It also provides random access with the ElementAt<T> method, which will work with O(1) complexity if the actual type implements IList<T>:

If the type of source implements IList<T>, that implementation is used to obtain the element at the specified index. Otherwise, this method obtains the specified element.

Exposing IList<T> or ReadOnlyCollection<T> could be useful if you really wanted to stress the fact that random access with constant-time complexity is available. Deciding which one really depends on a particular scenario.

Expose IList or ReadOnlyCollection for Read-Only Property?

1 Answers1