.NET 4.5 introduced a new interface which is great if you want to expose a collection as readonly.

The problem with IEnumerable<T> is that it’s forward only. i.e. there is no guarantee that you can traverse it multiple times. The collection size might also be unknown, depending on the implementation.

So if you want to expose a collection which can be traversed multiple times and where the size is know there is as of .NET 4.5 another alternative. And that’s IReadOnlyList<T>. It inherits another new interface calledIReadOnlyCollection<out T>.

Code

public interface IReadOnlyCollection<out T> 
      : IEnumerable<T>, 
        IEnumerable
{
    int Count { get; }
}
public interface IReadOnlyList<out T> 
      : IReadOnlyCollection<T>
      , IEnumerable<T>
      , IEnumerable
{
    T this[int index] { get; }
}

Usage example

public class MyRepository
{
    public IReadOnlyList<User> FindUsers(string lastName)
    {
        // [...]
    }

    // [...]
}