using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel.Composition;

namespace Demo
{
    [Export(typeof(IUserProvider))]
    public sealed class UserProvider : IUserProvider
    {
        public ReadOnlyCollection<User> GetAllUsers()
        {
            return new List<User>
            {
                new User(0, "admin"),
                new User(1, "Dennis"),
                new User(2, "Samantha"),
            }.AsReadOnly();
        }
    }
}

This could be defined virtually anywhere; all that matters is that the application knows where to look for it (via the ComposablePartCatalogs it creates).