Ayende csv user data

userdata.cs

public class UserData
{
    public int Id { get; set; }
    public string Email { get; set; }
}

public class UserDataSearcher
{
    private readonly Dictionary<string, List<int>> userIdsByEmail;

    public UserDataSearcher(IEnumerable<UserData> usersData)
    {
        userIdsByEmail = new Dictionary<string, List<int>>(StringComparer.InvariantCultureIgnoreCase);
        foreach (var userData in usersData)
        {
            List<int> userIdList;
            if (! userIdsByEmail.TryGetValue(userData.Email, out userIdList))
            {
                userIdList = new List<int>();
                userIdsByEmail.Add(userData.Email, userIdList);
            }
            userIdList.Add(userData.Id);
        }
    }

    public IEnumerable<int> GetUserIdsByEmail(string email)
    {
        List<int> userIdList;
        if (userIdsByEmail.TryGetValue(email, out userIdList))
        {
            return userIdList;
        }
        return Enumerable.Empty<int>();
    }
}