/// <summary>
/// Add a new user. If the user has already
/// been added, an exception is thrown.
/// </summary>
/// <param name="userName">
/// User name for new user.
/// </param>
/// <param name="password">
/// Password for new user.
/// </param>
/// <param name="userType">
/// User type for new user.
/// </param>
public void AddUser(string userName, string password, string userType)
{
if (Users.ContainsKey(userName))
{
throw new ArgumentException(nameof(AddUser));
}
Users.Add(userName, new User(userName, password, userType));
_userTypes.AddUserType(userType);
}