Returning ArrayList from a WebService

These days I am working on a Project in which I need to implement a Service Oriented Architecture. The services are exposed as Web Services. I was planning to return the List<User> to the client but found out that List cannot be returned since its not serializable. I plan to use ArrayList and that worked. If you would like to return an ArrayList from the WebService you can do so check out the simple code below:

 [WebMethod]
    [System.Xml.Serialization.XmlInclude(typeof(User))]
    public  ArrayList GetAllUsers()
    {
        User user = new User();
        return user.GetAllUsers();
    }

Above is your WebMethod and below is the defination of the class User:

 [Serializable]
    public class User
    {           
        
        /* PRIVATE FIELDS */
        private int _userID;
        private string _firstName; 
        private string _lastName;

        /* PUBLIC PROPERTIES */
        public int UserID
        {
            get { return _userID; }
            set { _userID = value; } 
        }

Pretty Simple Right!