Chat Application using Web services in C#

using System;
using System.Web;
using System.Data;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Xml;

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class ChatService : System.Web.Services.WebService
{
    static protected ArrayList arrUsers = new ArrayList();
    static protected ArrayList arrMessage = new ArrayList();    
    
    public ChatService () {

        //Uncomment the following line if using designed components 
        //InitializeComponent();        
    }
    [WebMethod]
    public string GetUsers() 
    {
        string strUser = string.Empty;        
        for (int i = 0; i < arrUsers.Count; i++)
        {
            strUser = strUser + arrUsers[i].ToString() + "|";
        }
        return strUser;
    }

    [WebMethod]
    public void AddUser(string strUser)
    {        
        arrUsers.Add(strUser);                 
    }

    [WebMethod]
    public void RemoveUser(string strUser)
    {
        for (int i = 0; i < arrUsers.Count; i++)
        {
            if(arrUsers[i].ToString() == strUser)
                arrUsers.RemoveAt(i);
        }
    }

    [WebMethod]
    public void SendMessage(string strFromUser, string strToUser, string strMess)
    {
        arrMessage.Add(strToUser + ":" + strFromUser + ":" + strMess);        
    }

    [WebMethod]
    public string ReceiveMessage(string strUser)
    {
        string strMess = string.Empty;
        for (int i = 0; i < arrMessage.Count; i++)
        {
            string[] strTo = arrMessage[i].ToString().Split(':');
            if (strTo[0].ToString() == strUser)
            {
                for (int j = 1; j < strTo.Length; j++)
                {
                    strMess = strMess + strTo[j] + ":";
                }
                arrMessage.RemoveAt(i);
                break;
            }            
        }
        return strMess;
    }
    
}