[DataContract]
    public class MessageEntity
    {
        [DataMember]
        public string Content { get; set; }
    }

Code
 protected void btnSend_Click(object sender, EventArgs e)
        {
            MessageEntity message = new MessageEntity();
            message.Content = txtMessageContent.Text;

List<MessageEntity> messageList = HttpRuntime.Cache["MessageEntityList"] as List<MessageEntity>;
            if (messageList == null)
            {
                messageList = new List<MessageEntity>();
                messageList.Add(message);
                HttpRuntime.Cache.Add("MessageEntityList", messageList, null, System.Web.Caching.Cache.NoAbsoluteExpiration, System.Web.Caching.Cache.NoSlidingExpiration, System.Web.Caching.CacheItemPriority.Default, null);
            }
            else
            {
                messageList.Add(message);
                HttpRuntime.Cache["MessageEntityList"] = messageList;
            }

lbCacheCount.Text = messageList.Count.ToString();
        }

Code
[ServiceContract(CallbackContract = typeof(ISendMessageServiceCallBack))]
    public interface ISendMessageService
    {
        [OperationContract(IsOneWay = true)]
        void GetMessage();
    }

public interface ISendMessageServiceCallBack
    {
        [OperationContract(IsOneWay = true)]
        void ReceiveMessage(MessageEntity messageEntity);
    }

Code
 [ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
    public class SendMessageService : ISendMessageService
    {
        ISendMessageServiceCallBack callback;
        Timer heartTimer;
        Random random = new Random();

#region ISendMessageService 成员

public void GetMessage()
        {
            callback = OperationContext.Current.GetCallbackChannel<ISendMessageServiceCallBack>();
            heartTimer = new Timer(new TimerCallback(heartTimer_Elapsed), null, 3000, Timeout.Infinite);
        }

#endregion

private void heartTimer_Elapsed(object data)
        {
            List<MessageEntity> messageList = HttpRuntime.Cache["MessageEntityList"] as List<MessageEntity>;
            if (messageList != null && messageList.Count > 0)
            {
                MessageEntity message = messageList[0];
                messageList.Remove(message);
                HttpRuntime.Cache["MessageEntityList"] = messageList;
                callback.ReceiveMessage(message);
            }
            int interval = random.Next(2000, 5000);
            heartTimer.Change(interval, Timeout.Infinite);
        }
    }

Code
<service behaviorConfiguration="WebApp.SendMessageServiceBehavior" name="WebApp.SendMessageService">
                <endpoint address="" binding="wsDualHttpBinding" contract="WebApp.ISendMessageService">
                    <identity>
                        <dns value="localhost"/>
                    </identity>
                </endpoint>
                <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
            </service>

Code
public partial class GetMessageForm : Form,ISendMessageServiceCallback
    {
        SendMessageServiceClient client;

public GetMessageForm()
        {
            InitializeComponent();
            client = new SendMessageServiceClient(new System.ServiceModel.InstanceContext(this));
        }

private void btnStartGet_Click(object sender, EventArgs e)
        {
            client.GetMessage();
        }

private delegate void UpdateListBoxDelegate(string Message);
        private void UpdateListBox(string message)
        {
            this.listbMessage.Items.Add(message);
            listbMessage.SelectedIndex = listbMessage.Items.Count - 1;
        }
        #region ISendMessageServiceCallback 成员

public void ReceiveMessage(MessageEntity messageEntity)
        {
            if (true == listbMessage.InvokeRequired)
            {
                listbMessage.Invoke(new UpdateListBoxDelegate(UpdateListBox), messageEntity.Content);
            }
            else
            {
                UpdateListBox(messageEntity.Content);
            }
        }

#endregion

}