IChatService.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.ServiceModel;
using System.Text;

namespace ChatClient
{
    // our hosted interface
    [ServiceContract(CallbackContract = typeof(IChatService))]
    public interface IChatService
    {
        [OperationContract(IsOneWay = true)]
        void Join(string memberName);

        [OperationContract(IsOneWay = true)]
        void Leave(string memberName);

        [OperationContract(IsOneWay = true)]
        void SendMessage(string memberName, string message);
    }
}