如何保证DBContext实例上下文唯一

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Runtime.Remoting.Contexts;
using System.Runtime.Remoting.Messaging;
using System.Text;
using Dulei.Model;


namespace Dulei.DAL
{
     public class EFDbcontextFactory
    {
         public static DbContext GetcurrentDbcontext()
         {
             DbContext db = (DbContext) CallContext.GetData("Dbcontext");
             if (db==null)
             {
                 db = new DataModelContainer();
                 CallContext.SetData("Dbcontext",db);
             }
             //也可以使用HttpContext是一样的
             return db;
         }
    }
}