问找不到ObjectSet库？

EN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using ModelLayer.PocoModels;
using System.Data.Objects;

namespace ModelLayer
{
    public class NorthwindDataContext : ObjectContext
    {
        private ObjectSet<Category> _categories;
        private ObjectSet<Product> _products;
        public NorthwindDataContext()
            : base("name=NorthwindEntities",
                "NorthwindEntities")
        {
            _categories = CreateObjectSet<Category>();
            _products = CreateObjectSet<Product>();
        }
    }
}