//要比较的键值在 select new {....} 中
var products = (from c in context.Products select new { c.ID,c.Name }).ToList();// 一次性加载

//这里换成你的EXCEL数据集合
var stuffs = new List<Stuff> { new Stuff { name = "CCC1" }, new Stuff { name = "BBB1" } };

            foreach (var s in stuffs) {

                var o=products.Where(n => n.Name == s.name);//不会再查数据库

                if (o.Count(n => n.Name == s.name)==0)
                {
                    //insert
                    context.Products.Add(new NS.Models.Product { ID=Guid.NewGuid(), Name=s.name});
                }
                else {
                    //update
                    NS.Models.Product update=new NS.Models.Product();
                    update.ID = o.FirstOrDefault().ID;//必须设置
                    update.Name=s.name;
                    update.Count = 3;
                    var e=context.Entry(update);
                    if (e.State == System.Data.EntityState.Detached) {
                        e.State = System.Data.EntityState.Modified;
                    }
                }
            }
            context.SaveChanges();   //最后一次性更新