EntityFramework 如何更新 detached 实体

Question

如题，EntityFramework 如何更新 detached 实体，如下代码不能更新：Product p = null;using (EFDbContext context = new EFDbContext()) {    p = context.Products.First();}using (EFDbContext context = new EFDbContext()) {    p.Remark = "aaa";    context.SaveChanges(); //不能更新}