Entity framework在验证未通过的时候默认不会抛出详细异常，这给我们debug带来很大的困难，不过我们可以手动捕获，见代码：

private int Update(Employee entity)
        {
            string error = string.Empty;
            using (NorthwindEntities ctx = new NorthwindEntities())
            {
                try
                {
                    //ctx.Employees.Attach(entity);
                    ctx.Entry(entity).State = System.Data.EntityState.Modified;
                    return ctx.SaveChanges();
                }
                catch (DbEntityValidationException ex)
                {
                    foreach (var item in ex.EntityValidationErrors)
                    {
                        foreach (var item2 in item.ValidationErrors)
                        {
                            error = string.Format("{0}:{1}\r\n", item2.PropertyName, item2.ErrorMessage);
                        }
                    }
                }

            }
            return 1;
        }