定义对象为什不可以写到while语句外面。VS2017

        /// <summary>
        /// 绑定产品信息到网络列表
        /// </summary>
        private void BindProduct()
        {
            Pros = new List<ProductClass>();
            SqlDataReader sdr = SqlHelper.Read("SELECT Id,JWD,ProName,ProNum,ProStyle FROM dbo.product");
            while (sdr.Read())
            {
                ProductClass pro = new ProductClass();//为什么不能写到while语句外面
                pro.Id = Convert.ToInt32(sdr["Id"]);
                pro.JWD = sdr["JWD"].ToString();
                pro.ProName = sdr["ProName"].ToString();
                pro.ProNum = sdr["ProNum"].ToString();
                pro.ProStyle = sdr["ProStyle"].ToString();
                Pros.Add(pro);
            }
            sdr.Close();
            dgvProduct.DataSource = Pros;
        }