C# datatable 中 根据条件修改某个字段的值

DataSet ds = bll.GetList(where,index);

int i = 0;
foreach (DataRow item in ds.Tables[0].Rows)
{
if (ds.Tables[0].Rows[i]["CHECKIN_TIME"].ToString() !="")
{
string CHECKIN_TIME = Convert.ToDateTime(ds.Tables[0].Rows[i]["CHECKIN_TIME"].ToString()).ToString("yyyy-MM-dd");
item["CHECKIN_TIME"] = CHECKIN_TIME + " 16:30";
string CHECKOUT_TIME = Convert.ToDateTime(ds.Tables[0].Rows[i]["CHECKOUT_TIME"].ToString()).ToString("yyyy-MM-dd");
item["CHECKOUT_TIME"] = CHECKOUT_TIME + " 8:00";
i++;
}

}