SQL存储过程的SUM函数与Net的SqlDataReader可能产生DBNull的情况

ALTER PROCEDURE [dbo].[SizeCount]
@UId int
AS
BEGIN
	SELECT SUM(Photo_Size) as CountNum
	FROM [Photo]
	where UId=@UId END 

如果在sql存储过程中不判断UID的不存在性,而且下面也不判断DBNULL就可能会发生前台程序错误!

        /// <summary>
        /// 统计字节
        /// </summary>
        /// <returns>返回string数量</returns> 
        public static string Photo_SizeCount(int UId)
        {
            string Count_Num = "0";
            SqlParameter[] parameters = {
			 			 new SqlParameter("@UId", SqlDbType.Int,4)
			 			 };
            parameters[0].Value = UId;
            SqlDataReader sdr = SQLHelper.SqlDataReader_Procedure("SizeCount", parameters);
            if (sdr.Read())
            {
                if (sdr["CountNum"]!=DBNull.Value)
                {
                    Count_Num = Convert.ToString(sdr["CountNum"]);
                }
            }
            sdr.Close();
            return Count_Num;
        }