class CmdIterator : IEnumerable<byte[]>,IDisposable
    {
        IDbCommand cmd;
        IDataReader rdr;
        Encoding enc;
        
        public CmdIterator(IDbCommand cmd,Encoding enc)
        {
            this.cmd = cmd;
            this.enc = enc;
        }
        public IEnumerator<byte[]> GetEnumerator()
        {
            if (rdr != null) rdr.Dispose();
            rdr = cmd.ExecuteReader();
            yield return enc.GetPreamble();
            while (rdr.Read())
            {
                yield return format(rdr);
            }
            rdr.Dispose();
            rdr = null;
        }
        public byte[] format(IDataRecord r)
        {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < r.FieldCount; ++i)
            {
                // форматировать по желанию до потери пульса
                sb.Append(r.GetValue(i).ToString());
                if (i== r.FieldCount-1) sb.Append("\r\n"); else sb.Append(',');
            }
            return enc.GetBytes(sb.ToString());
        }
        IEnumerator IEnumerable.GetEnumerator()
        {
            return this.GetEnumerator();
        }

        public void Dispose()
        {
            if (rdr != null)
            {
                rdr.Dispose();
                rdr = null;
            }
        }
    }