public static bool CheckNumber( string number)
        {
            const string charcode = "0123456789A?BCDEFGHIJK?LMNOPQRSTU?VWXYZ";

            int sum = 0;

            for( int i = 0; i < 10; i++ )
            {
                string c = number.Substring( i, 1 ).ToUpper();
                int idx = charcode.IndexOf( c );

                if( idx == -1 || c == "?" ) return false;

                idx = idx * 2 ^ i;

                sum = sum + idx;
            }

            sum = ( sum % 11 ) % 10;

            if( Convert.ToInt32( number.Substring( 10, 1 ) ) != sum ) return false;

            return true;
        }

该方法红色部分，哪位大侠给解析一下