System.__DTString.GetSubString C# (CSharp) Method

        internal DTSubString GetSubString() {
            DTSubString sub = new DTSubString();
            sub.index = Index;
            sub.s = Value;
            while (Index + sub.length < len) {
                DTSubStringType currentType;
                Char ch = Value[Index + sub.length];
                if (ch >= '0' && ch <= '9') {
                    currentType = DTSubStringType.Number;
                }
                else {
                    currentType = DTSubStringType.Other;
                }

                if (sub.length == 0) {
                    sub.type = currentType;
                }
                else {
                    if (sub.type != currentType) {
                        break;
                    }
                }
                sub.length++;
                if (currentType == DTSubStringType.Number) {
                    // Incorporate the number into the value
                    // Limit the digits to prevent overflow
                    if (sub.length > DateTimeParse.MaxDateTimeNumberDigits) {
                        sub.type = DTSubStringType.Invalid;
                        return sub;
                    }
                    int number = ch - '0';
                    BCLDebug.Assert(number >= 0 && number <= 9, "number >= 0 && number <= 9");
                    sub.value = sub.value * 10 + number;
                }
                else {
                    // For non numbers, just return this length 1 token. This should be expanded
                    // to more types of thing if this parsing approach is used for things other
                    // than numbers and single characters
                    break;
                }
            }
            if (sub.length == 0) {
                sub.type = DTSubStringType.End;
                return sub;
            }

            return sub;
        }

        /*=================================ParseTimeZone==========================
        **Action: Parse the timezone offset in the following format:
        **          "+8", "+08", "+0800", "+0800"
        **        This method is used by DateTime.Parse().
        **Returns:      The TimeZone offset.
        **Arguments:
        **      str the parsing string
        **Exceptions:
        **      FormatException if invalid timezone format is found.
        ============================================================================*/

        private static bool ParseTimeZone(ref __DTString str, ref TimeSpan result) {
            // The hour/minute offset for timezone.
            int hourOffset = 0;
            int minuteOffset = 0;
            DTSubString sub;

            // Consume the +/- character that has already been read
            sub = str.GetSubString();
            if (sub.length != 1) {
                return false;
            }
            char offsetChar = sub[0];
            if (offsetChar != '+' && offsetChar != '-') {
                return false;
            }
            str.ConsumeSubString(sub);

            sub = str.GetSubString();
            if (sub.type != DTSubStringType.Number) {
                return false;
            }
            int value = sub.value;
            int length = sub.length;
            if (length == 1 || length == 2) {
                // Parsing "+8" or "+08"
                hourOffset = value;
                str.ConsumeSubString(sub);
                // See if we have minutes
                sub = str.GetSubString();
                if (sub.length == 1 && sub[0] == ':') {
                    // Parsing "+8:00" or "+08:00"
                    str.ConsumeSubString(sub);
                    sub = str.GetSubString();
                    if (sub.type != DTSubStringType.Number || sub.length < 1 || sub.length > 2) {
                        return false;
                    }
                    minuteOffset = sub.value;
                    str.ConsumeSubString(sub);
                }
            }
            else if (length == 3 || length == 4) {
                // Parsing "+800" or "+0800"
                hourOffset = value / 100;
                minuteOffset = value % 100;
                str.ConsumeSubString(sub);
            }
            else {
                // Wrong number of digits
                return false;
            }
            Contract.Assert(hourOffset >= 0 && hourOffset <= 99, "hourOffset >= 0 && hourOffset <= 99");
            Contract.Assert(minuteOffset >= 0 && minuteOffset <= 99, "minuteOffset >= 0 && minuteOffset <= 99");
            if (minuteOffset < 0 || minuteOffset >= 60) {
                return false;
            }

            result = new TimeSpan(hourOffset, minuteOffset, 0);
            if (offsetChar == '-') {
                result = result.Negate();
            }
            return true;
        }