System.Runtime.Serialization.Json.JavaScriptReader.ReadNumericLiteral C# (CSharp) Method

        private object ReadNumericLiteral()
        {
            var sb = new StringBuilder();

            if (PeekChar() == '-')
            {
                sb.Append((char)ReadChar());
            }

            int c;
            int x = 0;
            bool zeroStart = PeekChar() == '0';
            for (; ; x++)
            {
                c = PeekChar();
                if (c < '0' || '9' < c)
                {
                    break;
                }

                sb.Append((char)ReadChar());
                if (zeroStart && x == 1)
                {
                    throw JsonError(SR.ArgumentException_LeadingZeros);
                }
            }

            if (x == 0) // Reached e.g. for "- "
            {
                throw JsonError(SR.ArgumentException_NoDigitFound);
            }

            // fraction
            bool hasFrac = false;
            int fdigits = 0;
            if (PeekChar() == '.')
            {
                hasFrac = true;
                sb.Append((char)ReadChar());
                if (PeekChar() < 0)
                {
                    throw JsonError(SR.ArgumentException_ExtraDot);
                }

                while (true)
                {
                    c = PeekChar();
                    if (c < '0' || '9' < c)
                    {
                        break;
                    }

                    sb.Append((char)ReadChar());
                    fdigits++;
                }
                if (fdigits == 0)
                {
                    throw JsonError(SR.ArgumentException_ExtraDot);
                }
            }

            c = PeekChar();
            if (c != 'e' && c != 'E')
            {
                if (!hasFrac)
                {
                    int valueInt;
                    if (int.TryParse(sb.ToString(), NumberStyles.Float, CultureInfo.InvariantCulture, out valueInt))
                    {
                        return valueInt;
                    }

                    long valueLong;
                    if (long.TryParse(sb.ToString(), NumberStyles.Float, CultureInfo.InvariantCulture, out valueLong))
                    {
                        return valueLong;
                    }

                    ulong valueUlong;
                    if (ulong.TryParse(sb.ToString(), NumberStyles.Float, CultureInfo.InvariantCulture, out valueUlong))
                    {
                        return valueUlong;
                    }
                }

                decimal valueDecimal;
                if (decimal.TryParse(sb.ToString(), NumberStyles.Float, CultureInfo.InvariantCulture, out valueDecimal) && valueDecimal != 0)
                {
                    return valueDecimal;
                }
            }
            else
            {
                // exponent
                sb.Append((char)ReadChar());
                if (PeekChar() < 0)
                {
                    throw JsonError(SR.ArgumentException_IncompleteExponent);
                }

                c = PeekChar();
                if (c == '-')
                {
                    sb.Append((char)ReadChar());
                }
                else if (c == '+')
                {
                    sb.Append((char)ReadChar());
                }

                if (PeekChar() < 0)
                {
                    throw JsonError(SR.ArgumentException_IncompleteExponent);
                }

                while (true)
                {
                    c = PeekChar();
                    if (c < '0' || '9' < c)
                    {
                        break;
                    }

                    sb.Append((char)ReadChar());
                }
            }

            return double.Parse(sb.ToString(), NumberStyles.Float, CultureInfo.InvariantCulture);
        }