static String FormatBigInteger(BigInteger value, String format, NumberFormatInfo info) {
        int digits = 0;
        char fmt = ParseFormatSpecifier(format, out digits);
        if (fmt == 'x' || fmt == 'X')
            return FormatBigIntegerToHexString(value, fmt, digits, info);

        bool decimalFmt = (fmt == 'g' || fmt == 'G' || fmt == 'd' || fmt == 'D' || fmt == 'r' || fmt == 'R');

        if (!decimalFmt) {
            // Silverlight supports invariant formats only
            throw new FormatException(SR.GetString(SR.Format_InvalidFormatSpecifier));
        }

        if (value._bits == null) {
            if (fmt == 'g' || fmt == 'G' || fmt == 'r' || fmt == 'R') {
                if (digits > 0)
                    format = String.Format(CultureInfo.InvariantCulture, "D{0}", digits.ToString(CultureInfo.InvariantCulture));
                else
                    format = "D";
            }
            return value._sign.ToString(format, info);
        }