public override int GetMaxByteCount(int charCount)
        {
            if (charCount < 0)
               throw new ArgumentOutOfRangeException("charCount",
                    Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
            Contract.EndContractBlock();

            // Characters would be # of characters + 1 in case left over high surrogate is ? * max fallback
            long byteCount = (long)charCount + 1;

            if (EncoderFallback.MaxCharCount > 1)
                byteCount *= EncoderFallback.MaxCharCount;

            // Max 3 bytes per char.  (4 bytes per 2 chars for surrogates)
            byteCount *= 3;

            if (byteCount > 0x7fffffff)
                throw new ArgumentOutOfRangeException("charCount", Environment.GetResourceString("ArgumentOutOfRange_GetByteCountOverflow"));

            return (int)byteCount;
        }

        public override int GetMaxByteCount(int charCount)
        {
            if (charCount < 0)
               throw new ArgumentOutOfRangeException("charCount",
                    Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
            Contract.EndContractBlock();

            // Suppose that every char can not be direct-encoded, we know that
            // a byte can encode 6 bits of the Unicode character.  And we will
            // also need two extra bytes for the shift-in ('+') and shift-out ('-') mark.
            // Therefore, the max byte should be:
            // byteCount = 2 + Math.Ceiling((double)charCount * 16 / 6);
            // That is always <= 2 + 3 * charCount;
            // Longest case is alternating encoded, direct, encoded data for 5 + 1 + 5... bytes per char.
            // UTF7 doesn't have left over surrogates, but if no input we may need an output - to turn off
            // encoding if MustFlush is true.

            // Its easiest to think of this as 2 bytes to turn on/off the base64 mode, then 3 bytes per char.
            // 3 bytes is 18 bits of encoding, which is more than we need, but if its direct encoded then 3
            // bytes allows us to turn off and then back on base64 mode if necessary.

            // Note that UTF7 encoded surrogates individually and isn't worried about mismatches, so all
            // code points are encodable int UTF7.
            long byteCount = (long)charCount * 3 + 2;

            // check for overflow
            if (byteCount > 0x7fffffff)
                throw new ArgumentOutOfRangeException("charCount", Environment.GetResourceString("ArgumentOutOfRange_GetByteCountOverflow"));

            return (int)byteCount;
        }