System.Text.GB18030Encoding.GetMaxByteCount C# (CSharp) Method

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

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

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

            // We could have 4 bytes for each char, no extra for surrogates because 18030 can do whole unicode range.
            byteCount *= 4;

            if (byteCount > 0x7fffffff)
                throw new ArgumentOutOfRangeException(nameof(charCount), SR.ArgumentOutOfRange_GetByteCountOverflow);

            return (int)byteCount;
        }