Newtonsoft.Json.Bson.BsonReader.GetString C# (CSharp) Method

        private string GetString(int length)
        {
            if (length == 0)
            {
                return string.Empty;
            }

            EnsureBuffers();

            StringBuilder builder = null;

            int totalBytesRead = 0;

            // used in case of left over multibyte characters in the buffer
            int offset = 0;
            do
            {
                int count = ((length - totalBytesRead) > MaxCharBytesSize - offset)
                    ? MaxCharBytesSize - offset
                    : length - totalBytesRead;

                int byteCount = _reader.Read(_byteBuffer, offset, count);

                if (byteCount == 0)
                {
                    throw new EndOfStreamException("Unable to read beyond the end of the stream.");
                }

                totalBytesRead += byteCount;

                // Above, byteCount is how many bytes we read this time.
                // Below, byteCount is how many bytes are in the _byteBuffer.
                byteCount += offset;

                if (byteCount == length)
                {
                    // pref optimization to avoid reading into a string builder
                    // first iteration and all bytes read then return string directly
                    int charCount = Encoding.UTF8.GetChars(_byteBuffer, 0, byteCount, _charBuffer, 0);
                    return new string(_charBuffer, 0, charCount);
                }
                else
                {
                    int lastFullCharStop = GetLastFullCharStop(byteCount - 1);

                    if (builder == null)
                    {
                        builder = new StringBuilder(length);
                    }

                    int charCount = Encoding.UTF8.GetChars(_byteBuffer, 0, lastFullCharStop + 1, _charBuffer, 0);
                    builder.Append(_charBuffer, 0, charCount);

                    if (lastFullCharStop < byteCount - 1)
                    {
                        offset = byteCount - lastFullCharStop - 1;
                        // copy left over multi byte characters to beginning of buffer for next iteration
                        Array.Copy(_byteBuffer, lastFullCharStop + 1, _byteBuffer, 0, offset);
                    }
                    else
                    {
                        offset = 0;
                    }
                }
            } while (totalBytesRead < length);

            return builder.ToString();
        }