System.IO.StreamReader.ReadLineAsyncInternal C# (CSharp) Method

        private async Task<string> ReadLineAsyncInternal()
        {
            if (CharPos_Prop == CharLen_Prop && (await ReadBufferAsync().ConfigureAwait(false)) == 0)
            {
                return null;
            }

            StringBuilder sb = null;

            do
            {
                char[] tmpCharBuffer = CharBuffer_Prop;
                int tmpCharLen = CharLen_Prop;
                int tmpCharPos = CharPos_Prop;
                int i = tmpCharPos;

                do
                {
                    char ch = tmpCharBuffer[i];

                    // Note the following common line feed chars:
                    // \n - UNIX   \r\n - DOS   \r - Mac
                    if (ch == '\r' || ch == '\n')
                    {
                        string s;

                        if (sb != null)
                        {
                            sb.Append(tmpCharBuffer, tmpCharPos, i - tmpCharPos);
                            s = sb.ToString();
                        }
                        else
                        {
                            s = new string(tmpCharBuffer, tmpCharPos, i - tmpCharPos);
                        }

                        CharPos_Prop = tmpCharPos = i + 1;

                        if (ch == '\r' && (tmpCharPos < tmpCharLen || (await ReadBufferAsync().ConfigureAwait(false)) > 0))
                        {
                            tmpCharPos = CharPos_Prop;
                            if (CharBuffer_Prop[tmpCharPos] == '\n')
                            {
                                CharPos_Prop = ++tmpCharPos;
                            }
                        }

                        return s;
                    }

                    i++;
                } while (i < tmpCharLen);

                i = tmpCharLen - tmpCharPos;
                if (sb == null)
                {
                    sb = new StringBuilder(i + 80);
                }
                sb.Append(tmpCharBuffer, tmpCharPos, i);
            } while (await ReadBufferAsync().ConfigureAwait(false) > 0);

            return sb.ToString();
        }