read csv with task.cs

        public static IEnumerable<Memory<char>> TryReadLine(TextReader reader)
        {
            var bufferLength = (int)Math.Pow(2, 16);
            var buffer = ArrayPool<char>.Shared.Rent(bufferLength);

            var i = 0;
            var j = 0;
            var state = State.BeforeField;
            int c;

            FillBufferAsync(true);

            int FillBufferAsync(bool initial)
            {
                var len = i - j;
                if (initial == false)
                {
                    Array.Copy(buffer, j, buffer, 0, len);
                }

                var totalRead = reader.ReadAsync(buffer, len, bufferLength - len).GetAwaiter().GetResult();
                bufferLength = len + totalRead;

                i = len;
                j = 0;

                return totalRead;
            }

            var tried = false;

            int Peek()
            {
                if (i < bufferLength)
                    return buffer[i];

                tried = true;
                return -1;
            }

        preloop:

            j = i;
            state = State.BeforeField;

        reloop:

            void Loop()
            {
                while (Peek() >= 0)
                {
                    c = buffer[i++];

                    switch (state)
                    {
                        case State.BeforeField:

                            switch (c)
                            {
                                case '"':
                                    state = State.InQuotedField;
                                    break;
                                case ',':
                                    //  fields.Add(string.Empty);
                                    break;
                                case '\r':
                                    // fields.Add(string.Empty);
                                    if (Peek() == '\n')
                                    {
                                        i++;
                                    }
                                    state = State.LineEnd;
                                    return;

                                case '\n':
                                    // fields.Add(string.Empty);
                                    state = State.LineEnd;
                                    return;

                                default:
                                    // builder.Append((char)c);
                                    state = State.InField;
                                    break;
                            }
                            break;

                        case State.InField:
                            switch (c)
                            {
                                case ',':
                                    //  AddField(fields, builder);
                                    state = State.BeforeField;
                                    break;
                                case '\r':
                                    //  AddField(fields, builder);
                                    if (Peek() == '\n')
                                    {
                                        i++;
                                    }
                                    state = State.LineEnd;
                                    return;

                                case '\n':
                                    //    AddField(fields, builder);
                                    state = State.LineEnd;
                                    return;

                                default:
                                    //      builder.Append((char)c);
                                    break;
                            }
                            break;

                        case State.InQuotedField:
                            switch (c)
                            {
                                case '"':
                                    var nc = Peek();
                                    switch (nc)
                                    {
                                        case '"':
                                            //        builder.Append('"');
                                            i++;

                                            break;
                                        case ',':
                                            i++;

                                            //           AddField(fields, builder);
                                            state = State.BeforeField;
                                            break;
                                        case '\r':
                                            i++;

                                            //          AddField(fields, builder);
                                            if (Peek() == '\n')
                                            {
                                                i++;
                                            }
                                            state = State.LineEnd;
                                            return;

                                        case '\n':
                                            i++;
                                            //          AddField(fields, builder);
                                            state = State.LineEnd;
                                            return;

                                        default:
                                            throw new InvalidDataException("Corrupt field found. A double quote is not escaped or there is extra data after a quoted field.");
                                    }
                                    break;
                                default:
                                    //    builder.Append((char)c);
                                    break;
                            }
                            break;

                        //     case State.LineEnd:

                        case State.LineEnd:
                            state = State.BeforeField;
                            break;

                        default:
                            throw new NotImplementedException();
                    }
                }
            }

            Loop();

        afterLoop:


            if (i < bufferLength == false)
            {
                FillBufferAsync(false);

                if (i == 0)
                {
                    ArrayPool<char>.Shared.Return(buffer);
                    yield break;
                }

                if (state != State.LineEnd)
                    goto reloop;
            }

            switch (state)
            {
                case State.BeforeField:
                case State.InField:
                case State.LineEnd:
                    var ff = buffer.AsMemory(j, i - j);
                    j = i;
                    state = State.BeforeField;

                    var t1 = Task.Run(Loop);
                    yield return ff;
                    t1.Wait();
                    if (tried)
                    {
                        tried = false;
                        FillBufferAsync(false);

                        if (state == State.LineEnd)
                            state = State.InField;

                        goto reloop;
                    }

                    goto afterLoop;

                case State.InQuotedField:
                    throw new InvalidDataException("When the line ends with a quoted field, the last character should be an unescaped double quote.");
            }
        }