Dynamic.cs

using CsvHelper;
using CsvHelper.Configuration;
using System;
using System.Collections.Generic;
using System.Dynamic;
using System.Globalization;
using System.IO;
using System.Linq;

namespace ConsoleApp8
{
    class Program
    {
        static void Main(string[] args)
        {
            using var sw = new StringWriter();
            var config = new CsvConfiguration(CultureInfo.CurrentCulture)
            {
                HasHeaderRecord = false,
            };
            using var writer = new CsvWriter(sw, config);

            // 出力用のテストデータ
            var orders = Enumerable.Range(0, 10).Select(x => new Orders
            {
                Id = x,
                // 偶数行と奇数行で DiscountsDetails の件数を変えています。(3項演算子なので見づらいですが…)
                DiscountsDetails = x % 2 == 0 ?
                    new List<DiscountsDetails>
                    {
                        new DiscountsDetails{ dataA = "a value of dataA-1", dataB = "a value of dataB-1" },
                        new DiscountsDetails{ dataA = "a value of dataA-2", dataB = "a value of dataB-2" },
                    } :
                    new List<DiscountsDetails>
                    {
                        new DiscountsDetails{ dataA = "a value of dataA-1", dataB = "a value of dataB-1" },
                        new DiscountsDetails{ dataA = "a value of dataA-2", dataB = "a value of dataB-2" },
                        new DiscountsDetails{ dataA = "a value of dataA-3", dataB = "a value of dataB-3" },
                        new DiscountsDetails{ dataA = "a value of dataA-4", dataB = "a value of dataB-4" },
                    }
            });

            // CSV に出力するときは Orders クラスではなく OrdersDynamicObject にして CSV に出力する
            writer.WriteRecords(orders.Select(x => new OrdersDynamicObject(x)));

            Console.WriteLine(sw.ToString());
        }
    }

    // CSV に出力するクラス
    public class Orders
    {
        public int Id { get; set; }
        public List<DiscountsDetails> DiscountsDetails { get; set; }
    }
    public class DiscountsDetails
    {
        public string dataA { get; set; }
        public string dataB { get; set; }
    }

    // CsvHelper の dynamic オブジェクトを CSV に出力する機能を使って実行時に動的に CSV に出力するプロパティを決める DynamicObject
    // https://joshclose.github.io/CsvHelper/examples/writing/write-dynamic-objects/
    public class OrdersDynamicObject : DynamicObject
    {
        private readonly Orders _orders;

        public OrdersDynamicObject(Orders orders)
        {
            _orders = orders;
        }

        public override IEnumerable<string> GetDynamicMemberNames()
        {
            return new[] { nameof(Orders.Id) }
                .Concat(
                    _orders.DiscountsDetails
                        ?.SelectMany(
                            (x, i) => new[] { $"{nameof(DiscountsDetails)}${i}${nameof(DiscountsDetails.dataA)}", $"{nameof(DiscountsDetails)}${i}${nameof(DiscountsDetails.dataB)}" }) ?? Array.Empty<string>());
        }

        public override bool TryGetMember(GetMemberBinder binder, out object result)
        {
            if (binder.Name == nameof(Orders.Id))
            {
                result = _orders.Id;
                return true;
            }
            else
            {
                var tokens = binder.Name.Split('$');
                if (tokens.Length == 3 && tokens[0] == nameof(DiscountsDetails) && int.TryParse(tokens[1], out var index))
                {
                    var obj = _orders.DiscountsDetails[index];
                    var (ok, value) = tokens[2] switch
                    {
                        nameof(DiscountsDetails.dataA) => (true, obj.dataA),
                        nameof(DiscountsDetails.dataB) => (true, obj.dataB),
                        _ => (false, null),
                    };
                    result = value;
                    return ok;
                }
            }

            // 正しくないプロパティ名
            result = null;
            return false;
        }
    }
}

output.csv

0,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2
1,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2,a value of dataA-3,a value of dataB-3,a value of dataA-4,a value of dataB-4
2,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2
3,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2,a value of dataA-3,a value of dataB-3,a value of dataA-4,a value of dataB-4
4,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2
5,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2,a value of dataA-3,a value of dataB-3,a value of dataA-4,a value of dataB-4
6,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2
7,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2,a value of dataA-3,a value of dataB-3,a value of dataA-4,a value of dataB-4
8,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2
9,a value of dataA-1,a value of dataB-1,a value of dataA-2,a value of dataB-2,a value of dataA-3,a value of dataB-3,a value of dataA-4,a value of dataB-4