using System;

using System.IO;

using System.Linq;

using System.Collections.Generic;

using System.Globalization;

using CsvHelper;

using CsvHelper.Configuration;

​

public class Program

{

    public static void Main()

    {

        var foos = new List<Foo>

        {

            new Foo

            {

                FooID = 1,

                Bars = new List<Bar>

                {

                    new Bar { BarID = "A", BarProperty1 = "Red", BarProperty2 = "Red", BarProperty3 = "Green" },

                }

            },

            new Foo

            {

                FooID = 2,

                Bars = new List<Bar>

                {

                    new Bar { BarID = "A", BarProperty1 = "Green", BarProperty2 = "Green", BarProperty3 = "Red" },

                    new Bar { BarID = "B", BarProperty1 = "Red", BarProperty2 = "Purple", BarProperty3 = "Orange" },

                    new Bar { BarID = "C", BarProperty1 = "White", BarProperty2 = "Black", BarProperty3 = "Red" },

                }

            },

        };

​

        string content = null;

​

        using (var memory = new MemoryStream())

        using (var writer = new StreamWriter(memory))

        using (var csv = new CsvWriter(writer, CultureInfo.GetCultureInfo("en")))

        {

            csv.Context.RegisterClassMap<BarMap>();

            var bars = foos

                .SelectMany(foo => foo.Bars.Select(bar => new Bar { BarID = $"{foo.FooID}{bar.BarID}", BarProperty1 = bar.BarProperty1, BarProperty2 = bar.BarProperty2, BarProperty3 = bar.BarProperty3 }));

            csv.WriteRecords(bars);

​

            writer.Flush();

            memory.Position = 0;

​

            using (var reader = new StreamReader(memory))

            {

                content = reader.ReadToEnd();

            }

        }

​

        Console.WriteLine(content);     

    }

}

​

public class Foo

{

    public int FooID { get; set; }

    public List<Bar> Bars { get; set; }

}

​

public class Bar

{

    public string BarID { get; set; }

    public string BarProperty1 { get; set; }

    public string BarProperty2 { get; set; }

    public string BarProperty3 { get; set; }

}

​

public class BarMap : ClassMap<Bar>

{

    public BarMap()

    {

        Map(bar => bar.BarID).Name("Id");

        Map(bar => bar.BarProperty1).Name("Prop1");

        Map(bar => bar.BarProperty2).Name("Prop2");

        Map(bar => bar.BarProperty3).Name("Prop3");

    }

}