using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {       

        List<Test> tmpList = new List<Test>();

        tmpList.Add(new Test() {id = 1, ilosc=11, kwota = 111});

        tmpList.Add(new Test() {id = 2, ilosc=22, kwota = 222});    

        var v = String.Join("|", tmpList.Select(x => $"{x.id}, {x.ilosc}, {x.kwota}"));

        Console.WriteLine(v);

    }

}

​

public class Test

{

    public int id, kwota, ilosc;    

}