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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var customers = new List<Customer>(){
                new Customer{CustomerId=2,ParentCustomerId=1},
                new Customer{CustomerId=1,ParentCustomerId=0},   
                new Customer{CustomerId=4,ParentCustomerId=0},
                new Customer{CustomerId=5,ParentCustomerId=0},
                new Customer{CustomerId=3,ParentCustomerId=1},
                new Customer{CustomerId=6,ParentCustomerId=5}
            };
            
            var ordered = customers.OrderBy(c => c.ParentCustomerId == 0 ? c.CustomerId : c.ParentCustomerId).ThenBy(c => c.CustomerId);
            foreach(var item in ordered){
                Console.WriteLine("Customer {0} - Parent - {1}",item.CustomerId,item.ParentCustomerId);
            }
        }
    }
    
    public class Customer
    {
        public int CustomerId{get;set;}
        public int ParentCustomerId{get;set;}
    }
        
}

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Compilation time: 0.16 sec, absolute running time: 0.09 sec, cpu time: 0.09 sec, average memory usage: 16 Mb, average nr of threads: 4