using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        // Student collection

        var list = new List<S>() { 

                new S() { Id = 1, Name = "Mobile", ParentId = null} ,

                new S() { Id = 2, Name = "Transport", ParentId = 1} ,

                new S() { Id = 3, Name = "Payment", ParentId = 2} ,

                new S() { Id = 4, Name = "X", ParentId = null} ,

                new S() { Id = 5, Name = "Y", ParentId = 4} ,

                new S() { Id = 6, Name = "Z", ParentId = 5} ,

                new S() { Id = 7, Name = "ZZZ", ParentId = null} 

            };

        //var qq = list

        //  .Where(x=>x.ParentId==null)

        //  .Select(x=> x.Name +"-"+ getChilderen(x.Id,list))

        //  .ToList();

        List<string> Result = new List<string>();

        getChilderen(0,list,Result,"");

        foreach(var q in Result)

            Console.WriteLine(q);

    }

    public static void  getChilderen (int parentId,List<S> children, List<string> Result,string name) {

        //if(children.Where(x=>x.ParentId==parentId).Count()>0)

        //  return children.Where(x=>x.ParentId==parentId)

        //  .Select(x=>x.Name +"-"+ getChilderen(x.Id,children)).FirstOrDefault().ToString();

        //  else

        //return  "";

        List<S> lst;

        if(parentId == 0){

            lst = children.Where(c=>c.ParentId == null).ToList();

            foreach(S item in lst)

            {

                Result.Add(name==""?item.Name : name + "-" + item.Name);         

            getChilderen(item.Id,children,Result,name + "-" +item.Name); 

            }                

        }

        else

        {

            lst = children.Where(c=>c.ParentId == parentId).ToList();

            if(lst.Count()>0)

        {

            Result.Add(name==""?lst.ElementAt(0).Name : name + "-" + lst.ElementAt(0).Name);         

            getChilderen(lst.ElementAt(0).Id,children,Result,name + "-" +lst.ElementAt(0).Name); 

        }

        }

        //else

        //{

        //  Result.Add(""); ; 

        //}

    }

}

​

public class S{

​

   public int Id { get; set; }

    public string Name { get; set; }

    public int? ParentId { get; set; }

}