using System;

using System.Collections.Generic;

​

class Test : IComparable<Test>

{

    public int Id { get; set; }

    public int ParentId { get; set; }

    public bool HasParentObject { get; set; }

​

    public int CompareTo(Test other)

    {

        if(other.HasParentObject && this.HasParentObject)

            return ParentId.CompareTo(other.ParentId);

        else if(other.HasParentObject)

            return 1;

        else 

            return -1; 

    }

}

public class Program

{

    public static void Main()

    {

        var tests = new List<Test>

        {

            new Test { Id = 1, ParentId = 0, HasParentObject = false},  

            new Test { Id = 2, ParentId = 1, HasParentObject = true},

            new Test { Id = 3, ParentId = 2, HasParentObject = true}

        };

        foreach(var test in tests)

            Console.WriteLine(test.Id);

        Console.WriteLine("\n");

        tests.Sort();

        foreach(var test in tests)

            Console.WriteLine(test.Id);

    }

}