using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list = new [] {

            new { id="153_2" , order=1 },

            new { id="153_3" , order=2 },

            new { id="58_75" , order=1 },

            new { id="58" , order=1 },

            new { id="58_35" , order=3 },

            new { id="153" , order=3 },

            new { id="55_60" , order=2 },

            new { id="55" , order=2 },

            new { id="58_35" , order=1 },

        };

        var newList = list

            .GroupBy(c => c.id.BeforeOrSame('_'))

            .Select(g => g.OrderBy(d => d.id.Contains('_'))

                        .ThenBy(d => d.order))

            .OrderBy(c => c.First().order)

            .SelectMany(c => c);

        foreach(var item in newList)

            Console.WriteLine(item.id);

    }

}

​

public static class Extensions

{

    public static string BeforeOrSame(this string str, char c)

    {

        var i = str.IndexOf(c);

        return i < 0 ? str : str.Substring(0, i);

    }

}