using System;

using System.Collections.Generic;

​

using System.Linq;              

public class Program

{

    public static void Main()

    {

         var listAll = new List<List<sig>> { new List<sig>(){new sig{key="501", name = "list1"}},

                new List<sig>(){new sig{key="505", name ="list2"}},

                new List<sig>(){new sig{key="249", name="list3"}}};

​

            string[] keys = { "505", "501", "249" };

​

            listAll =  listAll.OrderBy(e => Array.IndexOf(keys, e.FirstOrDefault()?.key)).ToList();

​

            Console.WriteLine(String.Join(",", listAll.SelectMany(e => e.Select(x => x.name))));

    }

}

public class sig {

    public string key { get; set; }

    public string value { get; set; }

    public string name { get; set; }

}