[FG28_Kodiak]

Yes a Singleton Pattern in a DLL.
http://en.wikipedia.org/wiki/Singleton_pattern

Example Singleton (threadsafe)

   public sealed class Singleton
    {
        public int planeCount { get; set; }
        
        private static readonly Lazy<Singleton> lazy = new Lazy<Singleton>(() => new Singleton());

        public static Singleton GetInstance { get { return lazy.Value; } }

        private Singleton()
        { 
             // Initialisations
        }
    }

in Mis-C# you use for example

//$reference parts/core/MyDll.dll
using System;
using System.Collections.Generic;
using maddox.game;
using maddox.game.world;
using MyNameSpace;

public class Mission : AMission
{
  
    Singleton s=Singleton.GetInstance;  // get access to the singleton or create it if not exist.


 public override void OnPlaceEnter(Player player, AiActor actor, int placeIndex)
    {
        base.OnPlaceEnter(player, actor, placeIndex);

        s.planeCount += 1; // example
    }
}

use Singleton s=Singleton.GetInstance; in every Scriptfile you need acces to the singleton. There is always only one Singleton present.