Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;
using System.Web.Script.Serialization;
using System.Windows;

namespace CitiesSkylinesLauncher
{
    class Program
    {
        static void Main(string[] args)
        {
            var gameDirIndex = Array.IndexOf(args, "--gameDir");

            if (gameDirIndex == -1)
            {
                var pathFile = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), @"Paradox Interactive\launcherpath");
                Directory.CreateDirectory(Path.GetDirectoryName(pathFile));
                File.WriteAllText(pathFile, Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) + "\\\n");
                MessageBox.Show("Launcher path set.\n\nStart the game as usual.", "", MessageBoxButton.OK, MessageBoxImage.Information);
            }
            else
            {
                var gameDir = args[gameDirIndex + 1];
                var json = new JavaScriptSerializer();
                var settings = json.Deserialize<LauncherSettings>(File.ReadAllText(Path.Combine(gameDir, "launcher-settings.json")));
                var argList = new List<string>(args);
                argList.RemoveRange(gameDirIndex, 2);
                Process.Start(Path.Combine(gameDir, settings.exePath), string.Join(" ", argList));
            }
        }
    }

    class LauncherSettings
    {
        public string exePath { get; set; }
    }
}