using System.Linq;

using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var source = new List<Controller>()   {

          new Controller {

              Actions = new List<Action> {

                  new Action {

                     Id = 1, IsSelected = true

                  },

                   new Action {

                     Id = 2, IsSelected = false

                  }

            }   

        },

        new Controller {

              Actions = new List<Action> {

                  new Action {

                     Id = 1, IsSelected = true

                  },

                  new Action {

                     Id = 3, IsSelected = false

                  },

                   new Action {

                     Id = 4, IsSelected = true

                  }

            }   

        }   

    };

         var toAdd = source

        .Where(x => x.Actions != null)

        .SelectMany(x => x.Actions.Where(z => z.IsSelected).Select(a => a.Id))

        .Distinct() // remove any duplicates

        .ToList();

        foreach(var result in toAdd) {

            Console.WriteLine(result);

        }

    }

}

​

public class Controller {

    public List<Action> Actions {get; set;}

}

​

public class Action {

    public int Id {get; set;}

    public bool IsSelected {get; set;}

}