using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

          List<Employee> employees = new List<Employee>

            {

                new Employee { Name = "foo", Type = "A" },

                new Employee { Name = "bar", Type = "B" },

            };

​

            List<Employee> employees2 = new List<Employee>

            {

                new Employee { Name = "abc", Type = "X" },

                new Employee { Name = "xyz", Type = "A" },

                new Employee { Name = "jkl", Type = "A" },  

            };

​

            List<Dept> depts = new List<Dept>

            {

                new Dept { Name = "Sales", Category = "Finance", Employees = employees },

                new Dept { Name = "Physics", Category = "Science", Employees = employees2 },

                new Dept { Name = "HR", Category = "HR", Employees = employees2 },

            };

​

            var result = depts.SelectMany(x => x.Employees.Where(z => z.Type == "A"), (DeptObj, empObj) =>

                                                new

                                                {

                                                    DeptObj.Category,

                                                    empObj

                                                }

                                              ).GroupBy(x => x.Category)

                                              .Select(x => new NewModel { Category = x.Key, EmpNames = x.Select(z => z.empObj.Name).ToList() });

          foreach(var item in result)

          {

              Console.WriteLine("Category: {0}", item.Category);

              foreach(var name in item.EmpNames)

              {

                  Console.WriteLine("\t{0}", name);

              }

          }

    }

     public class Employee

    {

        public string Name { get; set; }

        public string Type { get; set; }

    }

    public class Dept

    {

        public string Name { get; set; }

        public string Category { get; set; }

        public List<Employee> Employees { get; set; }

    }

​

    public class NewModel

    {

        public string Category { get; set; }

        public List<string> EmpNames { get; set; }

    }

}