linq IEnumerable Concat 方法

var empList = new Dictionary<int,List<Employee>>();

            Random r = new Random();

            int length = 10000;



            Console.WriteLine(string.Format("开始装载数据({0})...", length));

            List<Employee> _List = new List<Employee>();

            for (int i = 0; i < length; i++)

            {

                var e =new Employee()

                {

                    ID = i,

                    FName = "A" + i,

                    Age = r.Next(0, 100),

                    Sex = r.Next(0, 2) == 1 ? 'F' : 'M'

                };

                if (empList.TryGetValue(e.Age, out _List))

                    _List.Add(e);

                else

                    empList[e.Age] = new List<Employee>() { e };

            }



            IEnumerable<Employee> list = new List<Employee>();

            List<Employee> list1 = new List<Employee>();

            foreach (var item in empList)

            {

                IEnumerable<Employee> ie = item.Value.Where(p => p.Sex == 'F');

                list1.AddRange(ie);

                list.Concat(ie);

            }



            Console.WriteLine(list.ToList().Count);

            Console.WriteLine(list1.Count);

如上是我的代码，运行结果中，list.Count 为0 ，list1.Count 有值。
我想问一下这是为什么？
小弟刚接触Linq，请大神解惑
------解决方案--------------------
list = list.Concat(ie);