Enumerable.Empty和Enumerable.DefaultIfEmpty

class Program
    {
        static void Main(string[] args)
        {
            var noEmployeeList = GetIEnumerable();
            foreach (var emp in noEmployeeList) {
                Console.WriteLine(emp);
            }
            Console.Read();
        }

        static IEnumerable<Employee> GetIEnumerable()
        {
            if (IsOnActive()) {
                return new List<Employee> { new Employee() { Id = 1, Name = "zs", Salary = 10000 } };
            }
            // 比new List<Empolyee>()节省内存。
            return Enumerable.Empty<Employee>();
        }

        static bool IsOnActive()
        {
            return false;
        }
    }

    public sealed class Employee
    {
        public long Id { get; set; }

        public string Name { get; set; }

        public double Salary { get; set; }

        public override string ToString()
        {
            return string.Format("ID:{0},Name:{1},Salary:{2}", Id, Name, Salary);
        }
    }

转载自http://www.cnblogs.com/multiplesoftware/archive/2011/06/04/2072415.html

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            IEnumerable<string> enumerable = Enumerable.Empty<string>();
            int c = enumerable.Count();
            Console.WriteLine(c);

            IEnumerable<string> enumOne = Enumerable.DefaultIfEmpty<string>(enumerable);
            c = enumOne.Count();
            Console.WriteLine(c);
            foreach (var s in enumOne)
            {
                Console.WriteLine(string.IsNullOrEmpty(s));
            }

            IEnumerable<string> enumoned = Enumerable.DefaultIfEmpty(enumerable, "SUM");
            Console.WriteLine(enumoned.Count());
            foreach (var s in enumoned)
            {
                Console.WriteLine(s);
            }