C#에서 'is' 키워드는 어떻게 사용되나요?

using System; namespace DemoApplication{ class Program{ static void Main(){ Employee emp = new PermanentEmployee{ ID = 1, Name = "Martin" }; // Returns true as the derived type can be converted to base type. if (emp is Employee){ Console.WriteLine(emp.Name + " is Employee"); } else{ Console.WriteLine(emp.Name + " is not Employee"); } //Returns true, as the actual object is of type PermanentEmployee. if (emp is PermanentEmployee){ Console.WriteLine(emp.Name + " is PermanentEmployee"); } else{ Console.WriteLine(emp.Name + " is not PermanentEmployee"); } //Returns false, as PermanentEmployee object cannot be converted to //ContractEmployee. if (emp is ContractEmployee){ Console.WriteLine(emp.Name + " is ContractEmployee"); } else{ Console.WriteLine(emp.Name + " is not ContractEmployee"); } } } class Employee{ public int ID { get; set; } public string Name { get; set; } } class PermanentEmployee : Employee{ public int AnnualSalary { get; set; } } class ContractEmployee : Employee{ public int HourlySalary { get; set; } } }