通过关键字readonly可以设定类中的只读域，即不能修改此域：
/*
Example6_3.cs illustrates the use of readonly fields
*/
// declare the Car class
public class Car
{
// declare a readonly field
public readonly string make;
// declare a static readonly field
public static readonly int wheels = 4;
// define a constructor
public Car(string make)
{
System.Console.WriteLine("Creating a Car object");
this.make = make;
}
}
class Example6_3
{
public static void Main()
{
System.Console.WriteLine("Car.wheels = " + Car.wheels);
// Car.wheels = 5; // causes compilation error
// create a Car object
Car myCar = new Car("Toyota");
System.Console.WriteLine("myCar.make = " + myCar.make);
// myCar.make = "Porsche"; // causes compilation error
string i = System.Console.ReadLine();
}
}