readonly c#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication3
{
    public class ReadOnlyTest
    {
        class SampleClass
        {
            public int x;
            // Initialize a readonly field
            public readonly int y = 25;
            public readonly int z;

            public SampleClass()
            {
                // Initialize a readonly instance field
                z = 24;
            }

            public SampleClass(int p1, int p2, int p3)
            {
                x = p1;
                y = p2;
                z = p3;
            }
        }

        static void Main()
        {
            SampleClass p1 = new SampleClass(11, 21, 32);   // OK
            Console.WriteLine("p1: x={0}, y={1}, z={2}", p1.x, p1.y, p1.z);
            SampleClass p2 = new SampleClass();
            p2.x = 55;   // OK
            Console.WriteLine("p2: x={0}, y={1}, z={2}", p2.x, p2.y, p2.z);
        }
    }
}