Programming C#, Third Edition

readonly Fields

You might want to create a version of the Time class that is responsible for providing public static values representing the current time and date. Example 4-12 illustrates a simple approach to this problem.

Example 4-12. Using static public constants

public class RightNow
{
   // public member variables
   public static int Year;
   public static int Month;
   public static int Date;
   public static int Hour;
   public static int Minute;
   public static int Second;  

   static RightNow( )
   {
      System.DateTime dt = System.DateTime.Now;
      Year =      dt.Year;
      Month =     dt.Month;
      Date =      dt.Day;
      Hour =      dt.Hour;
      Minute =    dt.Minute;
      Second =    dt.Second;
   }
}

public class Tester
{
   static void Main( )
   {
      System.Console.WriteLine ("This year: {0}", 
             RightNow.Year.ToString( ));
      RightNow.Year = 2006;
      System.Console.WriteLine ("This year: {0}", 
             RightNow.Year.ToString( ));
   }
}

Output:
This year: 2005
This year: 2006

This works well enough, until someone comes along and changes one of these values. As the example shows, the RightNow.Year value can be changed, for example, to 2006. This is clearly not what we’d like.

We’d like to mark the static values as constant, but that is not possible, because we don’t initialize them until the static constructor is executed. C# provides the keyword readonly for exactly this purpose. If you change the class member variable declarations as follows:

public static readonly int Year;
public static readonly int Month;
public static readonly int Date;
public static readonly int Hour;
public static readonly int Minute;
public static readonly int Second;

then comment out the reassignment in Main( ):

// RightNow.Year = 2006; // error!

the program will compile and run as intended.