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.
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