Programming C#, Second Edition

Passing Parameters

By default, value types are passed into methods by value (see Section 4.1.2, earlier in this chapter). This means that when a value object is passed to a method, a temporary copy of the object is created within that method. Once the method completes, the copy is discarded. Although passing by value is the normal case, there are times when you will want to pass value objects by reference. C# provides the ref parameter modifier for passing value objects into a method by reference and the out modifier for those cases in which you want to pass in a ref variable without first initializing it. C# also supports the params modifier, which allows a method to accept a variable number of parameters. The params keyword is discussed in Chapter 9.

Passing by Reference

Methods can return only a single value (though that value can be a collection of values). Let’s return to the Time class and add a GetTime( ) method, which returns the hour, minutes, and seconds.

Because we cannot return three values, perhaps we can pass in three parameters, let the method modify the parameters, and examine the result in the calling method. Example 4-7 shows a first attempt at this.

Example 4-7. Returning values in parameters

public class Time
{
   // public accessor methods
   public void DisplayCurrentTime( )
   {
      System.Console.WriteLine("{0}/{1}/{2} {3}:{4}:{5}", 
         Month, Date, Year, Hour, Minute, Second);
   }

   public int GetHour( )
   {
      return Hour;
   }

     public void GetTime(int h, int m, int s)
     {
         h = Hour;