9.1 Overloading Methods

Often you'll want to have more than one method with the same name. The most common example of this is to have more than one constructor with the same name, which allows you to create the object with different parameters. For example, if you were creating a Time object, you might have circumstances where you want to create the Time object by passing in the date, hours, minutes, and seconds. Other times, you might want to create a Time object by passing in an existing Time object. Still other times, you might want to pass in just a date, without hours and minutes. Overloading the constructor allows you to provide these various options.

Chapter 8 explained that your constructor is automatically invoked when your object is created. Let's return to the Time class created in that chapter. It is possible to create a Time object by passing in a DateTime object to the constructor.

It would be convenient also to allow the client to create a new Time object by passing in year, month, date, hour, minute, and second values. Some clients might prefer one or the other constructor; you can provide both and the client can decide which better fits the situation.

In order to overload your constructor, you must make sure that each constructor has a unique signature. The signature of a method is composed of its name and its parameter list. Two methods differ in their signatures if they have different names or different parameter lists. Parameter lists can differ by having different numbers or types of parameters.

void myMethod(int p1);
void myMethod(int p1, int p2);      // different number
void myMethod(int p1, string s1);   // different types 
void someMethod(int p1);            // different name

The previous four lines of code show how you might distinguish methods by signature.

The first three methods are all overloads of the myMethod() method. The first differs from the second and third in the number of parameters. The second closely resembles the third version, but the second parameter in each is a different type. In the second method, the second parameter (p2) is an integer; in the third method, the second parameter (s1) is a string. These changes to the number or type of parameters are sufficient changes in the signature to allow the compiler to distinguish the methods.

The fourth method differs from the other three methods by having a different name. This is not method overloading, just different methods, but it illustrates that two methods can have the same number and type of parameters if they have different names. Thus, the fourth and first have the same parameter list, but their names are different.

A class can have any number of methods, as long as each one's signature differs from that of all the others. Example 9-1 illustrates a Time class with two constructors: one that takes a DateTime object and one that takes six integers.

Example 9-1. Overloading a method

using System;

namespace MethodOverloading
{
    public class Time
    {
        // private member variables
        private int Year;
        private int Month;
        private int Date;
        private int Hour;
        private int Minute;
        private int Second;

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

        // constructors
        public Time(System.DateTime dt) 
        { 
            Year =      dt.Year; 
            Month =     dt.Month; 
            Date =      dt.Day; 
            Hour =      dt.Hour; 
            Minute =    dt.Minute; 
            Second =    dt.Second; 
        } 

        public Time(int Year, int Month, int Date,  
            int Hour, int Minute, int Second) 
        { 
            this.Year =     Year; 
            this.Month =    Month; 
            this.Date =     Date; 
            this.Hour =     Hour; 
            this.Minute =   Minute; 
            this.Second =   Second; 
        } 
    }

   class Tester
   {
      public void Run()
      {
          System.DateTime currentTime = System.DateTime.Now;

          Time time1 = new Time(currentTime) ;
          time1.DisplayCurrentTime();

          Time time2 = new Time(2000,11,18,11,03,30) ;
          time2.DisplayCurrentTime();
      }

      static void Main()
      {
         Tester t = new Tester();
         t.Run();
      }
   }
}
 Output:
3/26/2002 16:17:32
11/18/2000 11:3:30

If a function's signature consisted only of the function name, the compiler would not know which constructors to call when constructing the new Time objects time1 and time2. However, because the signature includes the parameters and their types, the compiler is able to match the constructor call for time1 with the constructor whose signature requires a DateTime object.

System.DateTime currentTime 
Time time1 = new Time(currentTime);
public Time(System.DateTime dt)

Likewise, the compiler is able to associate the time2 constructor call with the constructor whose signature specifies six integer arguments.

Time time2 = new Time(2000,11,18,11,03,30);
public Time(int Year, int Month, int Date,
    int Hour, int Minute, int Second)

When you overload a method, you must change the signature (the name, number, or type of the parameters). You are free, as well, to change the return type, but this is optional. Changing only the return type does not overload the method, and creating two methods with the same signature but differing return types generates a compile error.