Creating the talk() Method for the CritterTalk Program
Take a look at the next version of the Critter program, Critter Talk. This version is on the CD−ROM as CritterTalk, if you want to look at the entire program.
class Critter { public string name;
public void talk(){
Console.WriteLine("The Critter says: My name is {0}", name);
} // end talk
}// end class
I added a method named talk() to the Critter class. The method is very simple. It sends a message to the screen. Note that, inside the method, the program refers to the name variable. It can do this because both the name variable and talk() method belong to the Critter class.
Changing the Menu to Use the talk() Method
Because the Critter class now has a talk() method, the main program can invoke myCritter.talk() to hear from the critter. Examine how I changed the code in the main() method to take advantage of the critter’s newfound verbal skills:
case 1: myCritter.talk(); break;
I didn’t change anything else in the program. You might notice that the main program becomes a little simpler as some of the functionality is shuffled off to the object. In object−oriented programming, as much detail as possible is handled by objects rather than by the main program.
Creating a Property in the CritterProp Program
The public instance variable already created for name was easy to make, but it has some serious weaknesses. Generally, you want to write your programs to prevent problems. By making the name public, you have no way to ensure that it will get an appropriate value. Figure 4.11 shows the next version of the Critter program, Critter Prop, featuring a characteristic called the property.
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Figure 4.11: You can change the name property so that the critter will reinforce special rules.
Also, you might want a characteristic to be read−only so that it can be changed only from within the class. For example, if you have a circle class with a radius property, users shouldn’t be able to set the area of the circle. They might set both values to 1, which should not be done. A circle with a radius of 1 must have an area of 2 * pi. It would be better for the area property to be read−only so that the user cannot change it inappropriately. Having many public variables hanging around is also considered dangerous because they are vulnerable to being changed inadvertently.
Instead of public instance variables, you can use a special entity of objects: the property. You’ve seen properties before, for example, the length property of a string object, which is written myString.Length. Properties are special characteristics of an object. They appear to be variables directly attached to a particular type of object. Critter.name seems like a property because you can refer to it like a property. However, you will see shortly that there is a better way to describe properties than simply assigning a variable to a class. Properties enable you to control access to information inside your class, which is a very powerful capability.
Examining the Critter Prop Program
I made one simple change to the menu code. Because I know that I’ll be changing the critter’s name, I implemented the code for changing the name in the menu:
case 4:
Console.WriteLine("Current name: {0}", myCritter.name); Console.Write("Change name to: ");
myCritter.name = Console.ReadLine(); break;
Notice that this code is exactly the same, whether the Critter class uses a private instance variable or a property. One of the hallmarks of good object−oriented programming is that you improve an object’s performance (as you will by adding a property) without requiring numerous changes to the programs that use the object.
Creating the Critter with a Name Property
I have made a number of changes to the Critter class, all of which involve the critter’s name. The name of the critter is stored in a private variable, which is accessed through some special methods. Take a look at the new code for the Critter class:
class Critter {
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private string pName;
public string Name { get {
return pName;
}// end get set{
if (value.Length > 8){
Console.WriteLine("The name can't be more than 8 characters"); pName = value;
pName = pName.Substring(0,8); Console.WriteLine("Changing name to {0}", pName);
}else {
pName = value;
}// end if
}// end set
}// end string property
public void talk(){
Console.WriteLine("The Critter says: My name is {0}", Name);
} // end talk
}// end class
Although the changes seem extensive, they make a lot of sense. To make a property, you create a private instance variable, and then you add special methods to your class to provide access to the private variable. I’ll describe these changes next.
Making a Private Instance Variable
The first thing I did was eliminate name as a public instance variable. (Remember, it’s called an instance variable because its value has meaning to the entire instance of the class.) It’s public because another program or programmer can have access to its value. I replaced the public instance variable Name with a private instance variable named pName. This private variable can only be accessed from within the Critter class.
Hint I used the p in pName to remind myself that this is a private instance variable. Many programmers use this convention so that they don’t confuse the public property Name with the private instance variable pName. Microsoft has initiated a new convention with C#. Public entities begin with an uppercase letter, and private ones begin with a lowercase. Your program will still run and compile just fine if you ignore this convention, but it’s worth adopting, because other C# programmers will expect the capitalization to provide a clue to whether a variable is public or private. I prefer the preceding p convention because it’s recognized in pretty much every language I use.
The pName variable holds the value of the critter’s name but is not directly accessible to the outside world. Instead, I will create some special methods that allow outside entities, such as another class or programmer, access to this variable.
Examining the Basic Design of a Property
A property is a pair of special methods, get() and set(), that control access to a private instance variable. Here’s the essential property description for the name property:
public string name {
get {
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return pName; } // end get
set {
pName = value;
} // end set
}// end name property
The name property is declared as public because anybody can have access to the property. Properties have types because they are methods that control access to variables. In essence, the get() and set() methods of a property combine to create a "wrapper" around a local variable. A property should have the same type as the variable it represents. A property contains a get() method and a set() method. The get() method is used when a program wants to get the value of the property from the class, and the set() method is used when a program wants to set the value of the property.
Trap The terminology here can sound crazy. You might think that the get() method would get a value for the property, but that’s not the way it works. The get() method is used to send the property out to the rest of the world, and set() is used to send a value to the property from the outside. If you take the point of view of the program that uses your object, it all makes sense.
The get() and set() methods aren’t exactly like other methods. They must always have the names get and set, and they exist only in the context of a property definition. Also, they do not have parameters like other methods.
Creating a get() Method
The get() method returns a value for a particular property. Therefore, a get() method always has a return statement. Usually, the get() method is very simple, as in this example, but it can become complex, especially if the property is not directly stored but is calculated from other elements.
Creating a set() Method
The set() method is designed to accept a potential value for a property and assign it to the private instance variable. The value that the calling program is trying to send to the program is stored in a special (undeclared) parameter named value. The simplest form of the set() method assigns value to the private instance variable your property surrounds. If you want a read−only property (one like area, described earlier, which can’t be changed from the outside), simply omit the set() method.
Using Properties as Filters
The great thing about properties is that they give you error−checking capabilities. Suppose that there is a rule stating that critter names cannot be more than eight characters long (maybe they are DOS critters). If name was a public instance variable, it would be difficult to ensure that this eight−character rule is enforced. However, if name was a property, making sure that critter names always follow the rule would be easy. Take another look at the set() method I built for the Critter class:
set{
if (value.Length > 8){
Console.WriteLine("The name can't be more than 8 characters"); pName = value;
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