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C# 30: A Beginner s Guide
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This program consists of two classes: Vehicle and VehicleDemo Inside VehicleDemo, the Main( ) method creates an instance of Vehicle called minivan Then the code within Main( ) accesses the instance variables associated with minivan, assigning them values and using those values It is important to understand that Vehicle and VehicleDemo are two separate classes The only relationship they have to each other is that one class creates an instance of the other Although they are separate classes, code inside VehicleDemo can access the members of Vehicle because they are declared public If they had not been given the public access specifier, their access would have been limited to the Vehicle class, and VehicleDemo would not have been able to use them Assuming that you call the preceding file UseVehiclecs, compiling this program creates a file called UseVehicleexe Both the Vehicle and VehicleDemo classes are automatically part of the executable file The program displays the following output:
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Minivan can carry 7 with a range of 336
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It is not necessary for both the Vehicle and the VehicleDemo classes to actually be in the same source file You could put each class in its own file, called Vehiclecs and VehicleDemocs, for example Just tell the C# compiler to compile both files and link them together For example, you could use this command line to compile the program if you split it into two pieces as just described:
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csc Vehiclecs VehicleDemocs
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This will create VehicleDemoexe because that is the class that contains the Main( ) method If you are using the Visual Studio IDE, you will need to add both files to your project and then build Before moving on, let s review a fundamental principle: Each object has its own copies of the instance variables defined by its class Thus, the contents of the variables in one object can differ from the contents of the variables in another There is no connection between the two objects, except for the fact that they are both objects of the same type For example, if you have two Vehicle objects, each has its own copy of Passengers, FuelCap, and Mpg, and the contents of these can differ between the two objects The following program demonstrates this fact:
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// This program creates two Vehicle objects using System; // A class that encapsulates information about vehicles class Vehicle { public int Passengers; // number of passengers public int FuelCap; // fuel capacity in gallons public int Mpg; // fuel consumption in miles per gallon } // This class declares two objects of type Vehicle class TwoVehicles {
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Introducing Classes, Objects, and Methods
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static void Main() { Vehicle minivan = new Vehicle(); Vehicle sportscar = new Vehicle(); int range1, range2; // Assign values to fields in minivan minivanPassengers = 7; minivanFuelCap = 16; minivanMpg = 21; // Assign values to fields in sportscar sportscarPassengers = 2; sportscarFuelCap = 14; sportscarMpg = 12;
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Remember, minivan and sportscar refer to separate objects
// Compute the ranges assuming a full tank of gas range1 = minivanFuelCap * minivanMpg; range2 = sportscarFuelCap * sportscarMpg; ConsoleWriteLine("Minivan can carry " + minivanPassengers + " with a range of " + range1); ConsoleWriteLine("Sportscar can carry " + sportscarPassengers + " with a range of " + range2); } }
The output produced by this program is shown here:
Minivan can carry 7 with a range of 336 Sportscar can carry 2 with a range of 168
As you can see, minivan s data is completely separate from the data contained in sportscar Figure 4-1 depicts this situation
Figure 4-1 One object s instance variables are separate from another s
C# 30: A Beginner s Guide
How Objects Are Created
Vehicle minivan = new Vehicle();
In the preceding programs, the following line was used to create an object of type Vehicle:
This declaration performs three functions First, it declares a variable called minivan of the class type Vehicle This variable is not, itself, an object Instead, it is simply a variable that can refer to an object Second, the declaration creates an actual, physical instance of the object This is done by using the new operator Finally, it assigns to minivan a reference to that object Thus, after the line executes, minivan refers to an object of type Vehicle The new operator dynamically allocates (that is, allocates at runtime) memory for an object and returns a reference to it This reference is then stored in a variable Thus, in C#, all class objects must be dynamically allocated As you might expect, it is possible to separate the declaration of minivan from the creation of the object to which it will refer, as shown here:
Vehicle minivan; // declare a reference to an object minivan = new Vehicle(); // allocate a Vehicle object
The first line declares minivan as a reference to an object of type Vehicle Thus, minivan is a variable that can refer to an object, but it is not an object itself The next line creates a new Vehicle object and assigns a reference to it to minivan Now, minivan is linked with an object The fact that class objects are accessed through a reference explains why classes are called reference types The key difference between value types and reference types is what a variable of each type means For a variable of a value type, the variable itself contains the value For example, given
int x; x = 10;
x contains the value 10 because x is a variable of type int, which is a value type However, in the case of
Vehicle minivan = new Vehicle();
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