print barcode with vb.net Introducing Classes, Objects, and Methods in C#

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Introducing Classes, Objects, and Methods
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Constructors
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In the preceding examples, the instance variables of each Vehicle object had to be set manually using a sequence of statements, such as:
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minivanPassengers = 7; minivanFuelCap = 16; minivanMpg = 21;
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An approach like this would never be used in professionally written C# code Aside from being error-prone (you might forget to set one of the fields), there is simply a better way to accomplish this task: the constructor A constructor initializes an object when it is created It has the same name as its class and is syntactically similar to a method However, constructors have no explicit return type The general form of a constructor is shown here: access class-name(param-list) { // constructor code } Typically, you will use a constructor to give initial values to the instance variables defined by the class, or to perform any other startup procedures required to create a fully formed object Often, access is public because a constructor is usually called from outside its class The parameter-list can be empty, or it can specify one or more parameters All classes have constructors, whether you define one or not, because C# automatically provides a default constructor that causes all member variables to be initialized to their default values For most value types, the default value is zero For bool, the default is false For reference types, the default is null However, once you define your own constructor, the default constructor is no longer used Here is a simple example that uses a constructor:
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// A simple constructor using System; class MyClass { public int x; public MyClass() { x = 10; } } class ConsDemo { static void Main() {
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The constructor for MyClass
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MyClass t1 = new MyClass(); MyClass t2 = new MyClass(); ConsoleWriteLine(t1x + " " + t2x); } }
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In this example, the constructor for MyClass is
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public MyClass() { x = 10; }
Notice that the constructor is specified as public because the constructor will be called from code defined outside of its class As mentioned, most constructors are declared public for this reason This constructor assigns the instance variable x of MyClass the value 10 The constructor is called by new when an object is created For example, in the line
MyClass t1 = new MyClass();
the constructor MyClass( ) is called on the t1 object, giving t1x the value 10 The same is true for t2 After construction, t2x has the value 10 Thus, the output from the program is
10 10
Parameterized Constructors
In the preceding example, a parameterless constructor was used While this is fine for some situations, most often, you will need a constructor that accepts one or more parameters Parameters are added to a constructor in the same way that they are added to a method: Just declare them inside the parentheses after the constructor s name For example, here, MyClass is given a parameterized constructor:
// A parameterized constructor using System; class MyClass { public int x; public MyClass(int i) { x = i; } } class ParmConsDemo { static void Main() { MyClass t1 = new MyClass(10);
This constructor has a parameter
4:
Introducing Classes, Objects, and Methods
MyClass t2 = new MyClass(88); ConsoleWriteLine(t1x + " " + t2x); } }
The output from this program is shown here:
10 88
In this version of the program, the MyClass( ) constructor defines one parameter called i, which is used to initialize the instance variable, x Thus, when this line executes,
MyClass t1 = new MyClass(10);
the value 10 is passed to i, which is then assigned to x
Add a Constructor to the Vehicle Class
We can improve the Vehicle class by adding a constructor that automatically initializes the Passengers, FuelCap, and Mpg fields when an object is constructed Pay special attention to how Vehicle objects are created
// Add a constructor to Vehicle using System; 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 is a constructor for Vehicle public Vehicle(int p, int f, int m) { Passengers = p; FuelCap = f; Mpg = m; } // Return the range public int Range() { return Mpg * FuelCap; } // Compute fuel needed for a given distance public double FuelNeeded(int miles) { return (double) miles / Mpg; } }
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