// default constructor in .NET framework

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// default constructor
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Part II
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Understanding the C# Language
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public double Area() { return Math.PI * radius * radius; } private int radius; }
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Note In C# parlance, the default constructor is a constructor that does not take any parameters. It does not matter whether the compiler generates it or you write it; it is still the default constructor. You can also write nondefault constructors (constructors that do take parameters), as you will see in the upcoming section titled Overloading Constructors. . Note that the constructor is marked as public. If this keyword is omitted, the constructor will be private (just like any other methods and elds). If the constructor is private, it cannot be used outside the class, which prevents you from being able to create Circle objects from methods that are not part of the Circle class. You might therefore think that private constructors are not that valuable. However, they do have their uses, but they are beyond the scope of the current discussion. You can now use the Circle class and exercise its Area method. Notice how you use dot notation to invoke the Area method on a Circle object:
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Circle c; c = new Circle(); double areaOfCircle = c.Area();
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Overloading Constructors
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You re almost nished, but not quite. You can now declare a Circle variable, point it to a newly created Circle object, and then call its Area method. However, there is still one last problem. The area of all Circle objects will always be 0 because the default constructor sets the radius to 0 and it stays at 0; the radius eld is private, and there is no way of changing its value after it has been initialized. One way to solve this problem is to realize that a constructor is just a special kind of method and that it like all methods can be overloaded. Just as there are several versions of the Console.WriteLine method, each of which takes different parameters, so too you can write different versions of a constructor. You can add a constructor to the Circle class, with the radius as its parameter, like this:
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class Circle { public Circle() { radius = 0; } // default constructor
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Creating and Managing Classes and Objects
public Circle(int initialRadius) // overloaded constructor { radius = initialRadius; } public double Area() { return Math.PI * radius * radius; } private int radius; }
Note The order of the constructors in a class is immaterial; you can de ne constructors in
whatever order you feel most comfortable with.
You can then use this constructor when creating a new Circle object, like this:
Circle c; c = new Circle(45);
When you build the application, the compiler works out which constructor it should call based on the parameters that you specify to the new operator. In this example, you passed an int, so the compiler generates code that invokes the constructor that takes an int parameter. You should be aware of a quirk of the C# language: if you write your own constructor for a class, the compiler does not generate a default constructor. Therefore, if you ve written your own constructor that accepts one or more parameters and you also want a default constructor, you ll have to write the default constructor yourself.
Partial Classes
A class can contain a number of methods, elds, and constructors, as well as other items discussed in later chapters. A highly functional class can become quite large. With C#, you can split the source code for a class into separate les so that you can organize the de nition of a large class into smaller, easier to manage pieces. This feature is used by Microsoft Visual Studio 2008 for Windows Presentation Foundation (WPF) applications, where the source code that the developer can edit is maintained in a separate le from the code that is generated by Visual Studio whenever the layout of a form changes.
Part II
Understanding the C# Language
When you split a class across multiple les, you de ne the parts of the class by using the partial keyword in each le. For example, if the Circle class is split between two les called circ1.cs (containing the constructors) and circ2.cs (containing the methods and elds), the contents of circ1.cs look like this:
partial class Circle { public Circle() // default constructor { radius = 0; } public Circle(int initialRadius) // overloaded constructor { radius = initialRadius; } }
The contents of circ2.cs look like this:
partial class Circle { public double Area() { return Math.PI * radius * radius; } private int radius; }
When you compile a class that has been split into separate les, you must provide all the les to the compiler.
In the following exercise, you will declare a class that models a point in two-dimensional space. The class will contain two private elds for holding the x- and y-coordinates of a point and will provide constructors for initializing these elds. You will create instances of the class by using the new keyword and calling the constructors.
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