C#. Class. Class definition. Object of class. Constructor of the class. Destructor. Keyword this

Class. Class definition. Object of class. Constructor of the class. Keyword this

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1. What is the class and object of the class?

A class is a template that determines the shape of the object. Classes are like types in a traditional programming language. The difference is that users can define new types in an object-oriented programming language.

If a class is like a type in a traditional programming language, then an object is like a variable. Sometimes objects are referred to as instance of a class.

Object class implements one of the possible variants of the class definition. When a class object is declared, memory is allocated for it.

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2. What difference between class declaration and object declaration of the class?

When class is declared, the memory is not allocated. Is formed only description of the class members. Class definition is called the class declaration. In the description of the class are specified data and program code.

As a rule, the program code executes the some work under the data in the class (operates data in the class). When class is defined, the memory for data and program code is not allocated. The class definition – this is an information about, what data are in this class, and what work is executed to these data.

If the object of a class is defined, then, based on the class definition, the memory is allocated for this object. Thus class is a physical abstraction. Physically, the class will be represented only after the declaring of the object of class.

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3. What is declared in the class?

When class is defined then data of this class are declared. Also is declared a program code which operates these data.

Also, a class can define only data or only the program code. But when you are working with the classes then the most classes define the data and a program code.

Data are encapsulated in the data members, which are defined in the class, but a code – in the member-functions. In C # are supported several types of data members and member functions.

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4. What is called the field of a class?

The data members are called the fields of class. The variables of the class instance and static variables are the data members. Class fields can be any valid type in the application.

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5. What are the C # language elements can be referred to member functions of a class?

To the member functions of a class can be referred:

methods;
constructors;
destructors;
indexers;
events;
operators;
properties.

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6. What is the general form of class definition?

Class is created by using a keyword class. The general form of a class definition is following:

class class_name
{
   // Declaring of variables of class instance.
   access type variable1;
   access type variable2;
   // ...
   access type variableN;

   // Declaring of methods.
   access returning_type method1(parameters)
   {
      // body of the method
   }

   access returning_type method2(parameters)
   {
       // body of the method
   }

   // ...
   access returning_type methodN(parameters)
   {
       // body of the method
   }
}

In the above description:

class_name – the name of a class. The class name defines a new data type. Name of the class can respond the entity of class. In turn, the class must define one (and only one) logical entity;
access – this is an access type. The C# language provides four access modifiers: public (public class member); private (private member of the class); protected (protected class member) and internal. The internal modifier defines the accessibility of class member in the all members of the assembly and its availability outside the assembly.
type – the type of variable that is a class member;
variable1, variable2, …, variableN – variables that are class members (data in the class);
method1, method2, …, methodN – methods (functions) that are class members;
returning_type – type that is returned by the class method;
parameters – the parameters of methods that are defined in the class.

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7. What are the types (modifiers) of access to the class members?

The access to the class members can be one of five types:

private – private access. In this case members (data or methods) of a class are accessible only into the class boundaries. This means that access to private members of the class, have only variables and methods in this class;
public – public access. In this case, the class members are accessible outside the class. It means that public class members can be used in other code snippets even outside the class;
protected – the protected access. The given type is used in the case of inheritance when class hierarchy is built. Such members of a class are open in the boundaries of the class hierarchy, but they are closed outside the class hierarchy;
internal – it determines the availability of a member of the class in all the assembly files and it inaccessibility outside the assembly. This means that the internal-class members are known only in the program, but not outside the program. This modifier is useful when you need to create the program components;
protected internal – this is a combination of the protected and internal modifiers using the logical operation “OR”. Such a member of the class is visible within the class or in inherited classes as well as in any methods of the current assembly. An inherited class can be placed in the same assembly or even in a different assembly.

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8. What access gives the modifier “protected internal“?

The paired access identifier “protected internal” consists of two separate modifiers “protected” and “internal“.

A member of the class that is declared as protected internal is available:

in methods of the current class;
in methods of a nested class if nested classes are used in the class;
in methods of a derived class (protected);
in methods of the current assembly (internal).

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9. Can be missed the access type modifier when members of a class are declared?

Answer: yes, it can be missed. In this case, the declared class member is private by default.

Example.

...

// declaration of class of MyPoint type
class MyPoint
{
    int x; // x - private member
    public int y;
}

...

// access to the class member
MyPoint p = new MyPoint();

// p.x = 25; - error!, access is closed
p.y = 13; // it works, because p.y declared as public

...

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10. Examples of simple classes declarations, that include data only.

Example 1. Class that includes only data. In this example, the new type MyPoint is created. The public class MyPoint is declared. This class defines a point on the coordinate plane.

...

public class MyPoint
{
    public int x; // coordinate x
    public int y; // coordinate y
}

...

Example 2. Definition of class MyBook, that describes data about a book.

...

public class MyBook
{
    public string Title; // the title of a book
    public string Author; // author name
    public int year;     // year of issue
    public float price;   // book cost
}

...

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11. How create the object (class instance)?

After class declaration the new type is created. In the previous paragraph the MyPoint type is created. The class object is defined as a regular variable of the some type.

Example 1. Creating the object of MyPoint type (see previously paragraph).

Way 1. Creating of the object of MyPoint type with name p.

MyPoint p = new MyPoint(); // is created variable p of MyPoint type
// is allocated the memory for class object MyPoint
// to the variable p is assigned the reference to the object

// filling the fields of the class
p.x = 25;
p.y = -10;

Way 2.

MyPoint p;

p = new MyPoint(); // memory allocation - is required!!!, else - error

// filling the fields of the class
p.x = 25;
p.y = -10;

Example 2. Creating the class instance of MyBook type (see the previous paragraph) with name b.

MyBook b = new MyBook();

// filling the fields of the class
b.Title = "Title of Book";
b.Author = "Author of Book";
b.year = 1998;
b.price = 0.01f;

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12. To which data types belong classes: to the type of value or reference types?

Classes belong to the reference types. The reference to the class instance is placed on the stack, and the class instance itself is placed on the heap. In the case of value types, an instance (variable) of this type is completely placed on the stack.

Example.

// variable z is the double type, which is the value-type
double z;
z = 2.85; // in the variable z is saved the value 2.85

MyPoint p; // is created the variable-reference p of type MyPoint
p = new MyPoint(); // the memory is allocated using the "new" operator
// for the variable p is assigned the reference on the allocated memory

In the given example, the address of created class instance in the memory is put to the variable p. The next figure shows the schematic difference between reference types and value types.

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13. Example of class definition, which contains data and methods (functions).

You can do some work above data in the class. It is realized by using a member functions.

In the next code snippets are described the classes with their own functions.

Example 1. Modification of MyPoint class, that includes of points coordinates. Four new member functions are added to the class:

function GetX(), that returns a x coordinate;
function GetY(), that returns a y coordinate;
function SetX(), which sets the x coordinate to the new value;
function SetY(), which sets the y coordinate to the new value.

The internal variables x and y of the class are declared as private.

class MyPoint
{
    // internal variables - are declared as private
    private int x;
    private int y;

    // member function of class – GetX()
    public int GetX()
    {
        return x;
    }

    // member function of class – GetY()
    public int GetY()
    {
        return y;
    }

    // member function of class SetX()
    public void SetX(int newx)
    {
        x = newx;
    }

    // member function of class – SetY()
    public void SetY(int newy)
    {
        y = newy;
    }
}

...

// using the class in the program
MyPoint p = new MyPoint();

// calling the functions - class members
p.SetX(5);
p.SetY(8);

// p.x and p.y - are unaccessible

// displaying the x-coordinate of class object p on the form
label1.Text = p.GetX().ToString();

...

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14. Example of using an array of class objects and implementation the destructor in a class

In the example below, are created two classes. First from their is of MyBook type. Second – array of books MyBooks. In class MyBooks is declared the destructor.

class MyBook
{
    // class fields - are declared as private
    private string Title;
    private string Author;
    private int year;
    private float price;

    // the class constructor without parameters
    public MyBook()
    {
        Title = "";
        Author = "";
        year = 1900;
        price = 0.00f;
    }

    // the class constructor with fours parameters
    public MyBook(string t, string a, int y, float p)
    {
        Title = t;
        Author = a;
        year = y;
        price = p;
    }
}

// class - array of books
class MyBooks
{
    MyBook[] mas_books; // array of books

    // the constructor of class MyBooks
    public MyBooks()
    {
        // memory allocation under the array
        mas_books = new MyBook[10];

        // memory allocation for separate item of array
        for (int i = 0; i < 10; i++)
        {
            // calling of the constructor with four parameters
            mas_books[i] = new MyBook("T" + i.ToString(),"A" + i.ToString(),i, (float)(i * 5));
        }
    }

    // destructor of class MyBooks
    ~MyBooks()
    {
        // actions that needs to be done before the destruction of the object
        // ...
    }

    // The internal method, that returns a book that given by the index.
    public MyBook GetBook(int index)
    {
        return (MyBook)mas_books[index];
    }
}

...

// using in the program
MyBooks MB = new MyBooks(); // in the constructor is created the array from 10 books of type MyBook


...

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15. What features of use the keyword “this” in the class?

In a class declaration, all members of the class receive an implicit reference this, which is a reference to the current instance of the class. More details about the cases and examples of using this in classes are described here.

Example. Declaration of the class DemoThis. There are two fields in the class. One of them is as private and is type “int” and is named “i“. The second field is public, it is of “double” type and named “d“. Two methods GetI1() and GetI2() are declared in the class. These methods return the value of field “i“. These methods do the same work, but access to the field “i” is realized differently. The same applies to GetD1() and GetD2(). In the class constructor DemoThis(int, double), that takes two parameters, the keyword “this” used for access to the fields i and d of class. This is due to the fact that the local parameters, that are variables of the constructor with the same names (i and d), hide the data members of declared class.

class DemoThis
{
    private int i;   // private field
    public double d; // public field

    // the constructor, which zeroes the fields
    public DemoThis()
    {
        i = 0; // without the word "this"
        this.d = 0.00; // by using "this"
    }

    // The given constructor demonstrates the benefit of using keyword "this".
    // The constructor takes two parameters, names of them are the same
    // as names of fields of class
    public DemoThis(int i, double d)
    {
        this.i = i; // this.i - class field: private int i;
        this.d = d; // this.d - class field: public double d;
    }

    // returning from the method without using "this"
    public int GetI1()
    {
        return i;
    }

    // using the reference "this" at the own class
    public int GetI2()
    {
        return this.i;
    }

    public double GetD1()
    {
        return d; // without reference "this"
    }

    public double GetD2()
    {
        return this.d; // using the reference "this"
    }
}

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16. Availability levels of a class in the assembly. Example

In C#, it is possible to hide a class (type) in the current assembly or make it accessible from other assemblies (different code).

The levels of class’s accessibility may be as follows:

public – open accessibility. When declaring a class with the public accessibility level, this class is accessible from any code in any assembly;
internal – in this case, the class is available within the current assembly.

If the word public or internal is absent before the word class, then the default access level is set within the current assembly (internal).

Приклад.

// Declaration of public class 
public class MyClass1
{
  // ...
}

// Declaring a class available within the current assembly
internal class MyClass2
{
  // ...
}

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