Introduction to Object Oriented Programming in C#

Transcription

1 Introduction to Object Oriented Programming in C# Chapters 7 and 8 ( ) Frank Ducrest

2 Introduction to Object-Oriented Programming in C# 2 1. Classes and Objects in C#

3 Introduction to Object-Oriented Programming in C# 3 C# Classes Definition of a type of thing; a data type Objects are created from classes; i. e. objects are variables of the data type defined by the class Classes are made up of members

4 Introduction to Object-Oriented Programming in C# 4 Class Relationships Classes can be related by Association simple interaction between classes; one class calls a method of another class Composition has a ; a class contains instances of other classes Specialization is a ; class is built from other classes; inheritance; a form of polymorphism Specialized class inherits properties from the general class Specialized class derived class; child class General class base class; parent class

5 Introduction to Object-Oriented Programming in C# 5 Defining a Class Creating a class Syntax of a class definition: [attributes] [access modifiers] class identifier [: base class] class body where identifier is the name of the class base class is the class that is being inherited class body is the set of members of the class

6 Introduction to Object-Oriented Programming in C# 6 Meta Data Data about data Attributes means of adding meta data to a C# class Reflection means for a running program to read meta data about its own components Meta data is, in general, a more advanced topic not covered in 207

7 Introduction to Object-Oriented Programming in C# 7 Access Modifiers Regulate what is visible to other classes; controls scope in object oriented programming A class and anything declared as a member of a class may be given an access modifier

8 Introduction to Object-Oriented Programming in C# 8 Access Modifiers (continued) List of access modifiers: public accessible to any method of any class private accessible only within the containing class; not inheritable; default access protected accessible only within the containing class and classes derived from the containing class; inheritable private internal accessible to methods of classes in the assembly (single executable or dll) of the class protected internal inheritable internal

9 Introduction to Object-Oriented Programming in C# 9 Class Body Contains the class members

10 Introduction to Object-Oriented Programming in C# 10 Class Members Anything declared at class level; i. e. anything declared immediately inside the code block of body of the class Any member is accessible throughout the class body Class members include Fields state variables and named constants Methods also called method functions; define capabilities and and responsibilities Properties a cross between a field and a method Constructors method like; used to initialize objects at declaration All objects of a class include all members of the class

11 Introduction to Object-Oriented Programming in C# 11 More on Variable Types Value type variable named variable created from an intrinsic type; the memory given to the variable contains the data of the type Reference type variable reference object; named variable created from a class type that can contain the address of an unnamed dynamic object created with new operator Classes are reference types Instance Object object variable; variable object; instance variable; unnamed variable created dynamically from a class type with new operator; may be referenced type a reference type variable

12 Introduction to Object-Oriented Programming in C# 12 More on Variable Types (continued) Value type variables and Reference type variables are copied by the assignment operator (=) and when passed as an actual parameter (parameter in the call) to a formal parameter (parameter of a method or constructor) All named variables are pass by value by default because the value of the variable is copied Only the address of an Instance Object is copied by the assignment operator (=) or when passed as an actual parameter (parameter in the call) to a formal parameter (parameter of a method or constructor) All instance objects are pass by reference because the reference address value (in the variable) is copied

13 Introduction to Object-Oriented Programming in C# 13 More on Variable Types (continued) What is copied by assignment or by parameter passing is always the same, i. e. what is copied is what is available to copy. The value of an intrinsic type variable is a value of the intrinsic type. The value of a reference type variable is an address. If a value of an expression is available, the value is copied. If an address is available, an address is copied.

14 Introduction to Object-Oriented Programming in C# 14 Constructors, Objects and Operator new Constructor a method like function that is called to initialize variables of a class with operator new Named for class No return type Multiple constructors are normal, each one having a different number of parameters All classes have a default constructor, either created by the class programmer or provided by the compiler

15 Introduction to Object-Oriented Programming in C# 15 Constructors, Objects and Operator new (continued) Given class Dog public class Dog... class body... A reference variable of type Dog can be created by Dog sheltie; But... this does not create an instance object of class Dog, just a reference type variable that can hold the address of the instance object

16 Introduction to Object-Oriented Programming in C# 16 Constructors, Objects and Operator new (continued) To create a variable of class Dog, new must be used to call a Dog class constructor: sheltie = new Dog(); This process creates two variables: the named reference variable and the unnamed dynamically created instance object Memory for reference variable sheltie address Memory for unnamed Dog instance object data

17 Introduction to Object-Oriented Programming in C# 17 The Memory Stack Memory Stack a region of memory given to a running program where memory is allocated contiguously in the sequence that methods are called A method call results in an allocation of memory from the stack; named variables (value type and reference objects) created inside the method receive use part of that allocated memory When a method completes (exit), all memory allocated to it is returned to the stack The memory of the calling method is always directly under the memory of the called method

18 Introduction to Object-Oriented Programming in C# 18 The Memory Heap Memory Heap a region of memory used to allocate memory for dynamic requests A call to operator new returns the address of a location on the heap that has been allocated to an instance object Garbage Collection automatic return of previously allocated memory to the heap All instance variables are garbage collected when no longer needed

19 Introduction to Object-Oriented Programming in C# Example Object Oriented Program

20 Introduction to Object-Oriented Programming in C# 20 Example of a Simple Class public class SimpleBook... class-body...

21 Introduction to Object-Oriented Programming in C# 21 Example of a Simple Class (continued): Field Declarations private string title; private int quantityonhand = 0; Assignment of a value during a field declaration is called an initializer.

22 Introduction to Object-Oriented Programming in C# 22 Example of a Simple Class (continued): Overriding the Default Constructor public SimpleBook() title = "-nothing-";

23 Introduction to Object-Oriented Programming in C# 23 Example of a Simple Class (continued): A Second Constructor public SimpleBook(string title, int quantity) this.title = title; quantityonhand = quantity; The assignment to quantityonhand in the constructor occurs after the assignment in the initializer. this is a hidden reference variable to the class field variable title; all field variables have a this Example of polymorphism Constructor used will depend on the number of parameters in the call (compiler depends on the signature)

24 Introduction to Object-Oriented Programming in C# 24 Example of a Simple Class (continued): Method Functions public void settitle(string t) if (t.length > 0) title = t; public string gettitle() return title; public int getquantityonhand() return quantityonhand; public void receivebook() quantityonhand++; public void sellbook() quantityonhand--;

25 Introduction to Object-Oriented Programming in C# 25 Using Class SimpleBook SimpleBook sb1 = new SimpleBook(); SimpleBook sb2 = new SimpleBook("The Atkins Diet", 10); Console.WriteLine("0, 1 copies in stock ", sb1.gettitle(), sb1.getquantityonhand()); Console.WriteLine("0, 1 copies in stock ", sb2.gettitle(), sb2.getquantityonhand()); Console.WriteLine(); sb1.settitle("paul Clifford"); for (int i=0; i<13; i++) sb1.receivebook(); sb2.receivebook(); sb2.receivebook(); Console.WriteLine("0, 1 copies in stock ", sb1.gettitle(), sb1.getquantityonhand()); Console.WriteLine("0, 1 copies in stock ", sb2.gettitle(), sb2.getquantityonhand());

26 Introduction to Object-Oriented Programming in C# 26 Expanding SimpleBook with a static Field public class SimpleBook // declaration of field variables... static private int totalbooks = 0;... public SimpleBook(string title, int quantity)... totalbooks += quantity;

27 Introduction to Object-Oriented Programming in C# 27 Expanding SimpleBook with a static Field... public void receivebook()... totalbooks++; public void sellbook()... totalbooks--; public int gettotalbooks() return totalbooks;

28 Introduction to Object-Oriented Programming in C# 28 Using Expanded Class SimpleBook class Program static void Main(string[] args) SimpleBook sb1 = new SimpleBook(); SimpleBook sb2 = new SimpleBook("The Atkins Diet", 10);... Console.WriteLine("sb1 reports thatthere are 0 copies " + "of all books in stock ", sb1.gettotalbooks()); Console.WriteLine("sb2 reports thatthere are 0 copies " + "of all books in stock ", sb2.gettotalbooks());...

29 Introduction to Object-Oriented Programming in C# Passing Intrinsic Types by Reference

30 Introduction to Object-Oriented Programming in C# 30 Passing an Intrinsic Type by Reference C# does not have a pass by reference operator for formal parameters (such as C++ does: &) Instead, either the ref or out modifiers are applied to both the formal and actual parameters to convert intrinsic types to reference types ref requires actual parameter initialization out does not require actual parameter initialization, but requires assignment of a value to formal parameter in method

31 Introduction to Object-Oriented Programming in C# 31 Passing an Intrinsic Type by Reference (Continued) private int a = 23; public void GetCopyA(ref int copya) copya = a; public Program() int b = 0; // must be initialized before call GetCopyA(ref b); System.Console.WriteLine("0", b); System.Console.ReadLine();

32 Introduction to Object-Oriented Programming in C# 32 Passing an Intrinsic Type by Reference (Continued) private int a = 23; public void GetCopyA(out int copya) copya = a; public Program() int b; // does not need initialized before call GetCopyA(out b); System.Console.WriteLine("0", b); System.Console.ReadLine();

33 Introduction to Object-Oriented Programming in C# is and as

34 Introduction to Object-Oriented Programming in C# 34 is Operator Boolean operator that returns true is an object is of a given type Example: SomeClass var1 = new SomeClass();... object obj = var1;... SomeClass var2; if (o is SomeClass) var2 = (SomeClass)var1;

35 Introduction to Object-Oriented Programming in C# 35 as Operator Expression operator that evaluates to a cast of the left operand to the type used as the right operand; if the types are not compatible, null is returned Example: SomeClass var1 = new SomeClass();... object obj = var1;... SomeClass var2 = obj as SomeClass; if (obj!= null) //... do something with var2...