object-oriented programming in c# object-oriented cse 668 prof. roger crawfis

Programming in C#

Object-OrientedObject-Oriented

CSE 668Prof. Roger Crawfis

Key Object-Oriented Concepts

Objects, instances and classesIdentity

Every instance has a unique identity, regardless of its data

EncapsulationData and function are packaged together Information hidingAn object is an abstraction

User should NOT know implementation details


Interfaces A well-defined contract A set of function members

Types An object has a type, which specifies its interfaces

and their implementations Inheritance

Types are arranged in a hierarchy Base/derived, superclass/subclass

Interface vs. implementation inheritance


Polymorphism The ability to use an object without knowing its

precise type Three main kinds of polymorphism

Inheritance Interfaces Reflection

Dependencies For reuse and to facilitate development, systems

should be loosely coupled Dependencies should be minimized

Programming in C#Inheritance and PolymorphismInheritance and Polymorphism


C# Classes

Classes are used to accomplish: Modularity: Scope for global (static) methods Blueprints for generating objects or instances:

Per instance data and method signatures

Classes support Data encapsulation - private data and

implementation. Inheritance - code reuse

Inheritance

Inheritance allows a software developer to derive a new class from an existing one.

The existing class is called the parent, super, or base class.

The derived class is called a child or subclass. The child inherits characteristics of the parent.

Methods and data defined for the parent class. The child has special rights to the parents methods

and data. Public access like any one else Protected access available only to child classes (and their

descendants). The child has its own unique behaviors and data.

Inheritance

Inheritance relationships are often shown graphically in a class diagram, with the arrow pointing to the parent class.

Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent.

Animal

Bird

Examples: Base Classes and Derived Classes

Base class Derived classes Student GraduateStudent

UndergraduateStudent

Shape Circle Triangle Rectangle

Loan CarLoan HomeImprovementLoan MortgageLoan

Employee FacultyMember StaffMember

Account CheckingAccount SavingsAccount

Declaring a Derived Class

Define a new class DerivedClass which extends BaseClass

class BaseClass { // class contents}

class DerivedClass : BaseClass { // class contents}

Controlling Inheritance

A child class inherits the methods and data defined for the parent class; however, whether a data or method member of a parent class is accessible in the child class depends on the visibility modifier of a member.

Variables and methods declared with private visibility are not accessible in the child class However, a private data member defined in the parent class is

still part of the state of a derived class. Variables and methods declared with public visibility

are accessible; but public variables violate our goal of encapsulation

There is a third visibility modifier that helps in inheritance situations: protected.

+ public- private# protected

The protected Modifier

Variables and methods declared with protected visibility in a parent class are only accessible by a child class or any class derived from that class

Book# pages : int

+ GetNumberOfPages() : void

Dictionary- definition : int

+ PrintDefinitionMessage() : void

Single Inheritance

Some languages, e.g., C++, allow Multiple inheritance, which allows a class to be derived from two or more classes, inheriting the members of all parents.

C# and Java support single inheritance, meaning that a derived class can have only one parent class.

Overriding Methods

A child class can override the definition of an inherited method in favor of its own

That is, a child can redefine a method that it inherits from its parent

The new method must have the same signature as the parent's method, but can have a different implementation.

The type of the object executing the method determines which version of the method is invoked.

Class Hierarchies

A child class of one parent can be the parent of another child, forming a class hierarchy Animal

Reptile Bird Mammal

Snake Lizard BatHorseParrot

Class Hierarchies

CommunityMember

Employee Student Alumnus

Faculty Staff

Professor Instructor

GraduateUnder

Class Hierarchies

Shape

TwoDimensionalShape ThreeDimensionalShape

Sphere Cube CylinderTriangleSquareCircle

Class Hierarchies

An inherited member is continually passed down the line Inheritance is transitive.

Good class design puts all common features as high in the hierarchy as is reasonable. Avoids redundant code.

References and Inheritance

An object reference can refer to an object of its class, or to an object of any class derived from it by inheritance.

For example, if the Holiday class is used to derive a child class called Christmas, then a Holiday reference can be used to point to a Christmas object.

Holiday day;day = new Holiday();…day = new Christmas();

Dynamic Binding

A polymorphic reference is one which can refer to different types of objects at different times. It morphs!

The type of the actual instance, not the declared type, determines which method is invoked.

Polymorphic references are therefore resolved at run-time, not during compilation.This is called dynamic binding.

Dynamic Binding

Suppose the Holiday class has a method called Celebrate, and the Christmas class redefines it (overrides it).

Now consider the following invocation:

day.Celebrate(); If day refers to a Holiday object, it

invokes the Holiday version of Celebrate; if it refers to a Christmas object, it invokes the Christmas version

Overriding Methods

C# requires that all class definitions communicate clearly their intentions.

The keywords virtual, override and new provide this communication.

If a base class method is going to be overridden it should be declared virtual.

A derived class would then indicate that it indeed does override the method with the override keyword.

Overriding Methods

If a derived class wishes to hide a method in the parent class, it will use the new keyword.

This should be avoided.

Overloading vs. Overriding

Overloading deals with multiple methods in the same class with the same name but different signatures

Overloading lets you define a similar operation in different ways for different data

Example:int foo(string[] bar);int foo(int bar1, float a);

Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature

Overriding lets you define a similar operation in different ways for different object types

Example:class Base { public virtual int foo() {} }class Derived { public override int foo() {}}

Polymorphism via Inheritance

StaffMember# name : string# address : string# phone : string+ ToString() : string+ Pay() : double

Volunteer

+ Pay() : double

Employee

# socialSecurityNumber : String# payRate : double

+ ToString() : string+ Pay() : double

Executive

- bonus : double

+ AwardBonus(execBonus : double) : void+ Pay() : double

Hourly

- hoursWorked : int

+ AddHours(moreHours : int) : void+ ToString() : string+ Pay() : double

Widening and Narrowing

Assigning an object to an ancestor reference is considered to be a widening conversion, and can be performed by simple assignment

Holiday day = new Christmas();

Assigning an ancestor object to a reference can also be done, but it is considered to be a narrowing conversion and must be done with a cast:

Christmas christ = new Christmas();Holiday day = christ;Christmas christ2 = (Christmas)day;

Widening and Narrowing

Widening conversions are most common. Used in polymorphism.

Note: Do not be confused with the term widening or narrowing and memory. Many books use short to long as a widening conversion. A long just happens to take-up more memory in this case.

More accurately, think in terms of sets: The set of animals is greater than the set of parrots. The set of whole numbers between 0-65535

(ushort) is greater (wider) than those from 0-255 (byte).

Type Unification

Everything in C# inherits from object Similar to Java except includes value types. Value types are still light-weight and handled

specially by the CLI/CLR. This provides a single base type for all

instances of all types. Called Type Unification

The System.Object Class

All classes in C# are derived from the Object class if a class is not explicitly defined to be the child of an existing

class, it is a direct descendant of the Object class The Object class is therefore the ultimate root of all class

hierarchies. The Object class defines methods that will be shared by

all objects in C#, e.g., ToString: converts an object to a string representation Equals: checks if two objects are the same GetType: returns the type of a type of object

A class can override a method defined in Object to have a different behavior, e.g., String class overrides the Equals method to compare the

content of two strings

Programming in C#

PropertiesProperties


Properties

Typical pattern for accessing fields.private int x;public int GetX();public void SetX(int newVal);

Elevated into the language:private int count;public int Count { get { return count; } set { count = value; }}

Typically there is a backing-store, but not always.

Properties

Using a property is more like using a public field than calling a function:FooClass foo;

int count = foo.Count; // calls get

int count = foo.count; // compile error

The compiler automatically generates the routine or in-lines the code.

Properties

Properties can be used in interfacesCan have three types of a property

read-write, read-only, write-onlyMore important with WPF and

declarative programming.// read-only property declaration

// in an interface.

int ID { get; };

Automatic Properties

C# 3.0 added a shortcut version for the common case (or rapid prototyping) where my get and set just read and wrote to a backing store data element.

Avoids having to declare the backing store. The compiler generates it for you implicitly.public decimal CurrentPrice { get; set; }

Programming in C#

InterfacesInterfaces


Interfaces

An interface defines a contract An interface is a type Contain definitions for methods, properties,

indexers, and/or events Any class or struct implementing an interface must

support all parts of the contract Interfaces provide no implementation

When a class or struct implements an interface it must provide the implementations

Interfaces

Interfaces provide polymorphismMany classes and structs may implement

a particular interface.Hence, can use an instance of any one of

these to satisfy a contract.Interfaces may be implemented either:

Implicitly – contain methods with the same signature. The most common approach.

Explicitly – contain methods that are explicitly labeled to handle the contract.

public interface IDelete { void Delete();}public class TextBox : IDelete { public void Delete() { ... }}public class Car : IDelete { public void Delete() { ... }}

TextBox tb = new TextBox();tb.Delete();

Car c = new Car();iDel = c;iDel.Delete();

Interfaces Example

Explicit Interfaces

Explicit interfaces require the user of the class to explicitly indicate that it wants to use the contract.

Note: Most books seem to describe this as a namespace conflict solution problem. If that is the problem you have an extremely poor software design. The differences and when you want to use them are more subtle.

Explicit Interfaces

namespace OhioState.CSE494R.InterfaceTest{ public interface IDelete { void Delete(); }

public class TextBox : IDelete { #region IDelete Members void IDelete.Delete() { ... } #endregion

}}

TextBox tb = new TextBox();tb.Delete(); // compile error

iDel = tb;iDel.Delete();

Explicit Interfaces

The ReadOnlyCollection<T> class is a good example of using an explicit interface implementation to hide the methods of the IList<T> interface that allow modifications to the collection.

Calling Add() will result in a compiler error if the type is ReadOnlyCollection.

Calling IList.Add() will throw a run-time exception .

interface IControl { void Paint();}interface IListBox: IControl { void SetItems(string[] items);}interface IComboBox: ITextBox, IListBox {}

Interfaces Multiple Inheritance

Classes and structs can inherit from multiple interfaces

Interfaces can inherit from multiple interfaces

Programming in C#

StructsStructs


Classes vs. Structs

Both are user-defined types Both can implement multiple interfaces Both can contain

Data Fields, constants, events, arrays

Functions Methods, properties, indexers, operators, constructors

Type definitions Classes, structs, enums, interfaces, delegates

Class Struct

Reference type Value type

Can inherit from any non-sealed reference type

No inheritance(inherits only from System.ValueType)

Can have a destructor No destructor

Can have user-defined parameterless constructor

No user-defined parameterless constructor

Classes vs. Structs

C++ Struct C# Struct

Same as C++ class, but all members are public

User-defined value type

Can be allocated on the heap, on the stack or as a member (can be used as value or reference)

Always allocated on the stack or in-lined as a member field

Members are always publicMembers can be public, internal or private

C# Structs vs. C++ Structs

Very different from C++ struct

public class Car : Vehicle {

public enum Make { GM, Honda, BMW }

private Make make;

private string vid;

private Point location;

Car(Make make, string vid, Point loc) {

this.make = make;

this.vid = vid;

this.location = loc;

}

public void Drive() { Console.WriteLine(“vroom”); }

}

Car c = new Car(Car.Make.BMW, “JF3559QT98”, new Point(3,7));c.Drive();

Class Definition

public struct Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public int X { get { return x; } set { x = value; } } public int Y { get { return y; } set { y = value; } }}

Point p = new Point(2,5);p.X += 100;int px = p.X; // px = 102

Struct Definition

Programming in C#

ModifiersModifiers


Static vs. Instance Members

By default, members are per instanceEach instance gets its own fieldsMethods apply to a specific instance

Static members are per typeStatic methods can’t access instance dataNo this variable in static methods

Singleton Design Pattern

public class SoundManager {

private static SoundManager instance;

public static SoundManager Instance {

get { return instance; }

}

private static SoundManager() {

instance = new SoundManager();

}

private SoundManager() {

…

}

}

Static property– returns the reference to an instance of a SoundManager

Access Modifiers

Access modifiers specify who can use a type or a member

Access modifiers control encapsulationClass members can be public, private,

protected, internal, or protected internalStruct members can be public, private or

internal

If the access modifier is

Then a member defined in type T and assembly A is accessible

public to everyone

private within T only

protected to T or types derived from T

internal to types within A

protected internal

to T or types derived from T-or- to types within A

Access Modifiers

Access Defaults

You should always explicitly mark what access you want.

Class definitions default to internal.Member fields, methods and events

default to private for classesMember methods and events for

interfaces must be public, so you can not specify an access modifier for interfaces.

Abstract Classes

An abstract class can not be instantiatedIntended to be used as a base classMay contain abstract and non-abstract

function membersA pure abstract class has no

implementation (only abstract members) and is similar to an interface.

Sealed Classes

A sealed class is one that cannot be used as a base class.

Sealed classes can not be abstractAll structs are implicitly sealedPrevents unintended derivationAllows for code optimization

Virtual function calls may be able to be resolved at compile-time

Programming in C#

Class InternalsClass Internals


The this keyword is a predefined variable available in non-static function members Used to access data and function members

unambiguouslypublic class Person { private string name; public Person(string name) { this.name = name; } public void Introduce(Person p) { if (p != this) Console.WriteLine(“Hi, I’m “ + name); }}

this

name is a parameter

and a field.

base

The base keyword can be used to access class members that are hidden by similarly named members of the current class

public class Shape { private int x, y; public override string ToString() { return "x=" + x + ",y=" + y; }}internal class Circle : Shape { private int r; public override string ToString() { return base.ToString() + ",r=" + r; }}

public class MyClass { public const string version = “1.0.0”; public const string s1 = “abc” + “def”; public const int i3 = 1 + 2; public const double PI_I3 = i3 * Math.PI; public const double s = Math.Sin(Math.PI); //ERROR ...}

Constants

A constant is a data member that is evaluated at compile-time and is implicitly static (per type)e.g. Math.PI

Fields

A field or member variable holds data for a class or struct

Can hold:A built-in value typeA class instance (a reference)A struct instance (actual data)An array of class or struct instances

(an array is actually a reference)An event

Readonly Fields

Similar to a const, but is initialized at run-time in its declaration or in a constructor Once initialized, it cannot be modified

Differs from a constant Initialized at run-time (vs. compile-time)

Don’t have to re-compile clients

Can be static or per-instance

public class MyClass { public static readonly double d1 = Math.Sin(Math.PI); public readonly string s1; public MyClass(string s) { s1 = s; } }

Methods

All code executes in a methodConstructors, destructors and operators are

special types of methodsProperties and indexers are implemented

with get/set methodsMethods have argument listsMethods contain statementsMethods can return a value

Methods may be virtual or non-virtual (default) Non-virtual methods are not polymorphic Abstract methods are implicitly virtual.

internal class Foo { public void DoSomething(int i) { ... }}

Virtual Methods

Foo f = new Foo();f.DoSomething(6);

public class Shape { public virtual void Draw() { ... }}internal class Box : Shape { public override void Draw() { ... }}internal class Sphere : Shape { public override void Draw() { ... }}

protected void HandleShape(Shape s) { s.Draw(); ...}

HandleShape(new Box());HandleShape(new Sphere());HandleShape(new Shape());

Virtual Methods

Abstract Methods

An abstract method is virtual and has no implementation

Must belong to an abstract classUsed as placeholders or handles where

specific behaviors can be defined.Supports the Template design pattern.

public abstract class Shape { public abstract void Draw();}internal class Box : Shape { public override void Draw() { ... }}internal class Sphere : Shape { public override void Draw() { ... }}

private void HandleShape(Shape s) { s.Draw(); ...}

HandleShape(new Box());HandleShape(new Sphere());HandleShape(new Shape()); // Error!

Abstract Methods

Method Versioning

Must explicitly use override or new keywords to specify versioning intent

Avoids accidental overridingMethods are non-virtual by defaultC++ and Java produce fragile base

classes – cannot specify versioning intent

Programming in C#

ConstructorsConstructors


Constructors

Instance constructors are special methods that are called when a class or struct is instantiated

Performs custom initialization Can be overloaded If a class doesn’t define any constructors, an

implicit parameterless constructor is created Cannot create a parameterless constructor for

a struct All fields initialized to zero/null

Constructor Initializers

One constructor can call another with a constructor initializer

Use the this keyword. The called constructor will execute before the body of the current constructor.

internal class B { private int h; public B() : this(12) { } public B(int h) { this.h = h; }}


The base keyword is also used to control the constructors in a class hierarchy:public class Volunteer : Employee

{

public Volunteer( string name )

: base(name)

{

}

}


internal class B { private int h; public B() : this(12) { } public B(int h) { this.h = h; }}internal class D : B { private int i; public D() : this(24) { } public D(int i) { this.i = i; } public D(int i, int h) : base(h) { this.i = i; }}

Static Constructors

A static constructor lets you create initialization code that is called once for the class

Guaranteed to be executed before the first instance of a class or struct is created and before any static member of the class or struct is accessed

No other guarantees on execution order Only one static constructor per type Must be parameterless

Singleton Design Pattern

public class SoundManager {

private static SoundManager instance;

public static SoundManager Instance {

get { return instance; }

}

private static SoundManager() {

instance = new SoundManager();

}

private SoundManager() {

…

}

}

Static constructor– called once per type– not user-callable (private)

Instance constructor – marked private

Destructors

A destructor is a method that is called before an instance is garbage collected

Used to clean up any resources held by the instance, do bookkeeping, etc.

Only classes, not structs can have destructors Also called Finalizers.

internal class Foo { private ~Foo() { Console.WriteLine(“Destroyed {0}”, this); }}

Destructors

Unlike C++, C# destructors are non-deterministic

They are not guaranteed to be called at a specific time

They are guaranteed to be called before shutdown

You can not directly call the destructor Slows down the garbage collection if you

define one, so don’t unless you have to.

Dispose Design Pattern

Use the using statement and the IDisposable interface to achieve deterministic clean-up of unmanaged resources.

The destructor optionally calls a public Dispose method, that is also user-callable.

Programming in C#

OperatorsOperators


User-defined operatorsMust be a static method

internal class Car { private string vid; public static bool operator ==(Car x, Car y) { return x.vid == y.vid; }}

Operator Overloading


Overloadable unary operators

+ - ! ~

true false ++ --

Overloadable binary operators+ - * / ! ~

% & | ^ == !=

<< >> < > <= >=


No overloading for member access, method invocation, assignment operators, nor these operators: sizeof, new, is, as, typeof, checked, unchecked, &&, ||, and ?:

Overloading a binary operator (e.g. *) implicitly overloads the corresponding assignment operator (e.g. *=)


public struct Vector { private int x, y; public Vector(int x,int y) { this.x = x; this.y = y; } public static Vector operator +(Vector a, Vector b) { return new Vector(a.x + b.x, a.y + b.y); } public static Vector operator*(Vector a, int scale) { return new Vector(a.x * scale, a.y * scale); } public static Vector operator*(int scale, Vector a) { return a * scale; }}

Can also specify user-defined explicit and implicit conversions

internal class Note { private int value; // Convert to hertz – no loss of precision public static implicit operator double(Note x) { return ...; } // Convert to nearest note public static explicit operator Note(double x) { return ...; }}

Note n = (Note)442.578;double d = n;

Conversion Operators

The is Operator

The is operator is used to dynamically test if the run-time type of an object is compatible with a given type

private static void DoSomething(object o) { if (o is Car) ((Car)o).Drive();}

The as Operator

The as operator tries to convert a variable to a specified type; if no such conversion is possible the result is null

More efficient than using is operatorCan test and convert in one operation

private static void DoSomething(object o) { Car c = o as Car; if (c != null) c.Drive();}

object-oriented programming in c# object-oriented cse 668 prof. roger crawfis

Documents

parent class

class contents

existing class

class diagram

baseclass class baseclass

child classes

data encapsulation data

new class derivedclass