Object-Oriented Programming in C# Object-Oriented CSE 668 Prof. Roger Crawfis

Key Object-Oriented Concepts Objects, instances and classes Identity Every instance has a unique identity, regardless of its data Encapsulation Data and function are packaged together Information hiding An object is an abstraction User should NOT know implementation details

Key Object-Oriented Concepts 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

Key Object-Oriented Concepts 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

Inheritance and Polymorphism Programming in C# Inheritance and Polymorphism CSE 668 Prof. Roger Crawfis

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

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 ReptileBirdMammal SnakeLizardBatHorseParrot

Class Hierarchies CommunityMember EmployeeStudentAlumnus FacultyStaff ProfessorInstructor GraduateUnder

Class Hierarchies Shape TwoDimensionalShapeThreeDimensionalShape SphereCubeCylinderTriangleSquareCircle

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 (ushort) is greater (wider) than those from (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

Properties Programming in C# Properties CSE 668 Prof. Roger Crawfis

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 interfaces Can have three types of a property read-write, read-only, write-only More 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; }

Interfaces Programming in C# Interfaces CSE 668 Prof. Roger Crawfis

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 polymorphism Many 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 class is a good example of using an explicit interface implementation to hide the methods of the IList 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

Structs Programming in C# Structs CSE 668 Prof. Roger Crawfis

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

ClassStruct Reference typeValue type Can inherit from any non-sealed reference type No inheritance (inherits only from System.ValueType ) Can have a destructorNo destructor Can have user-defined parameterless constructor No user-defined parameterless constructor Classes vs. Structs

C++ StructC# 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 public Members 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

Modifiers Programming in C# Modifiers CSE 668 Prof. Roger Crawfis

Static vs. Instance Members By default, members are per instance Each instance gets its own fields Methods apply to a specific instance Static members are per type Static methods can’t access instance data No 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 encapsulation Class members can be public, private, protected, internal, or protected internal Struct 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 classes Member 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 instantiated Intended to be used as a base class May contain abstract and non-abstract function members A 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 abstract All structs are implicitly sealed Prevents unintended derivation Allows for code optimization Virtual function calls may be able to be resolved at compile-time

Class Internals Programming in C# Class Internals CSE 668 Prof. Roger Crawfis

The this keyword is a predefined variable available in non-static function members Used to access data and function members unambiguously public 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 type A 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 method Constructors, destructors and operators are special types of methods Properties and indexers are implemented with get/set methods Methods have argument lists Methods contain statements Methods 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 class Used 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 overriding Methods are non-virtual by default C++ and Java produce fragile base classes – cannot specify versioning intent

Constructors Programming in C# Constructors CSE 668 Prof. Roger Crawfis

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; } }

Constructor Initializers The base keyword is also used to control the constructors in a class hierarchy: public class Volunteer : Employee { public Volunteer( string name ) : base(name) { }

Constructor Initializers 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.

Operators Programming in C# Operators CSE 668 Prof. Roger Crawfis

User-defined operators Must 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 +-!~ truefalse++-- Overloadable binary operators +-*/!~ %&|^==!= <<>><><=>=

Operator Overloading 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. *=)

Operator Overloading 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) ; 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 operator Can test and convert in one operation private static void DoSomething(object o) { Car c = o as Car; if (c != null) c.Drive(); }