C#: INTRODUCTION

FOR DEVELOPERS

Neal Stublen

nstublen@jccc.edu

INDEXERS

What’s an indexer?

An array has an indexer…

int[] myArray = new int[5];for (int index = 0; index < 5; ++index){ myArray[index] = 3;}

The [ ] is the indexer.

What’s an indexer?

An indexer accesses one value by using another value

A special property: public <type> this[<index_type> <index_value>]

The index_type is often an integer, but can be another data type

Implemented on collection classes Throws an ArgumentException

ArgumentNullExceptionArgumentOutOfRangeException

Indexer Exampleclass MyThing{}

class MyCollection{ public void AddThing(MyThing inThing) { ... }

public MyThing this[int index] { get { ... } set { ... } }}

DELEGATES AND EVENTS

Start with an Example

What’s a delegate?

A delegate specifes a method signature for an event

A special data type:public delegate <return_type> DelegateName([parameters_list]);

Delegate Example// The delegate type declarationpublic delegate void NameChangedEventHandler(object sender, EventArgs e);

class NameObject{ // The event member public event NameChangedEventHandler NameChanged;

public void setName(string inName) { mName = inName; if (NameChanged != null) { NameChanged(this); } }}

Connecting to an Eventclass SomeObject{ private NameObject mTheName = new NameObject();

public void Init() { mTheName.NameChanged += new NameChangedEventHandler(EventHandler) }

private void EventHandler(object sender) { }}

OPERATOR OVERLOADING

What operations?

Unary operators+, -, !, ++, --, true, falsepublic static <return_type>operator <operator>(<operand_type>)

Binary operators+, -, *, /, %, &, |, ==, !=, >, <, >=, <=public static <return_type>operator <operator>(<operand_type_1>,

<operand_type_2>)

Operator Exampleclass MyCollection{ private List<object> mList;

public static MyCollection operator +(MyCollection col, object newObject)

{ col.mList.Add(newObject); return col; }}

collection += new object();

Overloading ==

Must also overload Equals() Must also overload GetHashCode() Must also overload !=

Must overload relational operators in pairs (<, >), (<=, >=), (==, !=)

Practice Exercise

Exercise 13-1, p. 418

CLASS INHERITANCE

What is inheritance? One class inherits the attributes and

behaviors of another class The base class should be a larger

classification of the derived class (ex. a Control is a larger classification for Button)A Button “is-a” Control; a Button “has-a”

BackColorA Book “is-a” Product; a Book “has-a” Publisher

The derived class extends or overrides behavior

.NET Inheritance

All classes implicitly inherit from System.ObjectGetType()ToString()Equals()ReferenceEquals()GetHashCode()Finalize()MemberwiseClone()

How does inheritance work?

class Fourth class Third class Second class First

methodA

methodB methodB

methodC methodC methodC

methodD methodD

methodE methodE

Fourth fourth = new Fourth();fourth.methodA();fourth.methodB();fourth.methodC();fourth.methodD();fourth.methodE();

First first = new Fourth();first.methodA();first.methodB();first.methodC();first.methodD();first.methodE();

“Polymorphism”The base class can take many different

forms.

Polymorphism

List<First> myList = new List<First>();myList.Add(new First());myList.Add(new Second());myList.Add(new Third());myList.Add(new Fourth());

foreach (First item in myList){ item.methodB(); item.methodC();}

How do we “do” inheritance?

Keywords: virtual, override A base class declares a method as

virtual A derived class declares a method as

override public/private =>

public/private/protected/internal Reference base class using “base.”

Inherited Classesclass First{ public virtual void methodA() { }}

class Second : First{ public override void methodA() { base.methodA(); }}

Considerations for Inheritance

Confirm “is-a” versus “has-a” relationship

Does adding one or more properties make sense?

Would an interface be more beneficial? Implicit and explicit casting between

inherited types Using the “as” operator instead of

casting to avoid exceptions

Abstract Classes

Abstract classes cannot be instantiated, and can only serve as a base class

Abstract methods and properties must be overridden in a derived classYou know the method or property exists for

every object of this type, but there is no implementation at this level of abstraction.

Sealed Classes

Sealed classes cannot be inherited Sealed methods and properties cannot

be overridden

Start thinking about how objects you need to model may inherit from one another.

Are there any obvious heirarchies, common attributes, or shared behavior?

Suggestions

Try to work through Exercises 14-1 and 14-2 in the book (p. 457)