Neal Stublen

Tonight’s Agenda  Indexers  Delegates and events  Operator overloading  Class inheritance  Q&A

Review  What methods can you use to get data into and out of a form? Form.Tag property Add methods and properties to the form

Review  What issues might make you think twice about using the Form.Tag property? It’s a generic Object type, so there are no compiler checks on the data type

Review  Using the Debug class to trace program execution Stepping through class construction ○ Multiple constructors Stepping through properties ○ get and set Stepping through form construction and initialization ○ Construction and initialization

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 this[ ]  The index_type is often an integer, but can be another data type  Implemented on collection classes

What’s an indexer?  Indexers can create classes that act like “virtual arrays” They have the array syntax, but they are not actually allocated as arrays  A special class property: public this[ ]  The index_type is often an integer, but can be another data type

Invalid Indexes  Using an invalid index value throws an ArgumentException ArgumentNullException ArgumentOutOfRangeException

Files as Byte Arrays class FileArray { public byte this[long index] { get { // read byte at index } set { // write byte value at index }

Books as Page Arrays class Book { public Page this[long index] { get { // return page “index” } set { // insert page at “index” }

What’s a delegate?  A delegate specifes a method signature for an event  Declare a delegate data type: public delegate DelegateName([parameters_list]);

Delegate Example // The delegate type declaration public 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, new EventArgs()); } } }

Connecting to an Event class SomeObject { private NameObject mTheName = new NameObject(); public void Init() { mTheName.NameChanged += new NameChangedEventHandler(EventHandler) } private void EventHandler(object sender, EventArgs e) { } }

An Event Example  Reuse the SportsTeam class  One form updates a team’s record  Another form reports the team’s record

What operations?  Unary operators +, -, !, ++, --, true, false public static operator ( )  Binary operators +, -, *, /, %, &, |, ==, !=, >, =, <= public static operator (, )

Operator Example class MyCollection { private List 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

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” BackColor A Book “is-a” Product; a Book “has-a” Publisher  The derived class extends or overrides behavior

.NET Inheritance  All classes implicitly inherit from System.Object GetType() ToString() Equals() ReferenceEquals() GetHashCode() Finalize() MemberwiseClone()

How does inheritance work? class Fourthclass Thirdclass Secondclass First methodA methodB methodC methodD 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 myList = new List (); 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  Reference base class from a derived class using “base.”

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

Public vs. Private  “public” indicates a method or property can be accessed from outside a class  “private” indicates a method or property can be accessed only from within a class  “protected” indicates a method or property can be accessed from within a class or a derived class

Public/Protected/Private class MyClass { public void MyPublicMethod() { } private void MyPrivateMethod() { } protected void MyProtectedMethod() { } } class YourClass { public void Sample() { MyClass test; // Access from anywhere test.MyPublicMethod(); } }

Public/Protected/Private class MyClass { public void MyPublicMethod() { } private void MyPrivateMethod() { } protected void MyProtectedMethod() { } } class MyDerivedClass : MyClass { public void Sample() { // Access from a derived class MyProtectedMethod(); }

Public/Protected/Private class MyClass { public void MyPublicMethod() { } private void MyPrivateMethod() { } protected void MyProtectedMethod() { // Access from within the class MyPrivateMethod(); }

What about “internal”?  The internal keyword provides access to methods and properties, but only from within other files in the same.NET assembly

Casting Operations  A derived class type can be implicitly cast to any of its base class types  Base class types must be explicitly cast to a derived class type

Implicit Casting class Base { } class Derived : Base { } int DoSomething(Base inObject) { } Derived derivedObject = new Derived(); // Implicitly cast to Base class type DoSomething(derivedObject);

Explicit Casting class Base { } class Derived : Base { } int DoSomething(Derived inObject) { } Base someObject = new Derived(); // Explicit cast to Derived class type DoSomething((Derived)someObject); // May throw an exception

Explicit Casting class Base { } class Derived : Base { } int DoSomething(Derived inObject) { } Base someObject = new Derived(); // Explicit cast to Derived class type DoSomething(someObject as Derived); // May return null

Considerations for Inheritance  Confirm “is-a” versus “has-a” relationship  Does adding one or more properties to the base class make more sense?  Would an interface be more beneficial?  Implicit and explicit casting between inherited types  Using the “as” operator instead of casting to avoid exceptions

Inheritance Example  Example product heirarchy p. 429

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 class You know the method or property exists for every object of this type, but there is no implementation at this level of abstraction All Objects have a ToString() implementation, but the implementations are all independent of one another

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)

Inheritance Walkthrough