Chapter 12 - Interfaces

Interfaces 1 C# 30

C# 3.0C# 3.0

Chapter 12 Interfaces

Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 Israel

A Web Browser ExampleInterfaces 2 C# 30

A Web Browser Example Imagine that you are developing a Web Imagine that you are developing a Web

Browser Your browser should display many types of contents Text, Sound, animation, video and other objects You are absolutely not interested in the objects

implementation details You just require that any such object implement

certain methods required by the web browser to bl th it h dlienable the items handling


A Web Browser ExampleInterfaces 3 C# 30

A Web Browser Example Suppose our simple browser need only Suppose our simple browser need only

these operations from each element: Load to be activated when the page comes up Play to be activated when the user clicks on the

item

You are looking for a feature in the glanguage that will allow you to define the set of methods you require from any typeset of methods you require from any type that wants its objects to integrate into your

li tiapplication


Interfaces IntroductionInterfaces 4 C# 30

Interfaces IntroductionYo ant this f nctionalit to ser e as a You want this functionality to serve as a contract: A type that claims to adhere to the contract must actually

implement all the methods defined in the contractp As a client that uses such a class, you want to be ensured

that all methods are implemented according to their definition in the contract

The C# feature that should be used for defining such a contract is the interfa edefining such a contract is the interface


The Web Browser ExampleInterfaces 5 C# 30

The Web Browser ExampleWe define a Web Page class We define a Web Page class The web page defines an interface:interfaceIEmbeddable {

voidLoad();()voidPlay();

}

The Web Page contains a collection of IEmbeddablel b {classWebPage {privateList_items=

newList();newList();publicvoidAdd(IEmbeddable item){

_items.Add(item);}


}}


The Web Browser ExampleThe AnimationObj type claims to The AnimationObj type claims to implement the IEmbeddable interface:pclassAnimationObj :IEmbeddable{{publicvoidLoad(){//}

publicvoidPlay(){// }{//}

publicvoidDraw(){//}

}



The Web Browser ExampleIt is therefore possible to add an It is therefore possible to add an AnimationObj item to the web page:j p gclassWebPageApp {

staticvoidMain(){() {

AnimationObj animator=newAnimationObj();animator Dra ()animator.Draw();animator.Play();////

WebPage page=newWebPage();pa e Add(ani ator)page.Add(animator);//

}


}}

Interfaces Vs Abstract ClassInterfaces 8 C# 30

Interfaces Vs. Abstract ClassLooking at an interface definition Looking at an interface definition Isnt it actually a class?

To be more specific, an abstract base class that contains only pure virtual methods?

The main distinction between a class and an interface is:an interface is: A class is an entity,

probably representing an entity in the application domain The class is composed of operations and data

f f An interface is only a behavior definition A set of related abilities, just signatures, with no

i l t ti


implementation

Interfaces Vs Base ClassInterfaces 9 C# 30

Interfaces Vs. Base ClassA base class ill be defined either as A base class will be defined either as: A regular class : providing common functionality, or as an interface: defining behavior only

In addition: In addition: A derived class extends a base class

It extends and specializes its base structure and functionality

A class implements an interface It provides its way of implementing the general behavior

defined in the interface


Interface DeclarationInterfaces 10 C# 30

Interface DeclarationIn C# an interface is not a class ith In C#, an interface is not a class with special characteristicsp It is a first-class built-in concept having rules and conventions for definition and having rules and conventions for definition and

implementation

A i t f i d fi d i th k d An interface is defined using the keyword interface By convention, its name should be preceded with a

capital I followed by the interface name with the firstcapital I followed by the interface name with the first letter capitalized IEmbeddable,IEnumerable


IEmbeddable,IEnumerable


Interface Declaration An interface can contain methods An interface can contain methods,

properties, indexers and events, none of hi h i l t d i th i t fwhich are implemented in the interface

itself An interface is a reference type that is never directly

created and has no actual representationp Interface method declarations should not be prefixed

with an access modifier They are all, by default, public Interface methods are all, by default, pure virtual , y , p

methods



Interface DeclarationAn interface can contain also properties An interface can contain also properties, indexers and events The following is an enhanced version of the

IEmbeddable interfaceinterfaceIEmbeddable{{

voidLoad();voidPlay();y();

stringTitle{get;}tCli k dE t Cli k deventClickedEvent Clicked;

stringthis[int index]{get;set;}}


}

Interface ImplementationInterfaces 13 C# 30

Interface ImplementationA class that is defined to implement an A class that is defined to implement an interface must define all members declared in the interface:

Interface method signatures cannot be modified Interface method signatures cannot be modified Interface methods must be defined as public Interface methods can be defined to be virtual

The compiler ensures that anThe compiler ensures that an implementing class actually adheres to its i t f t tinterface contract


Using the Implemented ItemInterfaces 14 C# 30

Using the Implemented Item Instances of a class can be used by the Instances of a class can be used by the

client that requires the general interface with no assumptions on the specific types that

implement it They are called through a reference to the interface:

publicvoidLoad()p (){

foreach(IEmbeddable iteminitems)item.Load();item.Load();

}


Inheritance and InterfaceInterfaces 15 C# 30

Inheritance and Interface Consider the following graphical system: Consider the following graphical system:

The shapes class hierarchy is composed of:A b t t b l h d fi i h An abstract base class Shape, defining shapes common fields position and methods Draw(),Resize() and Move()()

Three derived classes : Circle, Rectangle andTriangle, each overrides required methods as well as adding its specific functionalityadding its specific functionality

A Desktop class is defined to manage shapesA Desktop class is defined to manage shapes It contains a list of shapes: List _shapes It enables several operations, common to all shapes, to be

obtained on the stored shapes


A Graphical System ExampleInterfaces 16 C# 30

A Graphical System Example publicabstractclassShape{//}publicabstractclassShape{//}publicclassCircle:Shape{//}publicclassRectangle:Shape{//}publicclassTriangle:Shape{//}

publicclassDesktop{

privateList shapes=newList;privateList_shapes=newList;publicShapethis[int index]//indexeronshapes{//}{//}publicbool Add(Shapeshape){//}

}



Later a new shape behavior was defined

A Graphical System Example Later, a new shape behavior was defined The ability to be drawn in 3D:

publicinterfaceIDraw3D{

voidDraw3D();

Two of the shape classes chose to implement it:

();}

publicclassCircle:Shape IDraw3D

publicclassRectangle:Shape IDraw3DShape,IDraw3D

{publicvoidDraw3D()

Shape,IDraw3D{publicvoidDraw3D()

Th f ti lit li d t f th h

publicvoidDraw3D()...}

publicvoidDraw3D()...}


The new functionality was applied to some of the shape classes without interfering with the overall hierarchy


A Graphical System ExampleThe Desktop added a method to enable The Desktop added a method to enable activating the 3D draw: But how should the Draw3D() be defined?

Lets start with a simple attempt:Let s start with a simple attempt:publicbool Draw3D(int index){{

if(index>=_shapes.Count)thrownewIndexOutOfRangeException();

h [i d ] D 3D()_shapes[index].Draw3D()//

}

Can you find any problems with this definition?

}



A Graphical System Example The statement shapes[index] Draw3D() The statement _shapes[index].Draw3D()

Gets the following compile error:

Our previous WebPage class stored its items in an'Shape'doesnotcontainadefinitionfor'Draw3D'

Our previous WebPage class, stored its items in an IEmbeddable reference list

ensuring all stored items implement IEmbeddableensuring all stored items implement IEmbeddable

The Desktop class, however, stores its shapes in a more heterogeneous array: a Shape reference listmore heterogeneous array: a Shape reference list

Only Shape class methods can be directly applied on Shapereferencesreferences

The Draw3D()method can only be activated through an IDraw3Dreference



A Graphical System Example Lets cast! Lets cast!

publicbool Draw3D(int index){

////IDraw3DthreeDShape =(IDraw3D)_shapes[index];threeDShape.Draw3D();//

}

How about this?


Querying for ImplementationInterfaces 21 C# 30

Querying for Implementation The second Draw3D() version would The second Draw3D() version would

compile However, when an index of a shape that does not

implement the IDraw3D interface is provided, a run-time exception I lidC tE ti is throwntime exception InvalidCastException is thrown

We need to find out if the required shape is of a type that implements IDraw3D We can use the operators is and as to query anWe can use the operators is and as to query an

object if it implements a specified interface

ID 3Dth DSh h [i] ID 3DIDraw3DthreeDShape =_shapes[i]asIDraw3D;if(threeDShape !=null)

threeDShape.Draw3D();


p ();

Inheritance / Interface RiddleInterfaces 22 C# 30

Inheritance / Interface Riddle The IDra 3D interface presented above The IDraw3D interface presented above

contained a single method named: gDraw3D() What would happen if the interfaces definers would

have called their method merely D ()?have called their method merely Draw()?

Do you see the problem that would arise in such a situation?


The Name Collision ProblemInterfaces 23 C# 30

The Name Collision Problem Lets revisit the Circle class definition: Let s revisit the Circle class definition:

publicclassCircle:Shape,IDraw3D{//publicvoidDraw3D(){//}

}

Which Draw() is the Circle class defining?

}

() g The base Shape Draw(), or the IDraw3DDraw()?

Can the class implement both definitionsCan the class implement both definitions of the method only once?

And if so the method has a completely different And if so, the method has a completely different meaning in the different contexts


The Name Collision ProblemInterfaces 24 C# 30

The Name Collision Problem In the current Circle class definition: In the current Circle class definition:

the Draw()method will be executed, both when activated in a Circle object (or Shape) context:activated in a Circle object (or Shape) context:Circlecircle =newCircle(5,10,15);circle Draw();

And in a IDraw3D context:circle.Draw();

IDraw3DthreeDShape =_shapes[index]asIDraw3D;if(threeDShape!=null)

threeDShape.Draw();

We would like to be able to define two threeDShape.Draw();

versions of the Draw() method and to ensure that each will be called in its context


Explicit Interface SpecificationInterfaces 25 C# 30

Explicit Interface Specification When implementing an interface method When implementing an interface method

It is possible to explicitly specify the name of the interface to which the method belongs:interface to which the method belongs:

publicclassCircle:Shape,IDraw3D{

publicoverridevoidDraw(){{

Console.Writeline("Iama2Dcircle!");}

voidIDraw3D.Draw(){

Console Writeline("IamaBall!");Console.Writeline("IamaBall!");}

}


The System InterfacesInterfaces 26 C# 30

The System Interfaces Being a key concept interfaces plays an Being a key concept, interfaces plays an

important role in the .NET framework There are several pre-defined interfaces

The String class, for example, supportsThe String class, for example, supports several members such as:

Concat() Replace()and Length Concat(), Replace()and Length In addition, it implements several interfaces:

publicsealedclassString:IComparable,ICloneable,IConvertible,IComparable,IEnumerable,IComparable,IEnumerable,IEnumerable,IEquatable


Interfaces: Another ViewpointInterfaces 27 C# 30

Interfaces: Another ViewpointInterfaces pro ide s ith the means for Interfaces provide us with the means for generic programmingg p g g Using interfaces, we can provide general algorithms

as type independent servicesas type independent services

For example, a sort algorithm can be applied to any type, so long as it provides an appropriate comparison methodan appropriate comparison method How will we define this requirement?

Ho ill it be enforced on instances pro ided as How will it be enforced on instances provided as arguments to our algorithm?


The IComparable InterfaceInterfaces 28 C# 30

The IComparable InterfaceThe IComparable interface is actually the The IComparable interface is actually the .NET version of the theoretical comparison pinterface idea we have just presented

While learning about the NET array we realized that While learning about the .NET array, we realized that the Array class provides a sorting methodW d it t t h ldi i t We used it to sort an array holding integers

Can this method be called for an array storing any element type?type?

Will it be possible, for example, to sort an array of Employeeobjects?objects?

Which compare method does the Sort() method use to perform the sorting algorithm?



The IComparable InterfaceTr ing to e ec te the emplo ees program Trying to execute the employees program with the following statements inside:gEmployee[]employees=...;//

Generates this run time exception:

//Array.Sort(employees);

Generates this run time exception:UnhandledException:pSystem.InvalidOperationException:SpecifiedIComparer threwanexception.>S t A tE ti System.ArgumentException:AtleastoneobjectmustimplementIComparable.



The IComparable Interface IComparable is the interface that is used IComparable is the interface that is used

to compare objects in the sorting methods IComparable contains a single method:

int CompareTo(objectobj );int CompareTo(objectobj ); Where: obj is the object to be compared with the current

instance The methods return value is an integer that indicates

the relative order of the compared operands Less than zero: This instance is less than obj Zero: This instance is equal to obj Greater than zero: This instance is greater than obj Greater than zero: This instance is greater than obj


The IComparable ExampleInterfaces 31 C# 30

The IComparable ExampleclassEmployee:IComparable {classEmployee:IComparable {

publicint CompareTo(objectobj){Employeeother=(Employee)obj;//Thedefaultsortingisbyname:returnString.Compare(_name,other._name);

}}}

classEmployeeManager {privateEmployee[]_employees;publicvoidSort(){

( )Array.Sort(_employees);}

}


}

Extension MethodsInterfaces 32 C# 30

Extending an Interface//I S t dll//InSystem.dllpublicinterfaceIComparable {int CompareTo(objectobj);p ( j j);

}//InMyExtensions.dll

M E t i {namespaceMyExtensions {staticclassComparableExtensions {publicstaticbool IsGreater(this IComparable obj,p ( p j,

objectother){returnobj.CompareTo(other)>0;

}}publicstaticbool IsEqual(this IComparable obj,

objectother){j ) {returnobj.CompareTo(other)==0;

}}


}}//endnamespace

Extending an InterfaceInterfaces 33 C# 30

Extending an Interface//InBigInt.dllclassBigInteger :IComparable {//Implementation

}}

//Inmycode:usingMyExtensions;//Importextensions

BigInt b1=newBigInt(10000000);BigInt b1=newBigInt(10000000);BigInt b2=newBigInt(10000001);

b1.IsEqual(b2);//CompareExtensions.IsEqualb1.IsGreater(b2);//CompareExtensions.IsGreater



The IComparable Interface We will learn Generics later in this course We will learn Generics later in this course

It is something similar to C++ templates

Using IComparable is expensive in most cases and error pronecases and error prone We have to cast from object to our type For value types, we pay the box/unbox operations In .NET 2.0, new generic interfaces were introducedg

IComparable is one of them:publicinterfaceIComparablepublicinterfaceIComparable{

int CompareTo(Tother);


( )}

Additional Comparing Methods: Interfaces 35 C# 30

The IComparer The IComparable interface is the default The IComparable interface is the default

sorting interface The CompareTo() method therefore define the

default compare method It is possible to provide other comparing methods, to

sort based on different sorting criteriaThi b d b d fi i h l l th t i l t This can be done by defining helper classes that implement the IComparer or IComparer interfaces Compare() method

An instance of such a helper class is passed as an additional parameter to the Sort() method

When this is done the provided Compare() method will be When this is done, the provided Compare() method will be used, instead of the default CompareTo() method

This lets us tune the sorting algorithm dynamically


g g y y

The IEnumerable InterfaceInterfaces 36 C# 30

The IEnumerable InterfaceC# arra and collections elements can be C# array and collections elements can be iterated using the foreach constructg Is foreach implemented specifically for the

framework collections?framework collections?Or

Does it define a general contract of which any class Does it define a general contract of which any class that adheres to it can join the foreach game?Can we apply the f h statement usage on our Can we apply the foreach statement usage on our user-defined collection class?



The IEnumerable Interface Tr ing to compile the follo ing (some hat Trying to compile the following (somewhat nave) statement on our original ) gEmployeeManager class:foreach (Employeeein_employees)

Console.WriteLine("Name:"+e.Name);

Will generate the following compile error:foreach statementcannotoperateonvariablesoftype'employeeTools.EmployeesManager'becausetype employeeTools.EmployeesManager because'employeeTools.EmployeesManager'doesnotcontainadefinitionfor'GetEnumerator



The IEnumerable Interface It seems that to use the foreach It seems that to use the foreach

statement We should implement the interface: IEnumerable

The IEnumerable interface contains oneThe IEnumerable interface contains one method:IE t G tE t ()

It returns an object of a type that f

IEnumerator GetEnumerator();

implements the IEnumerator interface


The IEnumerator InterfaceInterfaces 39 C# 30

The IEnumerator Interface The IEnumerator interface is the actual The IEnumerator interface is the actual

iteration interface, containing the following bmembers:

bj tC t{ t }objectCurrent{get;}bool MoveNext();voidReset();();


IEnumerable & IEnumeratorInterfaces 40 C# 30

IEnumerable & IEnumerator It seems that we have two separate It seems that we have two separate

entities: The container class and an enumerator class

The enumerable container does notThe enumerable container does not directly implement the enumeration operations but rather defines a separateoperations, but rather defines a separate enumerator class, as a nested type: This helper class implements the actual enumeration

operations on its containerWh th t i i k d f ti i When the container is asked for enumeration services (GetEnumerator()), it provides an object of its enumerator class to be used for enumeration


enumerator class to be used for enumeration

Enumerator ExampleInterfaces 41 C# 30

Enumerator ExampleclassEmployeeManager :IEnumerableclassEmployeeManager :IEnumerable{

////IEnumerable requiresthisfromanenumerableclass://ItreturnsanobjectofIEnumerable type,//thatprovidestheactualenumerationservices.publicIEnumerator GetEnumerator(){{

returnnewEnumerator(employees);}}//



Enumerator Example//Nestedclass//NestedclasspublicclassEnumerator:IEnumerator {publicEnumerator(IList employees){_employees=employees;ResetCurrent();

}publicobjectCurrent{{

get{if( current==1|| current> employees.Count1)(_cu e t || _cu e t > _e p oyees.Cou t )

thrownewInvalidOperationException();return_employees[current];

}}



Enumerator Examplepublicbool MoveNext(){publicbool MoveNext(){

if(_current

Interfaces and CollectionsInterfaces 45 C# 30

Interfaces and Collections IEnumerable and IComparable are two IEnumerable and IComparable are two

examples of collection-related interfaces The .NET framework provides several collection

classes in System.Collections.Generici d i l List,SortedList,Queue,

Stack,Dictionary and much more All NET collections implement several interfaces that All .NET collections implement several interfaces that

represent basic element management, such as the ICollection &ICollection,IEnumerable &,IEnumerable,ICloneable interfaces

Different collections implement other, more specific interfaces, such as: IList,IList and IDictionary,IDictionary


The IFormattable InterfaceInterfaces 46 C# 30

The IFormattable Interface Recall the string formatting discussion Recall the string formatting discussion

from the second course chapter In this discussion we presented the ability to control

the output format of some .NET pre-defined types:Console.WriteLine("Currencyformat:{0:C}",8888);Console.WriteLine(Longdateformat:{0:D}",DateTime.Now);

This formatting technique was also used to output enumerated type values as bit flags:

enum Characteristics{Smart=0x0001,//

}}Characteristicscharacter=Characteristics.Tall |

Characteristics.Kind;l i i (" h h i { }" h )


Console.WriteLine("Thecharacteris:{0:F}",character);

The IFormattable InterfaceInterfaces 47 C# 30

The IFormattable InterfaceCan I pro ide m sers s ch control o er Can I provide my users such control over the output format printing of my objects?p p g y j Can I enable them produce different output formats

based on provided format characters?based on provided format characters?

The answer is implement IFormattable The IFormattable Interface contains a single

method:stringToString(stringformat,

IFormatProvider formatProvider );


IFormattable ExampleInterfaces 48 C# 30

IFormattable ExampleclassEmployee:Iformattable {classEmployee:Iformattable {

privateint _id;privatestring name;privatestring_name;privateint _salary;privateint seniority;p ate t _se o ty;privatechar_gender;//publicoverridestringToString(){returnString.Format("Id:{0},Name:{1},Salary:{2}",

_id,_name,_salary);}



IFormattable ExamplepublicstringToString(stringfmt IFormatProvider fp)publicstringToString(stringfmt,IFormatProvider fp){

switch(fmt)switch(fmt){case(null):case ( u ):

returnToString();case"F":

returnString.Format(...);default:

returnToString();}

}}



IFormattable ExampleusingSystem;usingSystem;classEmployeesApp {

staticvoidMain(){staticvoidMain(){Employeee1=newEmployee("Jack",10000,'M');

Employeee2=newEmployee("Jane",12000,'F');Console.WriteLine("Employeeno.1{0}",e1);Console.WriteLine("Employeeno.3{0:F}",e2);

} Employeeno.1:Id:0,Name:Jack,Salary:M}

p y , , yEmployeeno.2:Id:1Name:JaneGender:F


Seniority:0Salary:12000

Interfaces 51 C# 30

Chapter 12 Exercise 1

THE ENHANCED WEEKLY SCHEDULE EXERCISESCHEDULE EXERCISE


Chapter SummaryInterfaces 52 C# 30

Chapter SummaryInterfaces define contracts Interfaces define contracts Classes define behavior

Querying a type for an interface implementation can be done with the isimplementation can be done with the isand as operators

Interfaces can be explicitly implemented .NET has many pre-defined interfaces

Many of those interfaces serve as the contractMany of those interfaces serve as the contract between collections and generic algorithms


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Chapter 12 - Interfaces