Chapter02 Core C# Programming Construct

7/27/2019 Chapter02 Core C# Programming Construct

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Hoang Anh [email protected]

HaNoi University of Technology

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Chapter 2. Core C#

Programming Constructs


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Objectives

This chapter surveys the C# language syntax. I introduceyou to the two fundamental kinds of types within theCLR: value types and reference types. This chapter alsodescribes namespaces and how you can use them tologically partition types and functionality within yourapplications.

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Roadmap

2.1. C# Is a strongly Typed Language

2.2. Expression

2.3. Statements and Expressions2.4. Types and Variabless

2.5. NameSpaces

2.6. Control Flows

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Every variable and object instance in the system is of a

well-defined type

This enables the compiler to check that the operations to

perform on variables and object instance are valid

It is always best to find bugs at compile time rather than

run time

Example:

Method ComputeAvg(): computes the average of two integersand returns the result

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double ComputeAvg( int param1, int param2 ){

return (param1 + param2) / 2.0;}

object ComputeAvg( object param1, object param2 )

{

return ((int) param1 + (int) param2) / 2.0;

}

ComputeAvg accepts two

integers and returns a double. If

passing an instance of Apple

type, the compiler will complain

and stop

Convert objects into

integers

Passing an instance of

Apple type causes an

exception( the instance

cant be convert into

integer

objectkeyword: an alias of

System.Object class

Object is not a numeric type

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Roadmap


2.2. Expression

2.3. Statements and Expressions 2.4. Types and Variabless

2.5. NameSpaces

2.6. Control Flows

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2.2. Expressions

Expressions in C# are identical to expressions in C++ andJava.

Are built using operands, eg. variables or types within anapplication, and operators

Operators can be overloaded Operators can have different meaning in different contexts

Eg: the + operator can mean string concatenation using with string operands Example : OverandOver.csc

C# operator precedence: When an expression contains multiple operators, theprecedence of the

operators controls the order in which the individual operators are evaluated Entries at the top of the table have higher precedence Operators within the same category have equal precedence.

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Category Expression Description

Primary x.m Member access

x(...) Method and delegate invocation

x[...] Array and indexer access

x++ Post-increment

x-- Post-decrement

new T(...) Object and delegate creation

new T(...){...} Object creation with initializer

new {...} Anonymous object initializer

new T[...] Array creation

typeof(T) Obtain System.Type object forT

checked(x) Evaluate expression in checked context

unchecked(x) Evaluate expression in unchecked contextdefault(T) Obtain default value of type T

delegate {...} Anonymous function (anonymous method)

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Category Expression DescriptionUnary +x Identity

-x Negation

!x Logical negation

~x Bitwise negation

++x Pre-increment

--x Pre-decrement(T)x Explicitly convert x to type T

Multiplicative x * y Multiplication

x / y Division

x % y Remainder

Additive x + y Addition, string concatenation, delegate combination

xy Subtraction, delegate removal

Shift x > y Shift right

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Category Expression DescriptionRelational and

type testing

x < y Less than

x > y Greater than

x = y Greater than or equal

x is T Return true ifx is a T, false otherwisex as T Return x typed as T, ornull ifx is not a T

Equality x == y Equal

x != y Not equal

Logical AND x & y Integer bitwise AND, boolean logical AND

Logical XOR x ^ y Integer bitwise XOR, boolean logical XORLogical OR x | y Integer bitwise OR, boolean logical OR

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Roadmap


2.2. Expression


2.5. NameSpaces

2.6. Control Flows

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2.3 Statements and Expressions

The actions of a program are expressed using

statements

Several different kinds of statements:

A block: consists of a list of statements written between the

delimiters { and }

Declarat ion statements:are used to declare local variables andconstants

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{x=69;y=96}

int a;

int b = 2, c = 3;


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Statements and Expressions(2)

Epression statementsare used to evaluate expressions

Select ion statementsare used to select one of a number of

possible statements for execution based on the value of someexpressionif, switch

Iteration statement sare used to repeatedly execute an

embedded statementwhile,do,for,foreach

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while (i ++< Length) {

Console.WriteLine(args[i]);

}

if(continue == true) {x=69;}

else {x=96;}

Console.WriteLine(Goodbye);


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Jump statementsare used to transfer control -break,continue, goto, throw, return, andyield

The try...catch statement is used to catch exceptions that

occur during execution of a block, and the try...finallystatement is used to specify finalization code that is alwaysexecuted, whether an exception occurred or not

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1. while (true) {

2. int n = Console.Read();3. if (n == 69) break;

4. Console.WriteLine(n);

5. }


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The checkedand uncheckedstatements are used to control

the overflow checking context for integral-type arithmeticoperations and conversions. Ex : Exam.csc

Thelock

statement is used to obtain the mutual-exclusion lock

for a given object, execute a statement, and then release thelock

The using statement is used to obtain a resource, execute a

statement, and then dispose of that resource

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Roadmap


2.2. Expression


2.5. NameSpaces

2.6. Control Flows

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Value Types and Reference Types

Value types: Living places:

On the stack

On the heap: only if they are members of reference types or if theyare boxed

Are copied by value by default when passed as parameters tomethods or assigned to other variables

Reference types:

Living place: on the heap Variables used to manipulate them are references to objects onthe managed heap

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Value Types

Contain directly their value and are customarily created

statically

Initialization: using the newstatement

Derive from System.ValueType

Primitives: int, float, char and bool

Others: enum, struct

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Reference Types

The lifetime of the resulting object is controlled be

garbage collection services provided by CLR

The reference holds the location of an object created on

the managed heap

Derive from System.Object and created with the new

keyword

Types: interfaces, arrays and delegates

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Example:

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int i = 123;string s = "Hello world";123

s "Hello world"

ValueType

ReferenceType

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Value Types contain Reference Types

Example:

class ShapeInfo

{

public string infoString;

public ShapeInfo(string info){ infoString = info; }

}

Reference type

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struct Rectangle{

public ShapeInfo rectInfo;public int rectTop, rectLeft, rectBottom, rectRight;public Rectangle(string info, int top, int left, int bottom, int right){

rectInfo = new ShapeInfo(info);rectTop = top; rectBottom = bottom;rectLeft = left; rectRight = right;

}public void Display(){

Console.WriteLine("String = {0}, Top = {1}, Bottom = {2}," +"Left = {3}, Right = {4}",

rectInfo.infoString, rectTop, rectBottom, rectLeft, rectRight);}

}

Value type

The Rectangle structure

contains a reference

type member

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static void ValueTypeContainingRefType(){

Console.WriteLine("-> Creating r1");

Rectangle r1 = new Rectangle("First Rect", 10, 10, 50, 50);

.

Console.WriteLine("-> Assigning r2 to r1");Rectangle r2 = r1;

Console.WriteLine("-> Changing values of r2");

r2.rectInfo.infoString = "This is new info!";

r2.rectBottom = 4444;

.r1.Display();

r2.Display();

}

Create the

first

Rectangle.

Assign a new

Rectangle tor1

Change some

values of r2.

Print values

of both

rectangles

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Result:

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Default Variable Initialization

The following categories of variables are automatically

initialized to their default values:

Static variables.

Instance variables of class instances.

Array elements.

For a variable of a reference-type, the default value is

null

For a variable of a value-type, the default value is thesame as the value computed by the value-types default

constructor

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Default Variable Initialization(2)

Value-types default constructor:

All value types implicitly declare a public parameterless instance

constructor called the defaul t con structor

Default constructor returns the default valuefor the value type:

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Value Type Default Value

sbyte, byte, short, ushort, int, uint,

long, and ulong

0

char '\x0000'

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Default Variable Initialization(3)

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Value Type Default Value

float 0.0f

double 0.0d

decimal 0.0m

bool false

enum-type E 0, converted to the type E

struct-type All value type fields: default value ofValue Type

All reference type fields: null

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Implicitly Typed Local Variables

In C#, every variable declared in the code must have an

explicit type associated with it. But sometimes, when

writing code for strongly typed languages, the amount of

typing needed to declare such variables can betedious

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Implicitly Typed Local Variables(2)

varis a new keyword in C# 3.0

Declaring a local variabl using the new var keyword asks

the compiler to reserve a local memory slot and attach

an inferred type to that slot

At compilation time, compiler can initialize variables

without asking the type explicitly

How to use:

varlocalVariant =

localVariant is init with type of

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using System;

using System.Collections.Generic;

public class EntryPoint{

static void Main()

{

var myList = new List();

myList.Add( 1 );

myList.Add( 2 );

myList.Add( 3 );

foreach( var i in myList )

{

Console.WriteLine( i );

}}

}

An implicitly typed

variable declaration must

include an initialzer

var newValue; // emits error CS0818

var a = 2, b = 1;var x, y = 4;

Not permited

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Implicitly Typed Local Variables(3)

Restriction:

varcannot use with multiple variable declarators

var x=69, y=9.6; //Error, Implicitly-typed local variables cannot have

multiple declarators

Declarator implicitly typed local variables must include a local-

variable-initializervar x; //Error, no initializer to infer type from

The local-variable-initializermust be an expression

The initializerexpression must have a compile-time typevar u = x => x + 1; //Error, anonymous functions do not have a type

The initializerexpression cannot refer to the declared variable

itself

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Type Conversion

Many times, its necessary to convert intstances of one

type to another

Implicit Conversion:

Explicit Conversion:

int defaultValue = 12345678;

long value = defaultValue;

int smallerValue = (int) value;

public class EntryPoint

{

static void Main()

{

int employeeID = 303;

object boxedID = employeeID;employeeID = 404;

int unboxedID = (int) boxedID;

System.Console.WriteLine( employeeID.ToString()

);

System.Console.WriteLine( unboxedID.ToString() );

}

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Implicit Conversions

One type of data is automatically converted into another

type of data

No data loss

Implicit Numerical Conversion

Implicit Enumeration Conversion Permit the decimal, integer, literal to be converted to any enum

type

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1. long x;

2. int y = 25;

3. x = y; //implicit numerical conversion


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Implicit Conversions(2)

Implicit Reference Conversion From any reference type to object.

From any class type D to any class type B, provided D is inherited from

B.

From any class type A to interface type I, provided A implements I.

From any interface type I2 to any other interface type I1, provided I2

inherits I1.

From any array type to System.Array.

From any array type A with element type a to an array type B with

element type b provided A & B differ only in element type (but the same

number of elements) and both a and b are reference types and animplicit reference conversion exists between a & b.

From any delegate type to System.Delegate type.

From any array type or delegate type to System.ICloneable.

From null type to any reference type.

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Implicit Conversions(3)

Boxing Conversions

Conversion of any value type to object type

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1. int x = 10;

2. object o = x; //Boxing


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Explicit Conversions

Using the casting operator()

May be loss data

Explicit Numerical Conversions

Explicit Enumeration Conversions

From sbyte, byte, short, ushort, int, uint, long, ulong, char, float,double or decimal to any enum type.

From any enum type to sbyte, byte, short, ushort, int, uint, long,

ulong, char, float, double or decimal.

From any enum type to any other enum type

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1. int x = (int) 26.45; //Explicit conversion

2. Console.WriteLine(x); // Displays only 26


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Explicit Conversions(2)

Explicit Reference Conversions

From object to any reference type.

From any class type B to any class type D, provided B is the

base class of D From any class type A to any interface type I, provided S is not

sealed and do not implement I.

From any interface type I to any class type A, provided A is not

sealed and implement I.

From any interface type I2 to any interface type I1, provided I2 isnot derived from I1.

From System.Array to any array type.

From System.Delegate type to any delegate type.

From System.ICloneable to any array or delegate type.37


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as and is Operators

The as operator

is used to perform conversions between compatible reference

types

The as operator is like a cast operation. However, if theconversion is not possible, as returns null instead of raising an

exception

The is operator

Checks if an object is compatible with a given type

An is expression evaluates to true if the provided expression is

non-null, and the provided object can be cast to the provided

type without causing an exception to be thrown

The is operator only considers reference conversions, boxing

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Generics

Support for generics is one of the most exciting new

additions to the C# language

Using the generic can define a type that depends upon

another type that is not specified at the point of definition

Example:

A collection may be a list, queue or stack

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1. public class List2. {3. private object[] elements;4. private int count;5.6. public void Add(object element) {

7. if (count == elements.Length) Resize(count*2);8. elements[count++] = element;9. }10.11. public object this[int index] {12. get { return elements[index]; }13. set { elements[index] = value; }

14. }15.16. public int Count {17. get { return count; }18. }19. }

1. public class List2. {3. private ItemType[] elements;4. private int count;5.6. public void Add(ItemType element) {

7. if (count == elements.Length) Resize(count*2);8. elements[count++] = element;9. }10.11. public ItemType this[int index] {12. get { return elements[index]; }13. set { elements[index] = value; }

14. }15.16. public int Count {17. get { return count; }18. }19. }

Generics

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1. List intList = new List();2.3. intList.Add(1);4. intList.Add(2);5. intList.Add("Three");6.7. int i = (int)intList[0];

1. List intList = new List();2.3. intList.Add(1); // Argument is boxed4. intList.Add(2); // Argument is boxed5. intList.Add("Three"); // Should be an error6.7. int i = (int)intList[0]; // Cast required

1. ListintList = new List();2.3. intList.Add(1); // No boxing4. intList.Add(2); // No boxing5. intList.Add("Three"); // Compile-time error6.7. int i = intList[0]; // No cast required


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Generics(2)

Why generics?

Type checking, no boxing, no downcasts

Reduced code bloat (typed collections)

How are C# generics implemented?

Instantiated at run-time, not compile-time

Checked at declaration, not instantiation

Work for both reference and value types Complete run-time type information

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Generics(3)

Can be used with various types

Class, struct, interface and delegate

Can be used with methods, parameters and return types

Support the concept of constraints

One base class, multiple interfaces, new()

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Generics(4)

Type parameters can be applied to Class, struct, interface, and delegate types

class Dictionary {...}

struct Pair {...}

interface IComparer {...}

delegate ResType Func(ArgType

arg);

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DictionarycustomerLookupTable;

DictionaryorderLookupTable;

Dictionary numberSpellings;


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1. class Array

2. {3. publicstatic T[] Create(int size) {4. returnnew T[size];5. }6.7. public static void Sort(T[] array) {

8. ...9. }10. }

1. string[] names = Array.Create(3);

2. names[0] = "Jones";3. names[1] = "Anderson";4. names[2] = "Williams";5. Array.Sort(names);

Generics(5)

Type parameters can be applied to Class, struct, interface, and delegate types

Methods

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1. interface IComparable{int CompareTo(object obj);}

2.3. class Dictionary

4. {5. public void Add(K key, V value) {

6. ...7. switch (((IComparable)key).CompareTo(x)) {

8. ...9. }10. }

11. }

1. interface IComparable{int CompareTo(object obj);}

2. class Dictionary where K: IComparable3. {

4. public void Add(K key, V value) {5. ...

6. switch (key.CompareTo(x)) {7. ...

8. }9. }10. }

1. interface IComparable {int CompareTo(T obj);}

2.3. class Dictionary: IDictionary where4. K: IComparable,5. V: IKeyProvider,6. V: IPersistable,7. V: new()

8. {9. ...10. }

Generics(6)

Constraints One base class, multiple interfaces, new()

Specified using where clause

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Roadmap


2.2. Expression

2.3. Statements and Expressions

2.4. Types and Variabless

2.5. NameSpaces

2.6. Control Flows

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2.5 Namespaces

Need for Namespaces

Using namespace directives

Using alias directives

Standard Namespaces in .NET

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Need for Namespaces

Namespaces allow you to create a system to organize

your code

A good way to organize your namespaces is via a

hierarchical system Placing code in different sub-namespaces can keep your

code organized

using keyword used to work with namespaces:

Create an alias for a namespace (a using alias).

Permit the use of types in a namespace, such that, you do not

have to qualify the use of a type in that namespace (a using

directive).

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Using namespace directives

1. using System;

2. classHello

3. {

4. static voidMain()

5. {

6. stringhello = Hello!;

7. Console.WriteLine(hello);8. }

9. }

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Console is a class ofnamespace System.

If not use using System; youmust use methodSystem.Console.WriteLine()to write hello


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Using alias directives

using identifier = namespace-or-type-name ;

For example:

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1. namespace N1.N2

2. {3. classA {}

4. }

5. namespace N3

6. {

7. usingA = N1.N2.A;

8. classB: A {}

9. }


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Standard Namespaces in .NET

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Fundamentals

System System.IO

System.AddIn System.Linq

System.Collections System.ReflectionSystem.ComponentModel System.Resources

System.Configuration System.Runtime

System.Diagnostics System.Security

System.DirectoryServices System.ServiceProcessSystem.EnterpriseServices System.Text

System.Globalization System.Threading

System.IdentityModel.Claims System.Transactions


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Standard Namespaces in .NET(2)

Windows Presentation Foundation

System.Windows

Windows Forms

System.Drawing System.Media

System.Windows.Forms

ASP.NET

System.Web

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Standard Namespaces in .NET(3)

Communications and Workflow

System.Messaging

System.Net

System.Net.Sockets System.ServiceModel

System.Web.Services

System.Workflow

DATA, XML and LINQ

System.Data

System.Xml

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Roadmap


2.2. Expression

2.3. Statements and Expressions

2.4. Types and Variabless

2.5. NameSpaces

2.6. Control Flows

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2.6 Control Flow

if-else, switch

while, do-while, and for

foreach

break, continue, goto, return, and throw

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if-else construct

if (condition)

statement(s)_1 //The original result

[else

statement(s)_2] //The alternative result

condi t ionis a relational or logical expression.

statement(s)_1is a statement (or a block of statements) that is

executed if the condition is true.

statement(s)_2is a statement (or a block of statements) that is

executed if the condition is false

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1. if (salary > 2000)2. Console.Write("Salary is greater than 2k");

1. if (salary > 2000)2. Console.Write("Salary is greater than 2k");

//The original result3. else4. Console.Write("Salary is less than or equal

to 2k"); // The alternative result


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Nested if-else Statements

if (condition_1)

statement_1;

else if (condition_2)

statement_2;

else if (condition_3)

statement_3;

...else

statement_n;

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switch (expression)

{

case constant-1: statement(s);

jump-statementcase constant-2: statement(s);

jump-statement

case constant-3:

...

[default: statement(s);jump-statement]

}

switch Constructexpression represents a valuethat corresponds to theassociated switch choice

statement(s) is astatement orblock ofstatements that is

executed if thecorrespondingcondition isevaluated true

jump-statement is

a branchingstatement totransfer controloutside the specificcase, such asbreak or goto(explained later)

default dealswith all theother cases

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1. using System;

2. classSwitch

3. {

4. static void main()

5. {

6. int n = 2;

7. switch (n)

8. {

9. case 1: Console.WriteLine("n=1");

10. break;

11. default: Console.WriteLine("n=2");

12. break;

13. }

14. }

15. }


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while Loop

while (control_expression)

statement(s);

control_expression is a condition be satisfied during

the loop execution.

statement(s) is a statement (or the block of statements)

to be executed.

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1. using System;2. classWhileLoop3. {4. static void Main()

5. {6. int counter=0;7. while(counter++


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do

statement(s)

while (control_expression);

control_expression is a condition to be satisfied during

the loop execution.

statement(s) is a statement (or the block of statements)

to be executed.

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do-while Loop

1. using System;2. classDoWhileLoop3. {4. static void Main()

5. {6. int counter=0;7. do8. {9. Console.WriteLine(counter);

10. }11. while(counter++


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for Loop

for ([initialization];[control_expression]; counter_update])statement(s)

initialization is the counter initialization statement.

control_expression is a condition to be satisfied during

the loop execution.

counter_update is the counter increment or decrement

statement.

statement(s) is the statement or block of statements to

be repeated.

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1. using System;2. classForLoop3. {4. static void Main()

5. {6. for (int counter = 1;counter


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The foreach Loop

foreach (type identifier in expression)statement(s);

type is the data type, such as int or string.

identifieris the variable name.

expression is the name of the array (or collection).

statement(s) is the statement or block of statements to

be executed.

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1. using System;2. classForeachLoop3. {4. staticvoid Main()

5. {6. int[,] myIntArray = {{1, 3, 5},7. {2, 4, 6} };8. foreach(int i in myIntArray)9. Console.Write("{0} ", i);

10. }11. }


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Branching Statements

break

terminates the closest enclosing loop or switch statement in

which it appears

goto: gotolabel;

is not recommended because it corrupts the program structure

gotocaseexpression; gotodefault; (in casestruct)

continue passes control to the next iteration of the enclosing iteration

statement in which it appears

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Branching Statements(2)

return

terminates execution of the method in which it appears and

returns control to the calling method

throw is used to signal the occurrence of an anomalous situation

(exception) during the program execution

Usually is used with try-catch or try-finally statements

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Summary

Youve learned some basic features of C# such as how

to use expressions and statements in C #

You are also informed about value types, reference

types as well as how to use variables in C# .

Documents

Chapter02 Core C# Programming Construct