Chương 2: Core C# Programming Construct pot

Object and delegate creation new T...{...} Object creation with initializer checkedx Evaluate expression in checked context uncheckedx Evaluate expression in unchecked context defaultT O

Hoang Anh Viet

VietHA@it-hut.edu.vn

HaNoi University of Technology

Chapter 2 Core C#

Programming Constructs

Objectives

“This chapter surveys the C# language syntax I introduce you to the two fundamental kinds of types within the CLR: value types and reference types This chapter also describes namespaces and how you can use them to logically partition types and functionality within your applications.”

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2.1 C# Is a strongly Typed Language

2.2 Expression

2.3 Statements and Expressions

2.4 Types and Variabless

2.5 NameSpaces

2.6 Control Flows

2.1 C# Is a strongly Typed Language

 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 integers and returns the result

4

double ComputeAvg( int param1, int param2 )

complain and stop

Convert objects into integers

Roadmap

 2.1 C# Is a strongly Typed Language

 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++ and

Java.

 Are built using operands, eg variables or types within an

application, 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

 C# operator precedence:

• When an expression contains multiple operators, the precedence 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

new T( ) Object and delegate creation

new T( ){ } Object creation with initializer

checked(x) Evaluate expression in checked context

unchecked(x) Evaluate expression in unchecked context

default(T) Obtain default value of type T

delegate { } Anonymous function (anonymous method)

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Category Expression Description

Additive x + y Addition, string concatenation, delegate combination

x – y Subtraction, delegate removal

x >> y Shift right

x as T Return x typed as T, or null if x is not a T

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

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 2.1 C# Is a strongly Typed Language

 2.2 Expression

 2.3 Statements and Expressions

 2.4 Types and Variabless

 2.5 NameSpaces

 2.6 Control Flows

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

Statements and Expressions(2)

 Epression statements are used to evaluate expressions

 Selection statements are used to select one of a number of

possible statements for execution based on the value of some expression – if, switch

 Iteration statements are used to repeatedly execute an

embedded statement – while , do , for , foreach

while (i ++< Length) {

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

else {x=96;}

Console WriteLine(“Goodbye”);

Statements and Expressions(3)

 Jump statements are used to transfer control - break,

continue, goto, throw, return, and yield

 The try catch statement is used to catch exceptions that occur during execution of a block, and the try finally

statement is used to specify finalization code that is always executed, whether an exception occurred or not

Statements and Expressions(4)

 The checked and unchecked statements are used to control the overflow checking context for integral-type arithmetic

operations and conversions

 The lock statement is used to obtain the mutual-exclusion lock for a given object, execute a statement, and then release the lock

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

statement, and then dispose of that resource

Roadmap

 2.1 C# Is a strongly Typed Language

 2.2 Expression

 2.3 Statements and Expressions

 2.4 Types and Variabless

 2.5 NameSpaces

 2.6 Control Flows

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Value Types and Reference Types

• Living place: on the heap

• Variables used to manipulate them are references to objects on the managed heap

Value Types

 Contain directly their value and are customarily created statically

 Initialization: using the new statement

 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

s "Hello world"

Value Type

Reference

Type

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Value Types contain Reference Types

 Example:

class ShapeInfo {

public string infoString;

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

}

Reference type

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;

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static void ValueTypeContainingRefType()

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

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

Assign a new Rectangle to

r1

Change some values of r2 Print values

of both

Result:

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Default Variable Initialization

 The following categories of variables are automatically initialized to their default values:

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

same as the value computed by the value-type’s default

constructor

Default Variable Initialization(2)

 Value-type’s default constructor:

 All value types implicitly declare a public parameterless instance

constructor called the default constructor

 Default constructor returns the default value for the value type:

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sbyte, byte, short, ushort, int, uint, long, and ulong 0

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Default Variable Initialization(3)

float 0.0f

double 0.0d

decimal 0.0m

enum-type E 0, converted to the type E

struct-type All value type fields: default value of

Value Type All reference type fields: null

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 be tedious…”

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Implicitly Typed Local Variables(2)

 var is 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:

var localVariant = <expression>

localVariant is init with type of <expression>

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:

 var cannot 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-initializer

var x; //Error, no initializer to infer type from

 The local-variable-initializer must be an expression

 The initializer expression must have a compile-time type

var u = x => x + 1; //Error, anonymous functions do not have a type

 The initializer expression cannot refer to the declared variable itself

long value = defaultValue;

int smallerValue = (int) value;

public class EntryPoint {

static void Main() {

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

1 long x;

2 int y = 25;

3 x = y; //implicit numerical conversion

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 an implicit 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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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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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

 From System.Array to any array type

 From System.Delegate type to any delegate type

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

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 the conversion

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 null, and the provided object can be cast to the provided type

non-without causing an exception to be thrown

 The is operator only considers reference conversions, boxing

conversions, and unboxing conversions

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 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…

1 public class List

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 List <ItemType>

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; }

3 intList.Add(1); // Argument is boxed

4 intList.Add(2); // Argument is boxed

5 intList.Add("Three"); // Should be an error 6.

7 int i = (int)intList[0]; // Cast required

1 List < int > intList = new List < int >();

 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

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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 Type parameters can be applied to

 Class, struct, interface, and delegate types

class Dictionary<KeyType, ValueType> { }

struct Pair<FirstType, SecondType> { }

1 class Array

3 public static T[] Create<T>(int size) {

4 return new T[size];

 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.

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

2 class Dictionary<K, V> where K: IComparable

 One base class, multiple interfaces, new()

 Specified using “where” clause

Roadmap

 2.1 C# Is a strongly Typed Language

 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

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

 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

If not use using System;

you must use method

Using alias directives

 using identifier = namespace-or-type-name ;

Standard Namespaces in NET

Standard Namespaces in NET(2)

 Windows Presentation Foundation

Standard Namespaces in NET(3)

 Communications and Workflow

Roadmap

 2.1 C# Is a strongly Typed Language

 2.2 Expression

 2.3 Statements and Expressions

 2.4 Types and Variabless

 2.5 NameSpaces

 2.6 Control Flows

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statement(s)_2] //The alternative result

 condition is a relational or logical expression.

 statement(s)_1 is a statement (or a block of statements) that is

executed if the condition is true.

 statement(s)_2 is a statement (or a block of statements) that is

executed if the condition is false

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2 Console.Write("Salary is greater than 2k");

2 Console.Write("Salary is greater than 2k");

// The original result

3 else

to 2k"); // The alternative result

Nested if-else Statements

jump-statement case constant-3:

[default: statement(s);

jump-statement]

}

switch Construct expression represents a value that corresponds to the

associated switch choice

statement(s) is a statement or block

of statements that

is executed if the corresponding condition is evaluated true

jump-statement is a branching

statement to transfer control outside the specific case, such as

break or goto (explained later)

default deals with all the other cases