.NET Fundamentals

November 20, 2003

Week 3

Class Agenda – November 20, 2003

Questions / Homework?

Types (Intrinsic and Reference), Classes, Objects Class Exercise (Demo derived classes

Class Exercise

Procedural vs Object Oriented Programming

Class Exercise

Homework Assignment

Questions / Homework ?

• Common Email List

• Homework ??

• Review public/private and static in class this week?

• Move last class from Thursday 12/18 to Monday 12/15? 

Course Schedule

Week Topics

1 Introduction to .NET Framework, C#

2 C#, Windows Forms Programming, Exceptions

3 Types in .NET

Classes

Objects and Writing Classes

Boxing and Unboxing

Week Topics4 ADO.NET (managed data access)

- Connections

- Adapters

- Data Sets

- Grid Control

5 Introduction to the Frameworks Class Library

Bootstrapping the Common Language Resource (CLR)

Modules and Assemblies

.Net Packaging

Public and Private Keys

Shared Assemblies

The Global Assemble Cache (GAC)

Strong Names

Week Topics

6 ASP.NET

Introduction to Web Services

Web Security

Advanced Concepts:

- Serialization

- Reflection

Web Services Class Project

7 Web Services Class Project (continued)

8 Web Services Class Project (continued)

Strongly Typed Object Oriented Programming Environment

Type Class Object

"In C# a type is defined by a class, while the individual instances of a class are known as objects." Jesse

Liberty

Types and Types in .NET

What is a type?

• Every variable has a type

• A type defines the variables general properties and behaviors

• A type can be complex like a form class or simple like an integer.

• Sometimes a type is tangible, like a button in a window

• Sometimes a type abstract, like a data table or a thread

Types in .NET

Previously, each programming language represented data types in its

own way. Now, the common type system provides every language in

Visual Studio .NET with a consistent set of data types.

In addition, every data type supports a minimum set of methods. Because all

languages use the same library of types, you can call one language from another without having to convert the type or the

call conventions.

The .NET run-time environment is designed to be safe and secure.

The .NET run-time environment enforces strict rules to guarantee the

safety of types.

type-safe code

• Type-safe code is code that accesses types only in well-defined, allowable ways.

• Type-safe code only accesses memory at fixed offsets corresponding to actual field members.

• Code that accesses memory at arbitrary offsets outside the range of memory that belongs to that object's publicly exposed fields, it is not type-safe.

.NET and type-safe code

• The Common Language Specification defines a set of programmatically verifiable rules.

• These rules govern the interoperation of types that are authored in different programming languages.

• These rules also establish requirements for Common Language Specification compliance.

• Visual Studio .NET languages such as Microsoft Visual Basic .NET and Microsoft Visual C# .NET comply with the Common Language Specification.

Type Fundamentals

• All objects in .NET ultimately derive from System.Object

• This means that every object of every type has a minimum set of methods

The Public Methods

• Equals - Determines whether two objectinstances are equal.

• GetHashCode - Serves as a hash function for a particular type, suitable for use in hashing algorithms and data structures like a hash table.

• ToString - Returns a string that represents the current object.

• GetType - Gets the type of the current instance.• ReferenceEquals - Determines whether the

specified object instances are the same instance.

The Protected Methods

• Methods only seen by derived classes. • MemberWiseClone - Creates a shallow copy of

the current object. Creates a new instance of the object and sets the new object’s fields to be identical to this object’s fields. Returns a reference to the new object.

• Finalize - Allows an object to attempt to free resources and perform other cleanup operations before the object is reclaimed by garbage collection.

Microsoft .NET supports two kinds of data types

• Value types. Types that are allocated in a stack or inline in a structure.

• Reference types. Types that are allocated in a heap.

Value Types

• Value types store the data directly on the stack. (simply, a last in first out list).

• You access this data directly.

• To create a copy of the value that is assigned, you can assign a value type to a variable.

• Value types are not inheritable.

• They are implicitly derived from the System.ValueType class, which derives from System.Object.

Value Types

Value types include:

• primitives

• enums

• structs

Primitives

Primitives are the foundation of data types. Primitives are the lowest types available.

You can identify primitives through keywords, which are aliases for predefined types in the System namespace. For example, the int or int32 data type is an alias for the System.Int32 object.

Because all data types are derived from System.Object, primitives are actually objects with a set of members that are available for each type. For example, the int32 data type has a member named MaxValue.

Primitive Types

Of these primitive types, only the string type is a reference type. All of the other primitive types are

value types.

- byte - short - int

- long - single - double

- decimal - bool - DateTime

- char - string

enumerators

The enum keyword is used to declare an enumeration, a distinct type consisting of a set of named constants called the enumerator list.

Every enumeration type has an underlying type, which can be any integral type except

char.

enum example

In this example, an enumeration, Days, is declared. Two enumerators are explicitly converted to int and

assigned to int variables.

enum Days {Sat=1, Sun, Mon, Tue, Wed, Thu, Fri}; int x = (int) Days.Sun; int y = (int) Days.Fri;

structs

• The struct data type inherits from the System.ValueType class.

• A struct type is a value type that can contain constructors, constants, fields, methods, properties, indexers, operators, events, and nested types.

• A struct is like a light weight class.

structs

The .NET structure data type is very similar to a class data type except that a structure data type is a value type (whereas a class data type is a reference type).

For increased efficiency, you may want to use structure data types if you do not need the overhead of maintaining a reference pointer. However, because a structure data type is a value type, it is not garbage collected.

Reference types

• Reference types store the data on the managed heap and store a pointer to the data on the stack

• You access the data in reference types through the reference pointer

• Are collected by the garbage collector when they are no longer in use.

• Are passed by reference.• Can be extended by inheritance.• Can specify Finalizers.• Reference pointer is type safe.• A variable of reference type always contains a reference

to a value of that type or a null reference.

Reference types include the following data types

• String

• Array

• Class

• Interface

• Delegate

String• .NET String data types are invariant.

• Because String data types are read-only after initialization, you cannot directly modify their content.

• The String variable contains a pointer to memory that contains the actual data. Any modification to the string deallocates the current memory block and allocates a new memory block for the new value

StringBuilder

NOTE: If the cost of deallocating and reallocating string greatly affects performance, you can use the StringBuilder class in Visual Studio .NET. You will notice performance benefits at approximately 300 string concatenations.

Array• Provides methods for creating, manipulating, searching, and sorting

arrays, thereby serving as the base class for all arrays in the common language runtime.

• All array types implicitly inherit from the System.Array class, which inherits from System.Object. Arrays are allocated on the managed heap.

• If all of the dimensions of the array are zero, the array has a length of zero, and the array is empty.

• In an array reference types are initialized to null, value types are initialized to the default value for their type (i.e. int members are initialized to zero). You can initialize arrays during declaration.

• By inheriting from System.Array, each array reference type automatically inherits a set of System.Array methods and properties such as Rank, Length, GetLength, and GetUpperBound.

Class

• A class is a data structure that may contain data members (such as constants and variables), function members (such as methods, properties, indexers, operators, events, and constructors), and nested types.

• Class types support inheritance. Inheritance is a mechanism whereby a derived class can extend and specialize a base class.

• Derived classes inherit and can extend the properties and the methods of the base class. Derived classes can also override inherited methods with new implementations.

Interface

• An interface is a contract.

• The class or a structure that implements the interface must adhere to the contract.

• The contract specifies the members that must be supplied by the class that implements the interface.

• The interface is a list of functions. The interface contains methods, properties, and events.

• An interface provides no implementation itself.

Delegate• The delegate reference type is central to the programming

model of the common language runtime.• Delegates are classes that hold references to procedures. • The Delegate reference provides a managed, type-safe

function pointer, which is the delegate type.• A delegate is derived from the System.Delegate class.• Delegates are the basis for events.• The .NET event model uses delegates to bind events to the

methods that are used to handle them. The delegate allows other classes to register for event notification by specifying a handler method. When the event occurs, the delegate calls its bound method.

Boxing and UnboxingIn some cases, you may want to treat an instance of a value type like an instance of a reference type. Boxing provides this mechanism.

Boxing converts a value type to a reference type by performing the following steps:

1. Allocates memory on the managed heap to store the value.

2. Copies the value to the managed heap.

3. Stores the reference to the data (address of the object) on the stack.

Unboxing converts an instance of a reference type back to its original value type by returning a pointer to the data within a boxed subject. The pointer does not include the usual overhead that is associated with a true object.

Boxing

• Here’s what happens when you box a value type:– Memory is allocated from the managed heap to hold the

value type plus the overhead members.– The value type’s fields are copied to the newly allocated

memory.– The address of the object is returned.– Note that a whole new object is returned – manipulating

the original value type will not change the boxed reference.

int value = 123;

object o = value; // box int into an object box int value2 = (int) o; // unbox into value2

Unboxing• Here’s what happens when a reference type is

unboxed– If the reference is null, a NullReferenceException is

thrown

– If the reference does not refer to an object that is a boxed value of the desired value type, an InvalidCastException is thrown

– A pointer to the value type is returned, which is frequently copied immediately to another value type.

int value = 123;object o = value; // box int into an object boxlong value2 = (long) o; // this throws a cast exception

Casting

There are two types of casting:

•Implicit casting. Implicit casting is transparent to users. The compiler automatically converts from one data type to another. The predefined, implicit conversions always succeed and never cause an error.

•Explicit casting. Explicit casts are called in the code. Explicit casts cannot guarantee success and may lose information if you cast from a larger type to a smaller type.

Type Class Object

"In C# a type is defined by a class, while the individual instances of a class are known as objects." Jesse

Liberty

In C# Everything happens within a Class!

class

• A class is a data structure that may contain data members (such as constants and variables), function members (such as methods, properties, indexers, operators, events, and constructors), and nested types.

• Class types support inheritance. Inheritance is a mechanism whereby a derived class can extend and specialize a base class.

• Derived classes inherit and can extend the properties and the methods of the base class. Derived classes can also override inherited methods with new implementations.

classClasses are declared using the keyword class. It takes the following form::[attributes] [modifiers] class identifier [:base-list] { class-

body }[;]

where: – attributes (Optional)– modifiers (Optional) The allowed modifiers are new,

abstract, sealed, and the four access modifiers. – identifier The class name. – base-list (Optional) A list that contains the one base class

and any implemented interfaces, all separated by commas. – class-body Declarations of the class members.

Common Class Members

• fields, which are the variables of the class.

• methods, which implement the computations and actions that can be performed by the class.

• properties, which define named characteristics associated with reading and writing those characteristics.

Instance versus Static Members

• Instance members are unique to each object instance and referenced by the object reference.

• Static members are limited to one copy, associated with the class type, being referenced by the class type.

Fields

• A field is a member that represents a variable associated with an object or class.

• Fields maintain class state

Methods

• A method is a member that implements a computation or action that can be performed by an object or class.

• Choose a name for your method based on the following guidelines.

– Use verbs or verb phrases to name methods

– The first letter in the identifier and the first letter of each subsequent concatenated word are capitalized.

Method Arguments

• Appear as local variables within the method

• Method arguments are private by default

Modifiers for Fields or Methods

Term Description

public Accessible to all methods in all assemblies.

private Method is visible only to member methods within the same class.

protected Method extends visibility to methods of derived classes.

internal Method extends visibility to any class in the same assembly.

protected internal protected or internal

Term Description

static The field is part of the type’s state, not the object’s state

readonly The field can be written to only in the constructor.

Modifiers for Fields

Modifiers for Methods

Term Description

static Method is associated with the type itself, not an instance of the type. The method may not access instance fields.

virtual Most-derived method is called, even if object is cast to a base type.

Applies only to non-static methods.

new Method should not override a virtual method defined by it’s base type – the method hides the inherited method. Applies only to virtual methods

Modifiers for Methods (continued)

Term Description

override Explicitly indicates the method is overriding as virtual method in it’s base type. Applies only to virtual methods.

abstract Indicates that a deriving type must implement the method. Need to mark the type abstract as well. Applies only to virtual methods.

sealed The derived type cannot override this method. Applies only to virtual methods.

Method Overloading• Method overloading occurs when a class contains two

methods with the same name, but different signatures. • Use method overloading to provide different methods

that do semantically the same thing. • Use method overloading instead of allowing default

arguments. Default arguments do not version well and therefore are not allowed in the Common Language Specification (CLS). The following code example illustrates an overloaded String.IndexOf method.

int String.IndexOf (String name);

int String.IndexOf (String name, int startIndex);

Method Overloading (continued)• Use default values correctly. In a family of overloaded

methods, the complex method should use parameter names that indicate a change from the default state assumed in the simple method. For example, in the following code, the first method assumes the search will not be case-sensitive. The second method uses the name ignoreCase rather than caseSensitive to indicate how the default behavior is being changed.

// Method #1: ignoreCase = false.MethodInfo Type.GetMethod(String name);

// Method #2: Indicates how the default behavior of method #1 is

// being changed.

MethodInfo Type.GetMethod (String name, Boolean ignoreCase);

Properties(get and set accessor)

get and set accessor

• Provides protection for variables in a class

• Define the variable as private

• Use a get and set accessor to access the variable

• The accessor of a property contains the executable statements associated with getting (reading or computing) or setting (writing) the property. The accessor declarations can contain a get accessor, a set accessor, or both.

Exampleusing System;

public class anyClass {

private string name;

public string Name {

get {

return name;

}

set {

name = value;

}

}

}

get accessor

• The body of the get accessor is similar to that of a method. It must return a value of the property type. The execution of the get accessor is equivalent to reading the value of the field.

• When you reference the property, except as the target of an assignment, the get accessor is invoked to read the value of the property.

• The get accessor must terminate in a return or throw statement, and control cannot flow off the accessor body.

set accessor• The set accessor is similar to a method that returns void. It

uses an implicit parameter called value, whose type is the type of the property. In the following example, a set accessor is added to the Name property:

• When you assign a value to the property, the set accessor is invoked with an argument that provides the new value. For example:

• It is an error to use the implicit parameter name (value) for a local variable declaration in a set accessor.

• When accessing a property using the set accessor, preserve the value of the property before you change it. This will ensure that data is not lost if the set accessor throws an exception.

accessors notes• A property is classified according to the accessors used as

follows: – A property with a get accessor only is called a read-

only property. You cannot assign a value to a read-only property.

– A property with a set accessor only is called a write-only property. You cannot reference a write-only property except as a target of an assignment.

– A property with both get and set accessors is a read-write property.

• In a property declaration, both the get and set accessors must be declared inside the body of the property.

accessors notes continued• It is a bad programming style to change the state of the

object by using the get accessor. For example, the following accessor produces the side effect of changing the state of the object each time the number field is accessed.public int Number { get { return number++; //

Don't do this } }

• The get accessor can either be used to return the field value or to compute it and return it. For example:

public string Name { get { return name != null ? name : "NA"; } }

• In the preceding code segment, if you don't assign a value to the Name property, it will return the value NA.

Constructors• Methods called whenever an object is instantiated

• Before the constructor is called, the object points to undifferentiated memory.

• After the constructor is called, the object points to valid instance.

• If a constructor is not defined the CLR creates one for you.

• Member variables are initialized to default values.

• Constructor method name is the same as the class name.

Constructor (continued)

• If you provide an overloaded constructor, you must provide a default constructor, even if it does nothing.

• Can call the base class constructor using the base keyword.

• Base class constructor always uses the most derived type.

• Constructors have no return type

• Typically declared public

• Arguments are defined just like any other method

Copy Constructor

• Constructor where the argument is another instance of the object

• Shallow copy – shares address of references defined in the class

• Deep copy – Duplicates all members throughout the hierarchy.

Initializers

• Setting member variables (i.e. fields) to specific values when the object is created.

int i = 12;

string tmpString = "abcd";

this pointer

• The this keyword refers to the current instance of the class.

• Static member functions do not have a this pointer.

• The this keyword can be used to access members from within constructors, instance methods, and instance accessors.

• It is an error to refer to this in a static method, static property accessor, or variable initializer of a field declaration.

Demo – Tic Tac Toe Program

• Walk through a windows form class

• View the static main method

• See how the controls are added to the form

• Implement a common button method

• View collection of controls

Class ExerciseCTime Class / Tester

• see handout.

Type Class Object

"In C# a type is defined by a class, while the individual instances of a class are known as objects." Jesse

Liberty

What is an object?

• A class is an abstract model.

• An object is the concrete realization or instance built on the model specified by the class.

• An object is created in the memory using the keyword 'new' and is referenced by an identifier called a "reference".

Creating an Object

MyClass myObjectReference = new MyClass();

• In the above, a instance of class MyClass is created with the variable name of myObjectReference.

The new operator• Allocates memory for the object from the

managed heap• Initializes the object’s overhead members (the

CLR uses these to manage the object).– The object’s pointer to the type’s method table.– A SyncBlockIndex (used to manage access to

the object by multiple thread).• Calls the type’s instance constructor, passing any

parameters specified. Most languages call the base class constructor, but this is not mandated by the CLR.

Object Oriented Programming

The essence of object-oriented programming is the creation of

new types.

What is an Object?

An object is a software bundle of related variables and methods.

Software objects are often used to model real-world objects you find in

everyday life.

Object Oriented Programming• Where programmers define not only the data type of a data

structure, but also the types of operations (functions) that can be applied to the data structure. The data structure becomes an object that includes both data and functions.

• Programmers create relationships between one object and another. Objects can inherit characteristics from other objects.

• A principal advantages of object-oriented programming over procedural programming is that programmers create modules that do not need to be changed when a new type of object is added. A programmer simply creates a new object that inherits many of its features from existing objects.

Procedural Programming

• Focus on sequential logic flow (i.e. steps in a procedure, open file, read records, calculate amount, write record, etc.)

• Focus on verbs (do this, then do this, etc.)

• Use flowcharts to work logic problems

Object Oriented Programming

• Focus on the nouns, the things (i.e. customer, stockroom, order, etc.)

• Focus on the relationships (i.e. orders for customer, parts for an order, etc.)

• Verbs are methods (Customer.PlaceOrder, Part.CheckInventory, etc.)

• Use Object Oriented Modeling Tools

Object Oriented Programming Languages

For a programming language to be a true OOP language, the language must meet the following criteria:

– Abstraction

– Encapsulation

– Polymorphism

– Inheritance

Abstraction

Abstraction manages the complexities of a business problem by allowing you to identify a set of objects involved with that business problem.

Encapsulation

Encapsulation hides the internal implementation of an abstraction within the particular object.

Polymorphism

Polymorphism provides for multiple implementations of the same method. For example, different objects can have a save method, each of which perform different processing.

Polymorphism

poly many morph forms

• When all derived types from a common base class provide the same method, with implementations specific to the derived class.

• To create polymorphic methods add virtual to the base class method.

• In the derived class add the word override.

Inheritance• The OO term for the ability of a type to use

and extend the functionality of a base type.

• A class inherits state and behavior from its superclass.

• Inheritance provides a powerful and natural mechanism for organizing and structuring software programs.

• .NET provides for true implementation inheritance whereby you can reuse the implementation of a class.

Implementation Inheritance

• A class can inherit from a superclass, and thereby the subclass derives the implementation of the methods in the superclass. The subclass can also override some of the methods and extend the behavior by adding more methods.

• One big advantage of implementation inheritance is code reuse.

• Unlike classic C++, the CLR (and hence C#) does not support multiple implementation inheritance.

Inheritance

To inherit from is to be a specialized version of –

Derivied Class

• Can not inherit a constructor

• Must implement its own constructor

• Can only access the base constructor by calling it implicitly

C# Programming – Writing Classes

Homework – Build a NameLister Class