VB.Net Lecture 2

MARWARI COLLEGE, RANCHI

(AN AUTONOMOUS UNIT OF RANCHI UNIVERSITY FROM 2009)

- Prakash Kumar, Dept. of CA

-Raju Manjhi, Dept. of CA 

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Programming in Visual Basic.NET

BCA/IT- Sem-IV(Old Course)

Type classification

The common type system supports two general categories of types, each of which is further divided into subcategories:

·         Value types

Value types directly contain their data, and instances of value types are either allocated on the stack or allocated inline in a structure. Value types can be built-in (implemented by the runtime), user-defined, or enumerations.

·         Reference types

Reference types store a reference to the value's memory address, and are allocated on the heap. Reference types can be self-describing types, pointer types, or interface types. The type of a reference type can be determined from values of self-describing types. Self-describing types are further split into arrays and class types. The class types are user-defined classes, boxed value types, and delegates. 

NOTE:

·         Variables that are value types each have their own copy of the data, and therefore operations on one variable do not affect other variables. Variables that are reference types can refer to the same object; therefore, operations on one variable can affect the same object referred to by another variable. All types derive from the System.Object base type.

·         Delegates: The runtime supports reference types called delegates that serve a purpose similar to that of function pointers in C++. Unlike function pointers, delegates are secure, verifiable, and type safe. A delegate type can represent any method with a compatible signature. While function pointers can only represent static functions, a delegate can represent both static and instance methods. Delegates are used for event handlers and callback functions in the .NET Framework.

Types and Assemblies

The runtime uses assemblies to locate and load types. The assembly manifest contains the information that the runtime uses to resolve all type references made within the scope of the assembly.

A type name in the runtime has two logical parts: 

1. the assembly name 
2. name of the type within the assembly. 

Two types with the same name but in different assemblies are defined as two distinct types.

Assemblies provide consistency between the scope of names seen by the developer and the scope of names seen by the runtime system. Developers author types in the context of an assembly. The content of the assembly a developer is building establishes the scope of names that will be available at run time.

Types and Namespaces

From the viewpoint of the runtime, a namespace is just a collection of type names. Particular languages might have constructs and corresponding syntax that help developers form logical groups of types, but these constructs are not used by the runtime when binding types. Thus, both the Object and String classes are part of the System namespace, but the runtime only recognizes the full names of each type, which are System.Object and System.String, respectively.

You can build a single assembly that exposes types that look like they come from two different hierarchical namespaces, such as System.Collections and System.Windows.Forms.

If you create a tool to represent types in an assembly as belonging to a hierarchical namespace, the tool must enumerate the types in an assembly or group of assemblies and parse the type names to derive a hierarchical relationship.

Object Oriented terminology:

An object is a structure containing data and methods that manipulate the data. Almost everything you do in Visual Basic is associated with objects. If you are new to object-oriented programming, the following terms and concepts will help you get started.

Classes :

A class is an abstract representation of something, whereas an object is a usable example of the thing the class represents.

“The one exception to this rule is shared class members, which are usable in both instances of a class and object variables declared as the type of the class.”

Fields : Classes are made of fields, properties, methods, and events. Fields and properties represent information that an object contains.

Properties :Properties are retrieved and set like fields, but are implemented using property Get and  Set procedures, which provide more control on how values are set or returned.

Methods :Methods represent actions that an object can perform. We define methods by adding procedures, either Sub routines or functions, to your class.

Events : Events are notifications an object receives from, or transmits to, other objects or applications. Events allow objects to perform actions whenever a specific occurrence takes place.

Instance : Initializes a new instance of the Instance class

Fields, properties, methods, and events are only one half of the object-oriented programming equation. 

True object-orient programming requires objects to support three qualities: encapsulation, inheritance, and polymorphism.

Encapsulation means that a group of related properties, methods, and other members are treated as a single unit or object. Objects can control how properties are changed and methods are executed. Encapsulation also makes it easier to change your implementation at a latter date by letting you hide implementation details of your objects, a practice called data hiding.

Inheritance describes the ability to create new classes based on an existing class. The new class inherits all the properties and methods and events of the base class, and can be customized with additional properties and methods.

Polymorphism means that you can have multiple classes that can be used interchangeably, even though each class implements the same properties or methods in different ways. Polymorphism is essential to object-oriented programming because it allows you to use items with the same names, no matter what type of object is in use at the moment.