Items in Visual Basic programs Program statement: A program statement is a line of code in a Visual Basic program, a self-contained instruction executed

Items in Visual Basic programs

Program statement: A program statement is a line of code in a Visual

Basic program, a self-contained instruction executed by the Visual Basic compiler that performs useful work within the application.

All program statements must follow syntax rules defined and enforced by the Visual Basic compiler.

In Visual Studio 2005, program statements can be composed of keywords, properties, object names, variables, numbers, special symbols, and other values.


Keyword: A keyword is a reserved word within the Visual

Basic language that is recognized by the Visual Basic compiler and performs useful work.

Keywords are one of the basic building blocks of program statements; they work together with objects, properties, variables, and other values to form complete lines of code and (therefore) instructions for the compiler and operating system.


Control: A control is a tool you use to create

objects on a Visual Basic form. You select controls from the Toolbox and use them to draw objects with the mouse on a form.

You use most controls to create user interface elements, such as buttons, picture boxes, and list boxes.


Object: An object is a user interface element that you

create on a visual Basic form with a control in the Toolbox. In Visual Basic, the form itself is also an object. You can move, resize, and customize objects by using property settings.

Objects have what is known as inherent functionality, they know how to operate and can respond to certain situations on their own. You can program Visual Basic objects by using customized event procedures for different situations in a program.


Class: A class is a blueprint or template for one or more

objects that defines what the object does. Accordingly, a class defines what an object can do but is not the object itself.

In Visual Basic 2005, you can use existing Visual Studio classes (like System,Math and System.Windows.Forms.Form), and you can build your own classes and inherit properties, methods, and events from them. (Inheritance allows one class to acquire the pre-existing interface and behaviour characteristics of another class.)


Property: A property is a value, or characteristic,

held by an object. For example, a button object has a

Text property to specify the text that appears on the button

and an Image property to specify the path to an image file that should appear on the button face.

In Visual Basic, properties can be set at design time by using the Properties window or at run time by using statements in the program code.


Event procedure : An event procedure is a block of code

that's executed when an object is manipulated in a program. For example,

when the Button1 object is clicked, the Button1_click event procedure is

executed. Event procedures typically evaluate and

set properties and use other program statements to perform the work of the program.


Method : A method is a special statement that performs

an action or a service for a particular object in a program.

In program code, the notation for using a method is object.Method( value) where

Object is the name of the object you want to work with, Method is the action you want to perform, and Value is an optional argument to be used by the

method.


Methods and properties are often identified by their position in a collection or object library, so don't be surprised if you see long references such as System.Drawing.Image.FromFile, which would be read as

"the FromFile method, which is a member of the Image class, which is a member of the System. Drawing object library."


Variable : A variable is a temporary storage location

for data in your program. You can use one or many variables in

your code, and they can contain words, numbers, dates, properties, or other values.

By using variables, you can assign a short and easy to remember name to each piece of data you plan to work with.

Variables can hold information entered by the user at run time, the result of a specific calculation, or a piece of data you want to display on your form. In short, variables are handy containers that you can use to store and track almost any type of information.

Using variables in a Visual Basic program requires some planning. Before you can use a variable, you must set aside memory in the computer for the variable's use.

Variable Declaration To declare a variable in Visual Basic 2005, type the

variable name after the Dim statement. (Dim stands for dimension.)

This declaration reserves room in memory for the variable when the program runs and lets Visual Basic know what type of data it should expect to see later.

Although this declaration can be done at any place in the program code (as long as the declaration happens before the variable is used), most programmers declare variables in one place at the top of their event procedures or code modules.

For example, the following statement creates space for a variable named LastName that will hold a textual, or string, value:

Dim LastName as String

Variable Declaration

Dim LastName as String

Note that in addition to identifying the variable by name, we've used the

As keyword to give the variable a particular type, and identified the type by using the keyword String.

A string variable contains textual information: words, letters, symbols-even numbers.

Why do you need to declare variables? Visual Basic wants you to identify the name and the type

of your variables in advance so that the compiler can set aside the memory the program will need to store and process the information held in the variables.

Finally, keep in mind another important characteristic of the variables is their scope.

Variable Naming Conventions You need to use names that are short but intuitive

and easy to remember. Begin each variable name with a letter or

underscore. This is a Visual Basic requirement. Variable names can contain only letters,

underscores, and numbers. Although variable names can be virtually any

length, try to keep them under 33 characters to make them easier to read.

Make your variable names descriptive by combining one or more words when it makes sense to do so.

Use a combination of uppercase and lowercase characters and numbers.

Variable Naming Conventions An accepted convention is to capitalize the first letter of

each word in a variable; for example, DateOfBirth. However, some programmers prefer to use so-called

camel casing (making the first letter of a variable name lowercase) to distinguish variable names from functions and module names, which usually begin with uppercase let ters.

Don't use Visual Basic keywords, objects, or properties as variable names. If you do, you'll get an error when you try to run your program.

Optionally, you can begin each variable name with a two-character or three character abbreviation corresponding to the type of data that's stored in the vari able. For example, use strName to show that the Name variable contains string data. (sometimes called the Hungarian Naming Convention)

Using Variables One practical use for a variable is to hold information from the

user. Although you can often use an object such as a list box or a text box to gather this information, at times you might want to deal directly with the user and save the input in a variable rather than in a property. One way to gather input is to use the InputBox function to display a dialog box on the screen and then use a variable to store the text the user types.

FullName = InputBox(prompt) You can display the contents of a variable by assigning the

variable to a property (such as the Text property of a label object) or by passing the variable as an argument to a dialog box function. One useful dialog box function for displaying output is the MsgBox function. When you call the MsgBox function, it displays a dialog box, sometimes called a message box, with various options that you can specify. Like InputBox, it takes one or more arguments as input, and the results of the function call can be assigned to a variable.

MsgBox(FullName, , “Result”)

BooleanBoolean True or false conditionsTrue or false conditions ValueValue TrueTrue

SelectSelectdata type…data type…

SelectSelectdata type…data type… To handle…To handle…To handle…To handle… CTS typeCTS typeCTS typeCTS type ExampleExampleExampleExample

Short, Integer,Long, Byte

Short, Integer,Long, Byte

Whole integersWhole integers ValueValue 23 (Integer)

23 (Integer)

Single, Double,Decimal

Single, Double,Decimal

Numbers with integersand fractional parts

Numbers with integersand fractional parts

ValueValue 9456.72(Decimal)

9456.72(Decimal)

DateDate Date and time valuesDate and time values ValueValue 02/12/200312:30:42 A.M.

02/12/200312:30:42 A.M.

StringString Printable and displayablecharacters

Printable and displayablecharacters

ReferenceReference “House”“House”

ObjectObject A pointer to the value ofan object

A pointer to the value ofan object

ReferenceReference myClassmyPerson

myClassmyPerson

Variable Types

Summary of Variable TypesVisual BasicVisual Basic

.NET type.NET typeVisual BasicVisual Basic

.NET type.NET typeStorageStorage

SizeSizeStorageStorage

SizeSize Range of ValuesRange of ValuesRange of ValuesRange of Values

BooleanBoolean

DateDate

DecimalDecimal

DoubleDouble

IntegerInteger

SingleSingle

2 bytes2 bytes

8 bytes8 bytes

16 bytes16 bytes

8 bytes8 bytes

4 bytes4 bytes

4 bytes4 bytes

True or FalseTrue or False

0:00:00 on January 1, 0001 to11:59:59 P.M. on December 31, 9999

0:00:00 on January 1, 0001 to11:59:59 P.M. on December 31, 9999

Up to 29 significant digits with valuesup to 7.9228 x 10 (signed)

Up to 29 significant digits with valuesup to 7.9228 x 10 (signed)

-4.94065645841246544E-324 to+1.79769313486231570E+308 (signed)

-4.94065645841246544E-324 to+1.79769313486231570E+308 (signed)

-2,147,483,648 to+2,147,483,647 (signed)

-2,147,483,648 to+2,147,483,647 (signed)

-3.4028235E+38 to 1.401298E-45(signed)

-3.4028235E+38 to 1.401298E-45(signed)

StringString VariesVaries 0 to approximately 2 billion Unicodecharacters (unsigned)

0 to approximately 2 billion Unicodecharacters (unsigned)

28

User-Defined Data Types

Visual Basic also lets you create your own data types. This feature is most useful when you're dealing with a group of data items that naturally fit together but fall into different data categories.

You create a user-defined type (UDT) by using the Structure statement, and you declare variables associated with the new type by using the Dim statement.

Be aware that the Structure statement cannot be located in an event procedure-it must be located at the top of the form along with other variable declarations, or in a code module.

For example, the following declaration creates a user-defined data type named Employee that can store the name, date of birth, and hire date associated with a worker:

structure Employee Dim Name AS StringDim DateofBirth As Date Dim HireDate As Date

End structure

Constants

If a variable in your program contains a value that never changes, you might consider storing the value as a constant instead of as a variable.

A constant is a meaningful name that takes the place of a number or a text string that doesn't change.

Constants are useful because they increase the readability of program code, they can reduce programming mistakes, and they make global changes easier to accomplish later.

Constants operate a lot like variables, but you can't modify their values at run time.

They are declared with the Const keyword, as shown in the following example: const pi As Double = 3.14159265

Scope

The location you choose for your declarations should be based on how you plan to use the constants or the variables.

Programmers typically keep the scope for declarations as small as possible, while still making them available for code that needs to use them.

For example, if a constant is needed only in a single event procedure, you should put the constant declaration within that event procedure.

However, you could also place the declaration at the top of the form's code, which would give all the event procedures in your form access to it

VB Operators

A formula is a statement that combines numbers, variables, operators, and keywords to create a new value.

Visual Basic contains several language elements designed for use in formulas.

Mathematical operators are the symbols used to tie together the parts of a formula.

Operators + - * / \ Mod ^ &

Shortcut Operators

Shortcut operators for mathematical and string operations that involve changing the value of an existing variable.

For example, if you combine the + symbol with the = symbol, you can add to a variable without repeating the variable name twice in the formula.

Thus, you can write the formula X = X + 6 by using the syntax X += 6.

Shortcut Operators

Addition(+) X=X+6 X+=6 Subtraction (-) X =X-6 X -= 6 Multiplication (*) X = X*6 X *= 6 Division (I) X = X / 6 X /= 6 Integer division (\) X = X \ 6 X \= 6 Exponentiation (^) X = X^ 6 X ^= 6 String concatenation (&) X = X & 'ABC

X &= 'ABC'

Precedence

Visual Basic lets you mix as many mathematical operators as you like in a formula, as long as each numeric variable and expression is separated from another by one operator. Total = 10 + 15 * 2 / 4 ^ 2

The formula processes several values and assigns the result to a variable named Total. But how is such an expression evaluated by Visual Basic?

In other words, what sequence does Visual Basic follow when solving the formula? You might not have noticed, but the order of evaluation matters a great deal in this example.

Precedence

Values within parentheses are always evaluated first

Exponentiation (raising a number to a power) is second.

Negation (creating a negative number) is third. Multiplication and division are fourth. Integer division is fifth. Remainder division is sixth. Addition and subtraction are last.

Conditional Expression

One of the most useful tools for processing information in an event procedure is a conditional expression.

A conditional expression is a part of a complete program statement that asks a True or-False question about a property, a variable, or another piece of data in the program code.

For example, the conditional expression price < 100

evaluates to True if the Price variable contains a value that is less than 100, and it evaluates to False if Price contains a value that is greater than or equal to 100.

Comparison Operators used in Conditional Expressions

= Equal to<> Not equal to > Greater than < less than>= Greater than or equal to <= Less than or equal to

Boolean expressions

Expressions that can be evaluated as True or False are also known as Boolean expressions, and the True or False result can be assigned to a Boolean variable or property.

You can assign Boolean values to certain object properties or Boolean variables that have been created by using the Dim statement and the As Boolean keywords.

Documents

Items in Visual Basic programs Program statement: A program statement is a line of code in a Visual Basic program, a self-contained instruction executed