Introduction to Visual Basic

What is Visual Basic?
•The "Visual" part refers to the method used
to create the graphical user interface (GUI).
Rather than writing numerous lines of code to
describe the appearance and location of
interface elements, you simply add pre-built
objects into place on screen
•The "Basic" part refers to the BASIC
(Beginners All-Purpose Symbolic Instruction
Code) language

Visual Basic Characteristics
• Rapid Application Development (RAD) tool
• Front-end applications.
•ActiveX technologies allow you to use the functionality
provided by other applications, such as Microsoft Word
word processor, Microsoft Excel spreadsheet, and other
Windows applications.
•Your finished application is a true .exe file.

Event-Driven programming Language?
In an event-driven application, the code doesn't follow a
predetermined path — it executes different code sections in
response to events.
Events can be triggered by the user's actions, by messages
from the system or other applications, or even from the
application itself.
The sequence of these events determines the sequence in
which the code executes, thus the path through the
application's code differs each time the program runs.
Your code can also trigger events during execution. For
example, programmatically changing the text in a text box
cause the text box's Change event to occur.

Interactive Development
The traditional application development process can be broken
into three distinct steps: writing, compiling, and testing code.
Unlike traditional languages, Visual Basic uses an interactive
approach to development, blurring the distinction between the
three steps.
With most languages, if you make a mistake in writing your
code, the error is caught by the compiler when you start to
compile your application. You must then find and fix the error
and begin the compile cycle again, repeating the process for
each error found.
Visual Basic interprets your code as you enter it, catching and
highlighting most syntax or spelling errors on the fly.

In addition to catching errors on the fly, Visual Basic also
partially compiles the code as it is entered. If the compiler
finds an error, it is highlighted in your code. You can fix the
error and continue compiling without having to start over.
Because of the interactive nature of Visual Basic, you'll find
yourself running your application frequently as you develop
it. This way you can test the effects of your code as you
work rather than waiting to compile later.

The Integrated Development Environment
Selecting the Project Type

Working with Visual Basic Projects
A project consists of :
•One project file that keeps track of all the components (.vbp).
•One file for each form (.frm).
•One binary data file for each form containing data for properties of
controls on the form (.frx). These files are not editable and are
automatically generated for any .frm file that contains binary
properties, such as Picture or Icon.
•Optionally, one file for each class module (.cls).
•Optionally, one file for each standard module (.bas).
The project file is simply a list of all the files and objects associated
with the project, as well as information on the environment options
you set.
You can convert the project into an executable file (.exe)

Form Modules
Form modules (.frm file name extension) can contain textual
descriptions of the form and its controls, including their property
settings. They also contain form-level declarations of constants,
variables and procedures; event procedures; and general
procedures.
Class Modules
Class modules (.cls file name extension) are similar to form modules,
except that they have no visible user interface. You can use class
modules to create your own objects, including code for methods and
properties.
Standard Modules
Standard modules (.bas file name extension) can contain public or
module-level declarations of types, constants, variables, external
procedures, and public procedures.

Variables
Variable is a placeholder in memory.
Variables are used to temporarily store values during the execution
of an application.
Variables have a name (the word you use to refer to the value the
variable contains) and a data type (which determines the kind of
data the variable can store).
A variable name:
•Must begin with a letter.
•Can't contain an embedded period.
•Must not exceed 255 characters.
•Must be unique within the same scope, which is the range from
which the variable can be referenced — a procedure, a form, and so
on.

Implicit Declaration
You don't have to declare a variable before using it.
Visual Basic automatically creates a variable with that name, which
you can use as if you had explicitly declared it. While this is
convenient, it can lead to subtle errors in your code if you misspell a
variable name.
Example:
x = “Sam”
a = 45
Explicit Declaration
To avoid the problem of misnaming variables, you can stipulate that
Visual Basic always warn you whenever it encounters a name not
declared explicitly as a variable.
Place this statement in the Declarations section of a class, form, or

Scope of Variables
A variable is scoped as either a procedure-level (local) or module-
level variable.
Scope
Procedure-level
Private: Variables are private to the procedure in which they
appear.
Public: Not applicable. You cannot declare public variables
within a procedure.
Module-level
Private: Variables are private to the module in which they
appear.
Public: Variables are available to all modules.

The Variant Data Type
Variant variables can hold any type of data described so far, and then
some.
Variables of this type take 16 bytes, in this format:
Bytes 0 and 1 hold an integer value that states which type of data is
stored in bytes 8 through 15.
Bytes 2 through 7 are unused (with only one exception, the Decimal
subtype), and in most cases not all the bytes in the second half of the
variable are used.
For example, if a Variant holds an Integer value, the first two bytes
contain the value 2-vbInteger, bytes 8 and 9 hold the actual 16-bit value,
and all other bytes are unused.

Variables Used Within a Procedure
You declare them with the Dim or Static keywords.
For example:
Dim intTemp As Integer
or
Static intPermanent As Integer
Variables Used Within a Module
You create module-level variables by declaring them with the
Private keyword in the Declarations section at the top of the
module.
Private intTemp As Integer
At the module level, there is no difference between Private and
Dim, but Private is preferred because it readily contrasts with
Public and makes your code easier to understand.

Variables Used by All Modules
To make a module-level variable available to other modules,
use the Public keyword to declare the variable.
The values in public variables are available to all procedures
in your application. Like all module-level variables,
Public variables are declared in the Declarations section at the
top of the module.
Public intTemp As Integer

Constants
The syntax for declaring a constant is:
[Public|Private] Const constantname[As type] = expression
A Const statement can represent a mathematical or date/time
quantity:
Const conPi = 3.14159
Public Const conMaxPlanets As Integer = 9
Const conReleaseDate = #1/1/95#
Const conCodeName = "Enigma"
You can place more than one constant declaration on a single line if
you separate them with commas:
Public Const conPi = 3.14, conMaxPlanets = 9

Data Types
By default, if you don't supply a data type, the variable is given the
Variant data type.
The Variant data type can represent many different data types in
different situations.
Declaring Variables with Data Types
You must use Private, Public, Dim or Static statement to declare it As
type.
Numeric Data Types
Visual Basic supplies several numeric data types
— Byte, Integer, Long
Single (single-precision floating point), Double (double-precision
floating point), and Currency.

The String Data Type
Private S As String
S = "Database"
You specify a fixed-length string with this syntax:
String * size
For example, to declare a string that is always 50 characters long, use
code like this:
Dim EmpName As String * 50
The Boolean Data Type
If you have a variable that will contain simple true/false
The default value of Boolean is False.
Dim blnRunning As Boolean

The Date Data Type
Date and time values can be contained both in the specific Date data type
and in Variant variables.
The Object Data Type
Object variables are stored as 32-bit (4-byte) addresses that refer to
objects within an application or within some other application.
A variable declared as Object is one that can subsequently be assigned
(using the Set statement) to refer to any actual object recognized by the
application.

Converting Data Types
Conversion functions to convert values into a specific data type.
Conversion function Converts an expression to
Cbool Boolean
Cbyte Byte
Ccur Currency
Cdate Date
CDbl Double
Cint Integer
CLng Long
CSng Single
CStr String
Cvar Variant

Arrays
Fixed-Size Arrays
When declaring an array, follow the array name by the upper bound in
parentheses.
The upper bound cannot exceed the range of a Long data type
(-2,147,483,648 to 2,147,483,647).
For example
Dim Counters(14) As Integer ' 15
elements.
To specify a lower bound, provide it explicitly using the To
keyword:
Dim Sums(100 To 120) As String

Multidimensional Arrays
Dim MatrixA(9, 9) As Double
Either or both dimensions can be declared with explicit lower bounds:
Dim MatrixA(1 To 10, 1 To 10) As Double

Dynamic Arrays
A dynamic array can be resized at any time.
To create a dynamic array
1. Declare the array as dynamic by giving it an empty dimension
list.Dim DynArray()
2. Allocate the actual number of elements with a ReDim
statement.ReDim DynArray(X + 1)
The ReDim statement can appear only in a procedure.
Unlike the Dim and Static statements, ReDim is an executable
statement — it makes the application carry out an action at run time.
Each ReDim can change the number of elements,
as well as the lower and upper bounds.
ReDim DynArray(4 to 12)

Preserving the Contents of Dynamic Arrays
Each time you execute the ReDim statement, all the values currently
stored in the array are lost.
Sometimes you may want to change the size of the array without losing
the data in the array.
You can do this by using ReDim with the Preserve keyword.
ReDim Preserve DynArray(UBound(DynArray) + 1)

User-Defined Types
A user-defined type (UDT) is a compound data structure that holds
several variables of simpler data types.
Before you can use a UDT variable, you must first define its structure,
using a Type directive in the declaration section of a module:
Private Type EmployeeUDT
Name As String Department
ID As Long
Salary As Double
End Type
The Type statement can be used only at module level.

Private Type EmployeeRecord ' Create user-defined type.
ID As Integer ' Define elements of data type.
Name As String * 20
Address As String * 30
Phone As Long
End Type
Private Sub Form_Load()
Dim MyRecord As EmployeeRecord ' Declare variable.
' Assignment to EmployeeRecord variable must occur in a
procedure.
MyRecord.ID = 12003
MsgBox MyRecord.ID
End Sub

Mathematical Operators
Operator Meaning
+, - Unary positive, negative
+ Addition
- Subtraction
* Multiplication
/ Division
^ exponentiation
Interestingly, the division operator (/) always converts both its operands
to Double, which can cause some unexpected overhead.
If you're dividing an Integer or Long number by another Integer or Long
number and you aren't interested in the decimal part of the quotient, you
should use the integer division operator (), which executes faster:

Comparison Operators
Visual Basic supports six comparison operators, which can be applied to
both numeric and string operands:
= < <= > >= <>
Boolean
Visual Basic for Applications supports a few Boolean operators,
which are especially useful for combining multiple Boolean
subexpressions.
The operators used most frequently are
AND, OR, XOR, and NOT.

Rounding and Truncating
The Int function truncates a number to the integer value equal or lower
than its argument.
Print Int(1.2) ' Displays "1"
Print Int(-1.2) ' Displays "-2"
The function that actually truncates the decimal part of a number is Fix:
Print Fix(1.2) ' Displays "1“
Print Fix(-1.2) ' Displays "-1"
Round, which lets you round a decimal number to the number of
digits you want (or to the nearest integer, if the second argument
is omitted):
Print Round(1.45) ' Displays "1"
Print Round(1.55) ' Displays "2"

Round cont…..
When the fractional part is exactly 0.5, it rounds up if the
integer portion is an odd number and rounds down if it's
even:
Print Round(1.5), Round(2.5) ' Both display "2".

Breaking a Single Statement Into Multiple Lines
You can break a long statement into multiple lines in the Code window
using the line-continuation character (a space followed by an
underscore).
Using this character can make your code easier to read, both online and
when printed.
S = _
"SELECT * FROM Titles, Publishers" _
& "WHERE Publishers.PubId = Titles.PubID" _
& "AND Publishers.State = 'CA'"
You can't follow a line-continuation character with a comment on the
same line.

Combining Statements on One Line
You can place two or more statements on a line if you use a
colon (:) to separate them:
Text1.Text = "Hello" : Red = 255
In order to make your code more readable, however, it's better
to place each statement on a separate line.
Adding Comments to Your Code
As you read through the examples in this guide, you'll often
come across the comment symbol ('). This symbol tells Visual
Basic to ignore the words that follow it.

Procedures
These can be of type Sub or Function, depending on whether they
return a value to the caller.
Each procedure has a unique name, a scope, a list of expected
arguments, and—if it's a function—a return value.
Scope can be Private, Public, or Friend.
A Private procedure can be invoked only from within the module in
which it's defined.
A Public procedure can be invoked from outside the module.
A Friend procedure is halfway between Private and Public: Such a
procedure can be called from anywhere in the current project, but
not from outside it. This difference becomes important only if you're
within a project of a type other than Standard EXE, one that
therefore exposes its classes to other applications in the form of
COM components.

Control Flow Branch Statements
If...Then
If condition Then statement
If condition Then
statements
End If
If...Then...Else
If condition1 Then
[statementblock-1]
[ElseIf condition2 Then
[statementblock-2]] ...
[Else
[statementblock-n]]
End If

' Single line version, without Else clause
If x > 0 Then y = x
' Single line version, with Else clause
If x > 0 Then y = x Else y = 0
' Single line, but with multiple statements separated by colons
If x > 0 Then y = x: x = 0 Else y = 0

' Multiline version of the above code (more readable)
If x > 0 Then
y = x
x = 0
Else
y = 0
End If
' An example of If..ElseIf..Else block
If x > 0 Then
y = x
ElseIf x < 0 Then
y = x * x
Else ' X is surely 0, no need to actually test it.
x = -1
End If

Select Case
A Select Case statement provides capability similar to the If...Then...Else
statement, but it makes code more readable when there are several
choices.
Select Case testexpression
[Case expressionlist1
[statementblock-1]]
[Case expressionlist2
[statementblock-2]]
.
.
.
[Case Else
[statementblock-n]]
End Select

The Select Case statement is less versatile than the If block in that it
can test only one expression against a list of values:
Select Case Mid$(Text, i, 1)
Case "0" To "9"
' It's a digit.
Case "A" To "Z", "a" To "z"
' It's a letter.
Case ".", ",", " ", ";", ":", "?"
' It's a punctuation symbol or a space.
Case Else
' It's something else.
End Select

Loop Structures
Do While condition
statements
Loop
Do
statements
Loop While condition
Do
statements
Loop Until condition
While condition
[statements]
Wend

For...Next
The syntax is:
For counter = start To end [Step increment]
statements
Next [counter]
The arguments counter, start, end, and increment are all numeric.
For Each...Next
Syntax :
For Each element In group
statements
Next element

IIf Function
Returns one of two parts, depending on the evaluation of an expression.
Syntax
IIf (expr, truepart, falsepart)
expr. Expression you want to evaluate.
truepart. Value or expression returned if expr is True.
falsepart. Value or expression returned if expr is False.
IIf always evaluates both truepart and falsepart, even though it returns
only one of them. Because of this, you should watch for undesirable side
effects.
For example, if evaluating falsepart results in a division by zero error, an
error occurs even if expr is True.

With Statement
Executes a series of statements on a single object or a user defined type.
Syntax
With object
[statements]
End With
Example:
With MyObject
.Height = 100 ' Same as MyObject.Height = 100.
.Caption = "Hello" ' Same as MyObject.Caption = "Hello".
With .Font
.Color = Red ' Same as MyObject.Font.Color = Red.
.Bold = True ' Same as MyObject.Font.Bold = True.
End With

Introduction to Visual Basic

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