Visual Basic/Variables and Types

This lesson will instruct you on the creation of variables in Visual Basic 6, as well as variable types. This lesson requires a very basic understanding of the interface in Visual Basic 6. If you have not done so already, it is recommended that you read the Introduction to VB6.

Setting Up The Project

1. Create a Standard EXE in the same way that you did in the previous lesson.
2. Double-click anywhere in the blank content of your form. This will bring up the code editor with the following code:

Private Sub Form_Load()

End Sub

The code that you type between these two lines will be executed when your program launches. Because of that, its a handy place to try out the new concepts like the ones you'll be learning in this article.

Object Types and Naming Scheme

These are very important things to remember in order to make your source code more easily intelligible.

File Types

You will use various file types while using VB6. These are some the most common ones.

• File type: What people call it
• Prefix: What people prefix the filename with
• Extension: What file extension is used
• Description: What its used for most often

File Type	Prefix	Extension	Description
Project		vbp	General project options
Form	frm	frm	GUI information and private code
BAS Module	mod	bas	Project-wide accessible functions
Class Module	cls	cls	Project-wide accessible subroutines
User Control	uc	ctl	Control object (Like an OCX with source code)
Property page	pag	pag	Property information
OLE Control		ocx	Compiled control object
Dynamic Link Lib		dll	Subs and functions accessible by other programs

Examples of common file names:

• OddCalc.vbp
• frmMain.frm
• frmAbout.frm
• frmPrintInvoice.frm
• modMain.bas
• modSettings.bas
• modDeclares.bas
• modWinsock.bas
• clsWinsock.cls
• ucCustomButton.ctl
• ucTreeView.ctl
• ucWinsock.ctl

Variables

A variable is a word or letter used to reference data used in a program. That data may be an integer like "23", a floating point number like "23.23", a string like "Hello World!", and many other data type.

Variable Names

The following are the requirements when naming the variables in Visual Basic:

• It must be less than 255 characters
• No spacing is allowed
• It must not begin with a number
• Period (dot) is not permitted

For the sake of making sure other people can look at your code and know what you were thinking:

• Prefix your variable name with the appropriate prefix for your variable's data type.
• Make sure the body of your variable name makes it easy to tell what its used for.
• Don't use an ambiguous name like "intUhhhh" or "strX" unless its use is within a very small scope of the program, and even then try not to do it much. Try using descriptive names. For example, to save the total marks sustained by a particular person in an integer, use something like int Marks/Marks Sustained rather than a random string of characters.

Using variables

In order to use a variable it must first be declared, then its usually modified to represent some sort of value, then its often outputted in some way so that you can see a result from it.

Example variable declaration, Setting, and Output:

 Dim strName as String ' Declares a string called strName
 strName = "Billy Bob" ' Sets the value of strName
 MsgBox strName        ' Outputs the value of strName to a Message Box

Example variable declaration, Setting, Modification, and Output:

 Dim strName as String ' Declares a string called strName
 strName = "Billy Bob" ' Sets the value of strName
 strName = strName & " Thornton" ' Appends " Thornton" to strName
 MsgBox strName        ' Outputs the value of strName to a Message Box

Now, go put those lines between Private Sub Form_Load() and End Sub in your program to execute those instructions when the program starts.

Variable Declaration Scope

The keyword used to declare a variable and the location in which its declared defines the scope and duration in which a variable is available.

Keyword:

• The first word used in a variable declaration

Declaration location:

• "Procedure" means the variable is declared within a Sub, Function, or Property
• "Module" means the variable is declared near the top of the module, not inside a procedure

Scope:

• "Procedure" means the variable is available only within the Sub, Function, or Property in which it was defined
• "Module" means the variable is available only within the module in which it was defined
• "Project" means the variable is available throughout all modules in the project

Duration:

• "Procedure" means the variable's value is gone once the procedure its available in has finished
• "Application" means the variable's value will remain throughout running of the program

Keyword	Declaration location	Scope	Duration
Dim	Procedure	Procedure	Procedure
Static	Procedure	Procedure	Application
Private	Module	Module	Application
Public	Module	Project	Application
Const	Procedure	Procedure	Procedure
Private Const	Module	Module	Application
Public Const	Module	Project	Application
Global	Module	Project	Application

Data Types

Data Types are classifications of what sort of data is contained inside of a variable. Variables of certain data types may only contain values which are acceptable for that data type. The following tables describe the data types available in VB6. You may select any of the words under the column "Type" in order to use that data type.

Examples:

 Dim intCount As Integer
 Dim lngHwnd As Long
 Dim sngPi as Single
 Dim strName As String
 Dim dtmBirth As Date
 Dim blnToggle As Boolean

Numeric Data Types

Type	Size	Range of Values	Prefix	Example Variable Name
Byte	1 byte	0 to 255	byt	bytFirstChar
Integer	2 bytes	-32,768 to 32,767	int	intCount
Long	4 bytes	-2,147,483,648 to 2,147,483,648	lng	lngHwnd
Single	4 bytes	Negative values: -3.402823E+38 to -1.401298E-45 Positive values: 3.402823E+38 to 1.401298E-45	sng	sngPi
Double	8 bytes	Negative values: -1.79769313486232e+308 to -4.94065645841247E-324 Positive values: 1.79769313486232e+308 to 4.94065645841247E-324	dbl	dblAngle
Currency	8 bytes	-922,337,203,685,477.5808 to 922,337,203,685,477.5807	cur	curTotalCost

Non-numeric Data Types

Type	Size	Range of Values	Prefix	Example Variable Name
String	Length of string	1 to 65,400 characters (fixed length)	str	strName
String	Length + 10 bytes	0 to 2 billion characters (variable length)	str	strHTM4
Date	8 bytes	January 1, 100 to December 31, 9999	dtm	dtmBirth
Boolean	2 bytes	True or False	bln	blnToggle
Object	4 bytes	Any embedded object	obj	objCurrent
Variant	16 bytes	Any value as large as Double (numeric)	vnt	vntNumber

Control Types

Control Type	Prefix
TextBox	txt
PictureBox	pic
Module	mod
Form	frm
Label	lbl
Frame	fra
CommandButton	cmd
CheckBox	chk
RadioButton	rad
ComboBox	cbo
ListBox	lst
Scroll Bar	sbr (no orientation needed)
Timer	tmr
DriveListBox	drv
DirListBox	dir
FileListBox	fil
Shape	shp
Image	img
Data	dat
OLE	ole
ListView	lvw
TreeView	tvw

Examples of common object names:

• txtName
• txtAddress
• cboYear
• cmdOK
• cmdCancel

Learning Visual Basic

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