CONTINUE..

VARIABLE-NAMING CONVENTIONS

When declaring variables, you should be aware of few naming conventions. A variable’s name:

Must begin with a letter Mustn’t exceed 255 characters. Must be unique within its scope Cannot include any special character except

“_” (underscore) Cannot include a space

VARIABLE-NAMING RULES:

A variable names in VB.NET are NOT case sensitive.

Variables can be prefixed to indicate their data type (prefixes indicate a variable's data type)

SCOPE AND LIFETIME OF VARIABLES

Scope = visibility of a variable is the section of the application that can see and manipulate the variable.

If a variable is declared within a procedure, only the code in the specific procedure has access to that variable.

This variable doesn’t exist for the rest of the application. When the variable’s scope is limited to a procedure, it’s called local.

ACCESS SPECIFIERS

Access specifiersAccess specifiers let's us specify how a let's us specify how a variable, method or a class can be used. The variable, method or a class can be used. The following are the most commonly used one's: following are the most commonly used one's:

Public: Gives variable public access which means that there is no restriction on their accessibility.

Private: Gives variable private access which means that they are accessible only within their declaration content.

ACCESS SPECIFIERS .. CONT

Protected: Protected access gives a variable accessibility within their own class or a class derived from that class.

Static: Makes a variable static which means that the variable will hold the value even the procedure in which they are declared ends.

Shared: Declares a variable that can be shared across many instances and which is not associated with a specific instance of a class or structure.

TYPES OF VARIABLES

Visual Basic.NET recognizes different categories of variables as shown in the previews Table & the most commonly used include:

Numeric Boolean String Character Date

NUMERIC VARIABLES

Note:Note: The data type of your variable can make a difference in the results of the calculations.

In our previous discussion of data types, we lumped all numeric data together; however, Visual Basic has several numeric data types.

BOOLEAN VARIABLES

The Boolean data type stores True/False values. Boolean variables are, integers that take the value 1 (for True) and 0 (for False). Actually, any non-zero value is considered True. Boolean variables are declared as:

 

Dim failure As BooleanDim failure As Boolean

   Boolean variables are used in testing

conditions, such as the following: If failure Then MessageBox.Show(“Couldn’t

complete the operation”)  

STRING VARIABLES

The String data type stores only text, and string variables are declared with the String type:

  Dim someText As StringDim someText As String    

You can assign any text to the variable someTxt. You can store nearly 2 GB of text in a string variable (that’s 2 billion characters and is much more text than you care to read on a computer screen).

 The following assignments are all valid:

Dim aString As StringDim aString As String

aString = “MIS is a cool course”aString = “MIS is a cool course”

aString = “” aString = “”

aString = “111, 000”aString = “111, 000”

  Assignment number 2 creates an empty string, and the

last one creates a string that just happens to contain numeric digits, which are also characters.

 

THE EMPTY STRING

The string "", which contains no characters, is called the empty string or the zero-length string.

The statement lstBox.Items.Add("") skips a line in the list box. The contents of a text box can be cleared with either the statement:

 txtBox.Clear()  or the statement txtBox.Text = ""  

USING TEXT BOXES FOR INPUT AND OUTPUT

The contents of a text box is always a string

Input example strVar = txtBox.Text  

Output example txtBox.Text = strVar 

DATA CONVERSION

Because the content of a text box or a label is always a stringstring, sometimes you must convert the input or output value into another type.

Table 2 shows the VB functions that perform data-type conversions. 

FunctionConverts Its Argument To:

CBoolBoolean

CByteByte

CCharUnicode character

CDateDate

CDblDouble

CDecDecimal

CIntInteger (4-byte integer, int32)

CLngLong (8-byte integer, int64)

CObjObject

CShortShort (2-byte integer, int16)

CSngSingle

CStrString

EXAMPLE:

numVar = CDbl(txtBox.Text)-Converts a String to a Double

 txtBox.Text = CStr(numVar)-Converts a number to a string

CHARACTER VARIABLES

Character variables store a single Unicode character in two bytes (16 bit).

you can use the CChart() function to convert integers to characters and the CInt() function to convert to their equivalent integer values.

DATE VARIABLES

Date and time values are stored internally in a special format, but you don’t need to know the exact format.

They are double-precision numbers: the integer part represents the date and the fractional part represents the time.

A variable declared as Date can store both date and time values with a statement like the following:

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Dim expiration As Date  The following are all valid assignments: expiration = #01/01/2004?expiration = # 1/2/2004 10;26:11 pm#expiration = “July 2, 2004”expiration = Now()