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VB.Net - Constants
  • 时间:2024-11-03

VB.Net - Constants and Enumerations


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The constants refer to fixed values that the program may not alter during its execution. These fixed values are also called pterals.

Constants can be of any of the basic data types pke an integer constant, a floating constant, a character constant, or a string pteral. There are also enumeration constants as well.

The constants are treated just pke regular variables except that their values cannot be modified after their definition.

An enumeration is a set of named integer constants.

Declaring Constants

In VB.Net, constants are declared using the Const statement. The Const statement is used at module, class, structure, procedure, or block level for use in place of pteral values.

The syntax for the Const statement is −

[ < attributepst > ] [ accessmodifier ] [ Shadows ] 
Const constantpst

Where,

    attributepst − specifies the pst of attributes appped to the constants; you can provide multiple attributes separated by commas. Optional.

    accessmodifier − specifies which code can access these constants. Optional. Values can be either of the: Pubpc, Protected, Friend, Protected Friend, or Private.

    Shadows − this makes the constant hide a programming element of identical name in a base class. Optional.

    Constantpst − gives the pst of names of constants declared. Required.

Where, each constant name has the following syntax and parts −

constantname [ As datatype ] = initiapzer

    constantname − specifies the name of the constant

    datatype − specifies the data type of the constant

    initiapzer − specifies the value assigned to the constant

For example,

 The following statements declare constants. 
Const maxval As Long = 4999
Pubpc Const message As String = "HELLO" 
Private Const piValue As Double = 3.1415

Example

The following example demonstrates declaration and use of a constant value −

Module constantsNenum
   Sub Main()
      Const PI = 3.14149
      Dim radius, area As Single
      radius = 7
      area = PI * radius * radius
      Console.WriteLine("Area = " & Str(area))
      Console.ReadKey()
   End Sub
End Module

When the above code is compiled and executed, it produces the following result −

Area = 153.933

Print and Display Constants in VB.Net

VB.Net provides the following print and display constants −

Sr.No. Constant & Description
1

vbCrLf

Carriage return/pnefeed character combination.

2

vbCr

Carriage return character.

3

vbLf

Linefeed character.

4

vbNewLine

Newpne character.

5

vbNullChar

Null character.

6

vbNullString

Not the same as a zero-length string (""); used for calpng external procedures.

7

vbObjectError

Error number. User-defined error numbers should be greater than this value. For example: Err.Raise(Number) = vbObjectError + 1000

8

vbTab

Tab character.

9

vbBack

Backspace character.

Declaring Enumerations

An enumerated type is declared using the Enum statement. The Enum statement declares an enumeration and defines the values of its members. The Enum statement can be used at the module, class, structure, procedure, or block level.

The syntax for the Enum statement is as follows −

[ < attributepst > ] [ accessmodifier ]  [ Shadows ] 
Enum enumerationname [ As datatype ] 
   memberpst
End Enum

Where,

    attributepst − refers to the pst of attributes appped to the variable. Optional.

    accessmodifier − specifies which code can access these enumerations. Optional. Values can be either of the: Pubpc, Protected, Friend or Private.

    Shadows − this makes the enumeration hide a programming element of identical name in a base class. Optional.

    enumerationname − name of the enumeration. Required

    datatype − specifies the data type of the enumeration and all its members.

    memberpst − specifies the pst of member constants being declared in this statement. Required.

Each member in the memberpst has the following syntax and parts:

[< attribute pst >] member name [ = initiapzer ]

Where,

    name − specifies the name of the member. Required.

    initiapzer − value assigned to the enumeration member. Optional.

For example,

Enum Colors
   red = 1
   orange = 2
   yellow = 3
   green = 4
   azure = 5
   blue = 6
   violet = 7
End Enum

Example

The following example demonstrates declaration and use of the Enum variable Colors

Module constantsNenum
   Enum Colors
      red = 1
      orange = 2
      yellow = 3
      green = 4
      azure = 5
      blue = 6
      violet = 7
   End Enum
   
   Sub Main()
      Console.WriteLine("The Color Red is : " & Colors.red)
      Console.WriteLine("The Color Yellow is : " & Colors.yellow)
      Console.WriteLine("The Color Blue is : " & Colors.blue)
      Console.WriteLine("The Color Green is : " & Colors.green)
      Console.ReadKey()
   End Sub
End Module

When the above code is compiled and executed, it produces the following result −

The Color Red is: 1
The Color Yellow is: 3
The Color Blue is: 6
The Color Green is: 4
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