Muokkaa

Jaa

Const Statement (Visual Basic)

  • Artikkeli

Declares and defines one or more constants.

Syntax

[ <attributelist> ] [ accessmodifier ] [ Shadows ]
Const constantlist

Parts

attributelist
Optional. List of attributes that apply to all the constants declared in this statement. See Attribute List in angle brackets ("<" and ">").

accessmodifier
Optional. Use this to specify what code can access these constants. Can be Public, Protected, Friend, Protected Friend, Private, or Private Protected.

Shadows
Optional. Use this to redeclare and hide a programming element in a base class. See Shadows.

constantlist
Required. List of constants being declared in this statement.

constant [ , constant ... ]

Each constant has the following syntax and parts:

constantname [ As datatype ] = initializer

Part Description
constantname Required. Name of the constant. See Declared Element Names.
datatype Required if Option Strict is On. Data type of the constant.
initializer Required. Expression that is evaluated at compile time and assigned to the constant.

Remarks

If you have a value that never changes in your application, you can define a named constant and use it in place of a literal value. A name is easier to remember than a value. You can define the constant just once and use it in many places in your code. If in a later version you need to redefine the value, the Const statement is the only place you need to make a change.

You can use Const only at module or procedure level. This means the declaration context for a variable must be a class, structure, module, procedure, or block, and cannot be a source file, namespace, or interface. For more information, see Declaration Contexts and Default Access Levels.

Local constants (inside a procedure) default to public access, and you cannot use any access modifiers on them. Class and module member constants (outside any procedure) default to private access, and structure member constants default to public access. You can adjust their access levels with the access modifiers.

Rules

  • Declaration Context. A constant declared at module level, outside any procedure, is a member constant; it is a member of the class, structure, or module that declares it.

    A constant declared at procedure level is a local constant; it is local to the procedure or block that declares it.

  • Attributes. You can apply attributes only to member constants, not to local constants. An attribute contributes information to the assembly's metadata, which is not meaningful for temporary storage such as local constants.

  • Modifiers. By default, all constants are Shared, Static, and ReadOnly. You cannot use any of these keywords when declaring a constant.

    At procedure level, you cannot use Shadows or any access modifiers to declare local constants.

  • Multiple Constants. You can declare several constants in the same declaration statement, specifying the constantname part for each one. Multiple constants are separated by commas.

Data Type Rules

  • Data Types. The Const statement can declare the data type of a variable. You can specify any data type or the name of an enumeration.

  • Default Type. If you do not specify datatype, the constant takes the data type of initializer. If you specify both datatype and initializer, the data type of initializer must be convertible to datatype. If neither datatype nor initializer is present, the data type defaults to Object.

  • Different Types. You can specify different data types for different constants by using a separate As clause for each variable you declare. However, you cannot declare several constants to be of the same type by using a common As clause.

  • Initialization. You must initialize the value of every constant in constantlist. You use initializer to supply an expression to be assigned to the constant. The expression can be any combination of literals, other constants that are already defined, and enumeration members that are already defined. You can use arithmetic and logical operators to combine such elements.

    You cannot use variables or functions in initializer. However, you can use conversion keywords such as CByte and CShort. You can also use AscW if you call it with a constant String or Char argument, since that can be evaluated at compile time.

Behavior

  • Scope. Local constants are accessible only from within their procedure or block. Member constants are accessible from anywhere within their class, structure, or module.

  • Qualification. Code outside a class, structure, or module must qualify a member constant's name with the name of that class, structure, or module. Code outside a procedure or block cannot refer to any local constants within that procedure or block.

Example 1

The following example uses the Const statement to declare constants for use in place of literal values.

' The following statements declare constants.
Const maximum As Long = 459
Public Const helpString As String = "HELP"
Private Const startValue As Integer = 5

Example 2

If you define a constant with data type Object, the Visual Basic compiler gives it the type of initializer, instead of Object. In the following example, the constant naturalLogBase has the run-time type Decimal.

Const naturalLogBase As Object = CDec(2.7182818284)
MsgBox("Run-time type of constant naturalLogBase is " &
    naturalLogBase.GetType.ToString())

The preceding example uses the ToString method on the Type object returned by the GetType Operator, because Type cannot be converted to String using CStr.

See also