Constant and Literal Data Types

發行項
11/18/2006

A literal is a value that is expressed as itself rather than as a variable's value or the result of an expression, such as the number 3 or the string "Hello". A constant is a meaningful name that takes the place of a literal and retains this same value throughout the program, as opposed to a variable, whose value may change.

When Option Strict is On, you must declare all constants explicitly with a data type. In the following example, the data type of MyByte is explicitly declared as data type Byte:

Option Strict On
Public Const MyByte As Byte = 2

Explicitly declaring data types is unambiguous, and typed code is easier to read and maintain than untyped code. When Option Strict is Off, however, the compiler uses the type of the expression used to initialize the constant. A numeric integer literal is cast by default to the Integer data type. The default data type for floating-point numbers is Double, and the keywords True and False specify a Boolean constant.

Literals and Type Coercion

In some cases, you might want to force a literal to a particular data type; for example, when assigning a particularly large integral literal value to a variable of type Decimal. The following example produces an error:

Dim myDecimal as Decimal
myDecimal = 100000000000000000000   ' This causes a compiler error.

The error results from the representation of the literal. The Decimal data type can hold a value this large, but the literal is implicitly represented as a Long, which cannot.

You can coerce a literal to a particular data type in two ways: by appending a type character to it, or by placing it within enclosing characters. A type character or enclosing characters must immediately precede and/or follow the literal, with no intervening space or characters of any kind.

To make the previous example work, you can append the D type character to the literal, which causes it to be represented as a Decimal:

MyDecimal = 100000000000000000000D

The following example demonstrates correct usage of type characters and enclosing characters:

Option Strict Off
Public Const DefaultInteger = 100   ' Default is Integer.
Public Const DefaultDouble = 54.3345612   ' Default is Double.
Public Const MyCharacter = "a"C   ' Forces constant to be a Char type.
Public Const MyDate = #01/15/01#   ' Demonstrates DateTime constants.
Public Const MyTime = #1:15:59 AM#
Public Const MyLong = 45L   ' Forces data type to be a Long.
Public Const MySingle = 45.55!   ' Forces data type to be a Single.

The following table shows the enclosing characters and type characters available in Visual Basic.

Data type	Enclosing character	Appended type character
Boolean	(none)	(none)
Byte	(none)	(none)
Char	"	C
Date	#	(none)
Decimal	(none)	D or @
Double	(none)	R or #
Integer	(none)	I or %
Long	(none)	L or &
Short	(none)	S
Single	(none)	F or !
String	"	(none)

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Constant and Literal Data Types

Literals and Type Coercion

See Also

Tasks

Reference

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