Constant and Literal Data Types (Visual Basic)
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 Infer is Off and 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 Class Sample
Public Const MyByte As Byte = 2
End Class
When Option Infer is On or Option Strict is Off, you can declare a constant without specifying a data type with an As clause. The compiler determines the type of the constant from the type of the expression. 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:
Dim MyDecimal As Decimal = 100000000000000000000D
The following example demonstrates correct usage of type characters and enclosing characters:
' Default to Integer.
Public Const DefaultInteger As Integer = 100
' Default to Double.
Public Const DefaultDouble As Double = 54.3345612
' Force constant to be type Char.
Public Const MyCharacter As Char = "a"c
' DateTime constants.
Public Const MyDate As DateTime = #1/15/2001#
Public Const MyTime As DateTime = #1:15:59 AM#
' Force data type to be Long.
Public Const MyLong As Long = 45L
' Force data type to be Single.
Public Const MySingle As Single = 45.55!
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) |
