17.3 Constants

A constant is a class member that represents a constant value: a value that
can be computed at compile-time. A
constant-declaration introduces one or more constants of a given type.
constant-declaration:
attributesopt constant-modifiersopt const type constant-declarators ;
constant-modifiers:
constant-modifier
constant-modifiers constant-modifier
constant-modifier:
new
public
protected
internal
private
constant-declarators:
constant-declarator
constant-declarators , constant-declarator
constant-declarator:
identifier = constant-expression
A constant-declaration may include a set of attributes (§24), a new
modifier (§17.2.2), and a valid combination
of the four access modifiers (§17.2.3). The attributes and modifiers apply
to all of the members declared by the
constant-declaration. Even though constants are considered static members,
a constant-declaration neither
requires nor allows a static modifier. It is an error for the same modifier
to appear multiple times in a constant
declaration.
The type of a constant-declaration specifies the type of the members
introduced by the declaration. The type is
followed by a list of constant-declarators, each of which introduces a new
member. A constant-declarator
consists of an identifier that names the member, followed by an ?=? token,
followed by a constant-expression
(§14.15) that gives the value of the member.
The type specified in a constant declaration must be sbyte, byte, short,
ushort, int, uint, long, ulong,
char, float, double, decimal, bool, string, an enum-type, or a
reference-type. Each constant-expression
must yield a value of the target type or of a type that can be converted to
the target type by an implicit conversion
(§13.1).
The type of a constant must be at least as accessible as the constant
itself (§10.5.4).
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220
The value of a constant is obtained in an expression using a simple-name (§1
4.5.2) or a member-access (§14.5.4).
A constant can itself participate in a constant-expression. Thus, a
constant may be used in any construct that
requires a constant-expression. [Note: Examples of such constructs include
case labels, goto case statements,
enum member declarations, attributes, and other constant declarations. end
note]
[Note: As described in §14.15, a constant-expression is an expression that
can be fully evaluated at compile-time.
Since the only way to create a non-null value of a reference-type other
than string is to apply the new operator,
and since the new operator is not permitted in a constant-expression, the
only possible value for constants of
reference-types other than string is null. end note]
When a symbolic name for a constant value is desired, but when the type of
that value is not permitted in a
constant declaration, or when the value cannot be computed at compile-time
by a constant-expression, a
readonly field (§17.4.2) may be used instead. [Note: The versioning
semantics of const and readonly differ
(§17.4.2.2). end-note]
A constant declaration that declares multiple constants is equivalent to
multiple declarations of single constants
with the same attributes, modifiers, and type. [Example: For example
class A
{
public const double X = 1.0, Y = 2.0, Z = 3.0;
}
is equivalent to
class A
{
public const double X = 1.0;
public const double Y = 2.0;
public const double Z = 3.0;
}
end example]
Constants are permitted to depend on other constants within the same
program as long as the dependencies are
not of a circular nature. The compiler automatically arranges to evaluate
the constant declarations in the
appropriate order. [Example: In the example
class A
{
public const int X = B.Z + 1;
public const int Y = 10;
}
class B
{
public const int Z = A.Y + 1;
}
the compiler first evaluates A.Y, then evaluates B.Z, and finally evaluates
A.X, producing the values 10, 11,
and 12. end example] Constant declarations may depend on constants from
other programs, but such
dependencies are only possible in one direction. [Example: Referring to the
example above, if A and B were
declared in separate programs, it would be possible for A.X to depend on
B.Z, but B.Z could then not
simultaneously depend on A.Y. end example]