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10.3 Constants

Visual Studio .NET 2003

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.

attributesopt   constant-modifiersopt   const   type   constant-declarators   ;
constant-modifiers   constant-modifier
constant-declarators   ,   constant-declarator
identifier   =   constant-expression

A constant-declaration may include a set of attributes (Section 17), a new modifier (Section 10.2.2), and a valid combination of the four access modifiers (Section 10.2.3). 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 (Section 7.15) 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 (Section 6.1).

The type of a constant must be at least as accessible as the constant itself (Section 3.5.4).

The value of a constant is obtained in an expression using a simple-name (Section 7.5.2) or a member-access (Section 7.5.4).

A constant can itself participate in a constant-expression. Thus, a constant may be used in any construct that requires a constant-expression. Examples of such constructs include case labels, goto case statements, enum member declarations, attributes, and other constant declarations.

As described in Section 7.15, 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.

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 (Section 10.4.2) may be used instead.

A constant declaration that declares multiple constants is equivalent to multiple declarations of single constants with the same attributes, modifiers, and type. 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;

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. 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. Constant declarations may depend on constants from other programs, but such dependencies are only possible in one direction. 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.