A constant-expression is an expression that can be fully evaluated at compile-time.

constant-expression

expression

The type of a constant expression can be one of the following: sbyte , byte , short , ushort , int , uint , long , ulong , char , float , double , decimal , bool , string, any enumeration type, or the null type. The following constructs are permitted in constant expressions:

• Literals (including the null literal).
• References to const members of class and struct types.
• References to members of enumeration types.
• Parenthesized sub-expressions, which are themselves constant expressions.
• Cast expressions, provided the target type is one of the types listed above.
• The predefined +, -, !, and ~ unary operators.
• The predefined +, -, *, /, %, <<, >>, &, |, ^, &&, ||, ==, !=, <, >, <=, and >= binary operators, provided each operand is of a type listed above.
• The ?: conditional operator.

Whenever an expression is of one of the types listed above and contains only the constructs listed above, the expression is evaluated at compile-time. This is true even if the expression is a sub-expression of a larger expression that contains non-constant constructs.

The compile-time evaluation of constant expressions uses the same rules as run-time evaluation of non-constant expressions, except that where run-time evaluation would have thrown an exception, compile-time evaluation causes a compile-time error to occur.

Unless a constant expression is explicitly placed in an unchecked context, overflows that occur in integral-type arithmetic operations and conversions during the compile-time evaluation of the expression always cause compile-time errors (14.5.12).

Constant expressions occur in the contexts listed below. In these contexts, a compile-time error occurs if an expression cannot be fully evaluated at compile-time.

• Constant declarations (17.3).
• Enumeration member declarations (21.30).
• case labels of a switch statement (15.7.2).
• goto case statements (15.9.3).
• Dimension lengths in an array creation expression (14.5.10.2) that includes an initializer.
• Attributes (24).

An implicit constant expression conversion (13.1.6) permits a constant expression of type int to be converted to sbyte , byte , short , ushort , uint , or ulong , provided the value of the constant expression is within the range of the destination type.