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An expression is either a conditional-expression or an assignment.
A constant-expression is an expression that can be fully evaluated at compile-time.
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.
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 (§7.5.12).
Constant expressions occur in the contexts listed below. In these contexts, an error occurs if an expression cannot be fully evaluated at compile-time.
Constant declarations (§10.3).
Enumeration member declarations (§14.3).
case labels of a switch statement (§8.7.2).
goto case statements (§8.9.3).
Dimension lengths in an array creation expression (§126.96.36.199) that includes an initializer.
An implicit constant expression conversion (§6.1.6) 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.
A boolean-expression is an expression that yields a result of type bool.
The controlling conditional expression of an if-statement (§8.7.1), while-statement (§8.8.1), do-statement (§8.8.2), or for-statement (§8.8.3) is a boolean-expression. The controlling conditional expression of the ?: operator (§7.12) follows the same rules as a boolean-expression, but for reasons of operator precedence is classified as a conditional-or-expression.
A boolean-expression is required to be of a type that can be implicitly converted to bool or of a type that implements operator true. If neither requirement is satisfied, a compile-time error occurs.
When a boolean expression is of a type that cannot be implicitly converted to bool but does implement operator true, then following evaluation of the expression, the operator true implementation provided by that type is invoked to produce a bool value.
The DBBool struct type in §11.4.2 provides an example of a type that implements operator true and operator false.
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