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7.18Constant expressions

A constant expression must be the null literal or a value with one of the following types: sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, string, or any enumeration type. Only 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.
  
• 
  References to const parameters or local variables
  
• 
  Parenthesized sub-expressions, which are themselves constant expressions.
  
• 
  Cast expressions, provided the target type is one of the types listed above.
  
• 
  checked and unchecked expressions
  
• 
  Default value expressions
  
• 
  The predefined +, –, !, and ~ unary operators.
  
• 
  The predefined +, –, *, /, %, <<, >>, &, |, ^, &&, ||, ==, !=, <, >, <=, and >= binary operators, provided each operand is of a type listed above.
  
• 
  The ?: conditional operator.
  

• 
  Identity conversions
  
• 
  Numeric conversions
  
• 
  Enumeration conversions
  
• 
  Constant expression conversions
  
• 
  Implicit and explicit reference conversions, provided that the source of the conversions is a constant expression that evaluates to the null value.
  

class C {

the initialization of iis an error because a boxing conversion is required. The initialization of str is an error because an implicit reference conversion from a non-null value is required.

Whenever an expression fulfills the requirements 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 (§7.18).

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 (§10.4).
  
• 
  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 (§7.5.10.4) that includes an initializer.
  
• 
  Attributes (§17).
  

7.19Boolean expressions

A boolean-expression is an expression that yields a result of type bool.

boolean-expression:
expression

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 (§7.13) 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.

8.Statements

C# provides a variety of statements. Most of these statements will be familiar to developers who have programmed in C and C++.

statement:
labeled-statement
declaration-statement
embedded-statement

embedded-statement:
block
empty-statement
expression-statement
selection-statement
iteration-statement
jump-statement
try-statement
checked-statement
unchecked-statement
lock-statement
using-statement
yield-statement

The embedded-statement nonterminal is used for statements that appear within other statements. The use of embedded-statement rather than statement excludes the use of declaration statements and labeled statements in these contexts. The example

void F(bool b) {
if (b)
int i = 44;
}

results in a compile-time error because an if statement requires an embedded-statement rather than a statement for its if branch. If this code were permitted, then the variable i would be declared, but it could never be used. Note, however, that by placing i’s declaration in a block, the example is valid.

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