Selection statements select one of a number of possible statements for
execution based on the value of some
expression.
selection-statement:
if-statement
switch-statement
15.7.1 The if statement
The if statement selects a statement for execution based on the value of a
boolean expression.
if-statement:
if ( boolean-expression ) embedded-statement
if ( boolean-expression ) embedded-statement else embedded-statement
boolean-expression:
expression
An else part is associated with the lexically nearest preceding if that is
allowed by the syntax. [Example:
Thus, an if statement of the form
if (x) if (y) F(); else G();
is equivalent to
Chapter 15 Statements
183
if (x) {
if (y) {
F();
}
else {
G();
}
}
end example]
An if statement is executed as follows:
? The boolean-expression (§14.16) is evaluated.
? If the boolean expression yields true, control is transferred to the
first embedded statement. When and
if control reaches the end point of that statement, control is transferred
to the end point of the if
statement.
? If the boolean expression yields false and if an else part is present,
control is transferred to the
second embedded statement. When and if control reaches the end point of
that statement, control is
transferred to the end point of the if statement.
? If the boolean expression yields false and if an else part is not
present, control is transferred to the
end point of the if statement.
The first embedded statement of an if statement is reachable if the if
statement is reachable and the
boolean expression does not have the constant value false.
The second embedded statement of an if statement, if present, is reachable
if the if statement is reachable
and the boolean expression does not have the constant value true.
The end point of an if statement is reachable if the end point of at least
one of its embedded statements is
reachable. In addition, the end point of an if statement with no else part
is reachable if the if statement is
reachable and the boolean expression does not have the constant value true.
15.7.2 The switch statement
The switch statement selects for execution a statement list having an
associated switch label that corresponds
to the value of the switch expression.
switch-statement:
switch ( expression ) switch-block
switch-block:
{ switch-sectionsopt }
switch-sections:
switch-section
switch-sections switch-section
switch-section:
switch-labels statement-list
switch-labels:
switch-label
switch-labels switch-label
switch-label:
case constant-expression :
default :
A switch-statement consists of the keyword switch, followed by a
parenthesized expression (called the
switch expression), followed by a switch-block. The switch-block consists
of zero or more switch-sections,
C# LANGUAGE SPECIFICATION
184
enclosed in braces. Each switch-section consists of one or more
switch-labels followed by a statement-list
(§15.2.1).
The governing type of a switch statement is established by the switch
expression. If the type of the switch
expression is sbyte, byte, short, ushort, int, uint, long, ulong, char,
string, or an enum-type,
then that is the governing type of the switch statement. Otherwise, exactly
one user-defined implicit
conversion (§13.4) must exist from the type of the switch expression to
one of the following possible
governing types: sbyte, byte, short, ushort, int, uint, long, ulong, char,
string. If no such
implicit conversion exists, or if more than one such implicit conversion
exists, a compile-time error occurs.
The constant expression of each case label must denote a value of a type
that is implicitly convertible
(§13.1) to the governing type of the switch statement. A compile-time
error occurs if two or more case
labels in the same switch statement specify the same constant value.
There can be at most one default label in a switch statement.
A switch statement is executed as follows:
? The switch expression is evaluated and converted to the governing type.
? If one of the constants specified in a case label in the same switch
statement is equal to the value of
the switch expression, control is transferred to the statement list
following the matched case label.
? If none of the constants specified in case labels in the same switch
statement is equal to the value of
the switch expression, and if a default label is present, control is
transferred to the statement list
following the default label.
? If none of the constants specified in case labels in the same switch
statement is equal to the value of
the switch expression, and if no default label is present, control is
transferred to the end point of the
switch statement.
If the end point of the statement list of a switch section is reachable, a
compile-time error occurs. This is
known as the ?no fall through? rule. [Example: The example
switch (i) {
case 0:
CaseZero();
break;
case 1:
CaseOne();
break;
default:
CaseOthers();
break;
}
is valid because no switch section has a reachable end point. Unlike C and
C++, execution of a switch
section is not permitted to ?fall through? to the next switch section, and
the example
switch (i) {
case 0:
CaseZero();
case 1:
CaseZeroOrOne();
default:
CaseAny();
}
results in a compile-time error. When execution of a switch section is to
be followed by execution of another
switch section, an explicit goto case or goto default statement must be
used:
Chapter 15 Statements
185
switch (i) {
case 0:
CaseZero();
goto case 1;
case 1:
CaseZeroOrOne();
goto default;
default:
CaseAny();
break;
}
end example]
Multiple labels are permitted in a switch-section. [Example: The example
switch (i) {
case 0:
CaseZero();
break;
case 1:
CaseOne();
break;
case 2:
default:
CaseTwo();
break;
}
is valid. The example does not violate the ?no fall through? rule because
the labels case 2: and default:
are part of the same switch-section. end example]
[Note: The ?no fall through? rule prevents a common class of bugs that
occur in C and C++ when break
statements are accidentally omitted. In addition, because of this rule, the
switch sections of a switch
statement can be arbitrarily rearranged without affecting the behavior of
the statement. For example, the
sections of the switch statement above can be reversed without affecting
the behavior of the statement:
switch (i) {
default:
CaseAny();
break;
case 1:
CaseZeroOrOne();
goto default;
case 0:
CaseZero();
goto case 1;
}
end note]
[Note: The statement list of a switch section typically ends in a break,
goto case, or goto default
statement, but any construct that renders the end point of the statement
list unreachable is permitted. For
example, a while statement controlled by the boolean expression true is
known to never reach its end
point. Likewise, a throw or return statement always transfers control
elsewhere and never reaches its end
point. Thus, the following example is valid:
switch (i) {
case 0:
while (true) F();
case 1:
throw new ArgumentException();
case 2:
return;
}
end note]
[Example: The governing type of a switch statement may be the type string.
For example:
C# LANGUAGE SPECIFICATION
186
void DoCommand(string command) {
switch (command.ToLower()) {
case "run":
DoRun();
break;
case "save":
DoSave();
break;
case "quit":
DoQuit();
break;
default:
InvalidCommand(command);
break;
}
}
end example]
[Note: Like the string equality operators (§14.9.7), the switch statement
is case sensitive and will execute a
given switch section only if the switch expression string exactly matches a
case label constant. end note]
When the governing type of a switch statement is string, the value null is
permitted as a case label
constant.
The statement-lists of a switch-block may contain declaration statements (§1
5.5). The scope of a local
variable or constant declared in a switch block is the switch block.
Within a switch block, the meaning of a name used in an expression context
must always be the same
(§14.5.2.1).
The statement list of a given switch section is reachable if the switch
statement is reachable and at least
one of the following is true:
? The switch expression is a non-constant value.
? The switch expression is a constant value that matches a case label in
the switch section.
? The switch expression is a constant value that doesn?t match any case
label, and the switch section
contains the default label.
? A switch label of the switch section is referenced by a reachable goto
case or goto default
statement.
The end point of a switch statement is reachable if at least one of the
following is true:
? The switch statement contains a reachable break statement that exits the
switch statement.
? The switch statement is reachable, the switch expression is a
non-constant value, and no default
label is present.
? The switch statement is reachable, the switch expression is a constant
value that doesn?t match any
case label, and no default label is present.