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The
if
,
if-else
and
switch
statements select statements to execute from many possible paths based on the value of an expression. The
if
statement
executes a statement only if a provided Boolean expression evaluates to
true
. The
if-else
statement
allows you to choose which of the two code paths to follow based on a Boolean expression. The
switch
statement
selects a statement list to execute based on a pattern match with an expression.
The
if
statement
An
if
statement can be any of the following two forms:
An
if
statement with an
else
part selects one of the two statements to execute based on the value of a Boolean expression, as the following example shows:
DisplayWeatherReport(15.0); // Output: Cold.
DisplayWeatherReport(24.0); // Output: Perfect!
void DisplayWeatherReport(double tempInCelsius)
if (tempInCelsius < 20.0)
Console.WriteLine("Cold.");
Console.WriteLine("Perfect!");
An
if
statement without an
else
part executes its body only if a Boolean expression evaluates to
true
, as the following example shows:
DisplayMeasurement(45); // Output: The measurement value is 45
DisplayMeasurement(-3); // Output: Warning: not acceptable value! The measurement value is -3
void DisplayMeasurement(double value)
if (value < 0 || value > 100)
Console.Write("Warning: not acceptable value! ");
Console.WriteLine($"The measurement value is {value}");
You can nest
if
statements to check multiple conditions, as the following example shows:
DisplayCharacter('f'); // Output: A lowercase letter: f
DisplayCharacter('R'); // Output: An uppercase letter: R
DisplayCharacter('8'); // Output: A digit: 8
DisplayCharacter(','); // Output: Not alphanumeric character: ,
void DisplayCharacter(char ch)
if (char.IsUpper(ch))
Console.WriteLine($"An uppercase letter: {ch}");
else if (char.IsLower(ch))
Console.WriteLine($"A lowercase letter: {ch}");
else if (char.IsDigit(ch))
Console.WriteLine($"A digit: {ch}");
Console.WriteLine($"Not alphanumeric character: {ch}");
In an expression context, you can use the
conditional operator
?:
to evaluate one of the two expressions based on the value of a Boolean expression.
The
switch
statement
The
switch
statement selects a statement list to execute based on a pattern match with a match expression, as the following example shows:
DisplayMeasurement(-4); // Output: Measured value is -4; too low.
DisplayMeasurement(5); // Output: Measured value is 5.
DisplayMeasurement(30); // Output: Measured value is 30; too high.
DisplayMeasurement(double.NaN); // Output: Failed measurement.
void DisplayMeasurement(double measurement)
switch (measurement)
case < 0.0:
Console.WriteLine($"Measured value is {measurement}; too low.");
break;
case > 15.0:
Console.WriteLine($"Measured value is {measurement}; too high.");
break;
case double.NaN:
Console.WriteLine("Failed measurement.");
break;
default:
Console.WriteLine($"Measured value is {measurement}.");
break;
At the preceding example, the
switch
statement uses the following patterns:
A
relational pattern
(available in C# 9.0 and later): to compare an expression result with a constant.
A
constant pattern
: test if an expression result equals a constant.
Important
For information about the patterns supported by the
switch
statement, see
Patterns
.
The preceding example also demonstrates the
default
case. The
default
case specifies statements to execute when a match expression doesn't match any other case pattern. If a match expression doesn't match any case pattern and there's no
default
case, control falls through a
switch
statement.
A
switch
statement executes the
statement list
in the first
switch section
whose
case pattern
matches a match expression and whose
case guard
, if present, evaluates to
true
. A
switch
statement evaluates case patterns in text order from top to bottom. The compiler generates an error when a
switch
statement contains an unreachable case. That is a case that is already handled by an upper case or whose pattern is impossible to match.
The
default
case can appear in any place within a
switch
statement. Regardless of its position, the
default
case is evaluated only if all other case patterns aren't matched or the
goto default;
statement is executed in one of the switch sections.
You can specify multiple case patterns for one section of a
switch
statement, as the following example shows:
DisplayMeasurement(-4); // Output: Measured value is -4; out of an acceptable range.
DisplayMeasurement(50); // Output: Measured value is 50.
DisplayMeasurement(132); // Output: Measured value is 132; out of an acceptable range.
void DisplayMeasurement(int measurement)
switch (measurement)
case < 0:
case > 100:
Console.WriteLine($"Measured value is {measurement}; out of an acceptable range.");
break;
default:
Console.WriteLine($"Measured value is {measurement}.");
break;
Within a
switch
statement, control can't fall through from one switch section to the next. As the examples in this section show, typically you use the
break
statement at the end of each switch section to pass control out of a
switch
statement. You can also use the
return
and
throw
statements to pass control out of a
switch
statement. To imitate the fall-through behavior and pass control to other switch section, you can use the
goto
statement
.
In an expression context, you can use the
switch
expression
to evaluate a single expression from a list of candidate expressions based on a pattern match with an expression.
Case guards
A case pattern may be not expressive enough to specify the condition for the execution of the switch section. In such a case, you can use a
case guard
. That is an additional condition that must be satisfied together with a matched pattern. A case guard must be a Boolean expression. You specify a case guard after the
when
keyword that follows a pattern, as the following example shows:
DisplayMeasurements(3, 4); // Output: First measurement is 3, second measurement is 4.
DisplayMeasurements(5, 5); // Output: Both measurements are valid and equal to 5.
void DisplayMeasurements(int a, int b)
switch ((a, b))
case (> 0, > 0) when a == b:
Console.WriteLine($"Both measurements are valid and equal to {a}.");
break;
case (> 0, > 0):
Console.WriteLine($"First measurement is {a}, second measurement is {b}.");
break;
default:
Console.WriteLine("One or both measurements are not valid.");
break;
The preceding example uses
positional patterns
with nested
relational patterns
.
C# language specification
For more information, see the following sections of the
C# language specification
:
The
if
statement
The
switch
statement
For more information about patterns, see the
Patterns and pattern matching
section of the
C# language specification
.
See also
C# reference
Conditional operator
?:
Logical operators
Patterns
switch
expression
Add missing cases to switch statement (style rule IDE0010)
