Selection statements (C# reference)
The following statements select statements to execute from a number of possible statements based on the value of an expression:
- The
if
statement: selects a statement to execute based on the value of 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 anelse
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."); } else { Console.WriteLine("Perfect!"); } }
An
if
statement without anelse
part executes its body only if a Boolean expression evaluates totrue
, 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}");
}
else
{
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 (available in C# 7.0 and later): to 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 is 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.
Note
The default
case can appear in any place within a switch
statement. Regardless of its position, the default
case is always evaluated last and only if all other case patterns aren't matched.
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 cannot 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.
Language version support
The switch
statement supports pattern matching beginning with C# 7.0. Since then, each major C# version adds new kinds of patterns. For more information, see Patterns.
In C# 6 and earlier, you use the switch
statement with the following limitations:
- A match expression must be of one of the following types: char, string, bool, an integral numeric type, or an enum type.
- Only constant expressions are allowed in
case
labels.
C# language specification
For more information, see the following sections of the C# language specification:
For more information about features introduced in C# 7.0 and later, see the following feature proposal notes:
See also
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