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:
C#
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($"Measured value is {measurement}."); break; } }
At the preceding example, the switch statement uses the following patterns:
- A relational pattern: to compare an expression result with a constant.
- A constant pattern: test if an expression result equals a constant.
The preceding example also demonstrates the default case. The default case specifies statements to execute when a match expression doesn't match any other case pattern. If a match expression doesn't match any case pattern and there's no default case, control falls through a switch statement.
A switch statement executes the statement list in the first switch section whose case pattern matches a match expression and whose case guard, if present, evaluates to true. A switch statement evaluates case patterns in text order from top to bottom. The compiler generates an error when a switch statement contains an unreachable case. That is a case that is already handled by an upper case or whose pattern is impossible to match.
You can specify multiple case patterns for one section of a switch statement, as the following example shows:
C#
DisplayMeasurement(-4); // Output: Measured value is -4; out of an acceptable range. DisplayMeasurement(50); // Output: Measured value is 50. DisplayMeasurement(13); // Output: Measured value is 132; out of an acceptable range. void DisplayMeasurement(int measurement) { switch (measurement) { case < 0: case > 100: Console.WriteLine($"Measured value is {measurement}; out of an acceptable range."); break; default: Console.WriteLine($"Measured value is {measurement}."); break; } }
Within a switch statement, control can't fall through from one switch section to the next. As the examples in this section show, typically you use the break statement at the end of each switch section to pass control out of a switch statement. You can also use the return and throw statements to pass control out of a switch statement. To imitate the fall-through behavior and pass control to other switch section, you can use the goto statement.
In an expression context, you can use the switch expression to evaluate a single expression from a list of candidate expressions based on a pattern match with an expression.
Case guards
A case pattern may be not expressive enough to specify the condition for the execution of the switch section. In such a case, you can use a case guard. That is an additional condition that must be satisfied together with a matched pattern. A case guard must be a Boolean expression. You specify a case guard after the when keyword that follows a pattern, as the following example shows:
C#
DisplayMeasurements(3, 4); // Output: First measurement is 3, second measurement is 4. DisplayMeasurements(5, 5); // Output: Both measurements are valid and equal to 5. void DisplayMeasurements(int a, int b) { switch ((a, b)) { case (> 0, > 0) when a == b: Console.WriteLine($"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; } }
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