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Patterns - Pattern matching using the is and switch expressions. - C# | Microsoft Learn
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狂野的橡皮擦
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You use the
is
expression
, the
switch statement
and the
switch expression
to match an input expression against any number of characteristics. C# supports multiple patterns, including declaration, type, constant, relational, property, list, var, and discard. Patterns can be combined using Boolean logic keywords
and
,
or
, and
not
.
The following C# expressions and statements support pattern matching:
is
expression
In those constructs, you can match an input expression against any of the following patterns:
var
pattern
: to match any expression and assign its result to a declared variable.
Logical , property , positional , and list patterns are recursive patterns. That is, they can contain nested patterns.
For the example of how to use those patterns to build a data-driven algorithm, see Tutorial: Use pattern matching to build type-driven and data-driven algorithms .
Declaration and type patterns
You use declaration and type patterns to check if the run-time type of an expression is compatible with a given type. With a declaration pattern, you can also declare a new local variable. When a declaration pattern matches an expression, that variable is assigned a converted expression result, as the following example shows:
object greeting = "Hello, World!";
if (greeting is string message)
Console.WriteLine(message.ToLower()); // output: hello, world!
A
declaration pattern
with type
T
matches an expression when an expression result is non-null and any of the following conditions are true:
The run-time type of an expression result is
T
.
The run-time type of an expression result derives from type
T
, implements interface
T
, or another
implicit reference conversion
exists from it to
T
. The following example demonstrates two cases when this condition is true:
var numbers = new int[] { 10, 20, 30 };
Console.WriteLine(GetSourceLabel(numbers)); // output: 1
var letters = new List<char> { 'a', 'b', 'c', 'd' };
Console.WriteLine(GetSourceLabel(letters)); // output: 2
static int GetSourceLabel<T>(IEnumerable<T> source) => source switch
Array array => 1,
ICollection<T> collection => 2,
_ => 3,
In the preceding example, at the first call to the
GetSourceLabel
method, the first pattern matches an argument value because the argument's run-time type
int[]
derives from the
Array
type. At the second call to the
GetSourceLabel
method, the argument's run-time type
List<T>
doesn't derive from the
Array
type but implements the
ICollection<T>
interface.
The run-time type of an expression result is a
nullable value type
with the underlying type
T
.
A
boxing
or
unboxing
conversion exists from the run-time type of an expression result to type
T
.
The following example demonstrates the last two conditions:
int? xNullable = 7;
int y = 23;
object yBoxed = y;
if (xNullable is int a && yBoxed is int b)
Console.WriteLine(a + b); // output: 30
If you want to check only the type of an expression, you can use a discard
_
in place of a variable's name, as the following example shows:
public abstract class Vehicle {}
public class Car : Vehicle {}
public class Truck : Vehicle {}
public static class TollCalculator
public static decimal CalculateToll(this Vehicle vehicle) => vehicle switch
Car _ => 2.00m,
Truck _ => 7.50m,
null => throw new ArgumentNullException(nameof(vehicle)),
_ => throw new ArgumentException("Unknown type of a vehicle", nameof(vehicle)),
For that purpose you can use a
type pattern
, as the following example shows:
public static decimal CalculateToll(this Vehicle vehicle) => vehicle switch
Car => 2.00m,
Truck => 7.50m,
null => throw new ArgumentNullException(nameof(vehicle)),
_ => throw new ArgumentException("Unknown type of a vehicle", nameof(vehicle)),
Like a declaration pattern, a type pattern matches an expression when an expression result is non-null and its run-time type satisfies any of the conditions listed above.
To check for non-null, you can use a
negated
null
constant pattern
, as the following example shows:
if (input is not null)
// ...
For more information, see the
Declaration pattern
and
Type pattern
sections of the feature proposal notes.
Constant pattern
You use a
constant pattern
to test if an expression result equals a specified constant, as the following example shows:
public static decimal GetGroupTicketPrice(int visitorCount) => visitorCount switch
1 => 12.0m,
2 => 20.0m,
3 => 27.0m,
4 => 32.0m,
0 => 0.0m,
_ => throw new ArgumentException($"Not supported number of visitors: {visitorCount}", nameof(visitorCount)),
In a constant pattern, you can use any constant expression, such as:
an
integer
or
floating-point
numerical literal
a
char
a
string
literal.
a Boolean value
true
or
false
an
enum
value
the name of a declared
const
field or local
The expression must be a type that is convertible to the constant type, with one exception: An expression whose type is
Span<char>
or
ReadOnlySpan<char>
can be matched against constant strings in C# 11 and later versions.
Use a constant pattern to check for
null
, as the following example shows:
if (input is null)
return;
The compiler guarantees that no user-overloaded equality operator
==
is invoked when expression
x is null
is evaluated.
You can use a
negated
null
constant pattern to check for non-null, as the following example shows:
if (input is not null)
// ...
For more information, see the
Constant pattern
section of the feature proposal note.
Relational patterns
You use a
relational pattern
to compare an expression result with a constant, as the following example shows:
Console.WriteLine(Classify(13)); // output: Too high
Console.WriteLine(Classify(double.NaN)); // output: Unknown
Console.WriteLine(Classify(2.4)); // output: Acceptable
static string Classify(double measurement) => measurement switch
< -4.0 => "Too low",
> 10.0 => "Too high",
double.NaN => "Unknown",
_ => "Acceptable",
In a relational pattern, you can use any of the
relational operators
<
,
>
,
<=
, or
>=
. The right-hand part of a relational pattern must be a constant expression. The constant expression can be of an
integer
,
floating-point
,
char
, or
enum
type.
To check if an expression result is in a certain range, match it against a
conjunctive
and
pattern
, as the following example shows:
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 3, 14))); // output: spring
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 7, 19))); // output: summer
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 2, 17))); // output: winter
static string GetCalendarSeason(DateTime date) => date.Month switch
>= 3 and < 6 => "spring",
>= 6 and < 9 => "summer",
>= 9 and < 12 => "autumn",
12 or (>= 1 and < 3) => "winter",
_ => throw new ArgumentOutOfRangeException(nameof(date), $"Date with unexpected month: {date.Month}."),
If an expression result is
null
or fails to convert to the type of a constant by a nullable or unboxing conversion, a relational pattern doesn't match an expression.
For more information, see the
Relational patterns
section of the feature proposal note.
Logical patterns
You use the
not
,
and
, and
or
pattern combinators to create the following
logical patterns
:
Negation
not
pattern that matches an expression when the negated pattern doesn't match the expression. The following example shows how you can negate a
constant
null
pattern to check if an expression is non-null:
if (input is not null)
// ...
Conjunctive
and
pattern that matches an expression when both patterns match the expression. The following example shows how you can combine
relational patterns
to check if a value is in a certain range:
Console.WriteLine(Classify(13)); // output: High
Console.WriteLine(Classify(-100)); // output: Too low
Console.WriteLine(Classify(5.7)); // output: Acceptable
static string Classify(double measurement) => measurement switch
< -40.0 => "Too low",
>= -40.0 and < 0 => "Low",
>= 0 and < 10.0 => "Acceptable",
>= 10.0 and < 20.0 => "High",
>= 20.0 => "Too high",
double.NaN => "Unknown",
Disjunctive
or
pattern that matches an expression when either pattern matches the expression, as the following example shows:
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 1, 19))); // output: winter
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 10, 9))); // output: autumn
Console.WriteLine(GetCalendarSeason(new DateTime(2021, 5, 11))); // output: spring
static string GetCalendarSeason(DateTime date) => date.Month switch
3 or 4 or 5 => "spring",
6 or 7 or 8 => "summer",
9 or 10 or 11 => "autumn",
12 or 1 or 2 => "winter",
_ => throw new ArgumentOutOfRangeException(nameof(date), $"Date with unexpected month: {date.Month}."),
As the preceding example shows, you can repeatedly use the pattern combinators in a pattern.
Precedence and order of checking
The pattern combinators are ordered from the highest precedence to the lowest as follows:
When a logical pattern is a pattern of an
is
expression, the precedence of logical pattern combinators is
higher
than the precedence of logical operators (both
bitwise logical
and
Boolean logical
operators). Otherwise, the precedence of logical pattern combinators is
lower
than the precedence of logical and conditional logical operators. For the complete list of C# operators ordered by precedence level, see the
Operator precedence
section of the
C# operators
article.
To explicitly specify the precedence, use parentheses, as the following example shows:
static bool IsLetter(char c) => c is (>= 'a' and <= 'z') or (>= 'A' and <= 'Z');
The order in which patterns are checked is undefined. At run time, the right-hand nested patterns of
or
and
and
patterns can be checked first.
For more information, see the
Pattern combinators
section of the feature proposal note.
Property pattern
You use a
property pattern
to match an expression's properties or fields against nested patterns, as the following example shows:
static bool IsConferenceDay(DateTime date) => date is { Year: 2020, Month: 5, Day: 19 or 20 or 21 };
A property pattern matches an expression when an expression result is non-null and every nested pattern matches the corresponding property or field of the expression result.
You can also add a run-time type check and a variable declaration to a property pattern, as the following example shows:
Console.WriteLine(TakeFive("Hello, world!")); // output: Hello
Console.WriteLine(TakeFive("Hi!")); // output: Hi!
Console.WriteLine(TakeFive(new[] { '1', '2', '3', '4', '5', '6', '7' })); // output: 12345
Console.WriteLine(TakeFive(new[] { 'a', 'b', 'c' })); // output: abc
static string TakeFive(object input) => input switch
string { Length: >= 5 } s => s.Substring(0, 5),
string s => s,
ICollection<char> { Count: >= 5 } symbols => new string(symbols.Take(5).ToArray()),
ICollection<char> symbols => new string(symbols.ToArray()),
null => throw new ArgumentNullException(nameof(input)),
_ => throw new ArgumentException("Not supported input type."),
A property pattern is a recursive pattern. That is, you can use any pattern as a nested pattern. Use a property pattern to match parts of data against nested patterns, as the following example shows:
public record Point(int X, int Y);
public record Segment(Point Start, Point End);
static bool IsAnyEndOnXAxis(Segment segment) =>
segment is { Start: { Y: 0 } } or { End: { Y: 0 } };
The preceding example uses the
or
pattern combinator
and
record types
.
Beginning with C# 10, you can reference nested properties or fields within a property pattern. This capability is known as an
extended property pattern
. For example, you can refactor the method from the preceding example into the following equivalent code:
static bool IsAnyEndOnXAxis(Segment segment) =>
segment is { Start.Y: 0 } or { End.Y: 0 };
For more information, see the
Property pattern
section of the feature proposal note and the
Extended property patterns
feature proposal note.
You can use the
Simplify property pattern (IDE0170)
style rule to improve code readability by suggesting places to use extended property patterns.
Positional pattern
You use a
positional pattern
to deconstruct an expression result and match the resulting values against the corresponding nested patterns, as the following example shows:
public readonly struct Point
public int X { get; }
public int Y { get; }
public Point(int x, int y) => (X, Y) = (x, y);
public void Deconstruct(out int x, out int y) => (x, y) = (X, Y);
static string Classify(Point point) => point switch
(0, 0) => "Origin",
(1, 0) => "positive X basis end",
(0, 1) => "positive Y basis end",
_ => "Just a point",
At the preceding example, the type of an expression contains the
Deconstruct
method, which is used to deconstruct an expression result. You can also match expressions of
tuple types
against positional patterns. In that way, you can match multiple inputs against various patterns, as the following example shows:
static decimal GetGroupTicketPriceDiscount(int groupSize, DateTime visitDate)
=> (groupSize, visitDate.DayOfWeek) switch
(<= 0, _) => throw new ArgumentException("Group size must be positive."),
(_, DayOfWeek.Saturday or DayOfWeek.Sunday) => 0.0m,
(>= 5 and < 10, DayOfWeek.Monday) => 20.0m,
(>= 10, DayOfWeek.Monday) => 30.0m,
(>= 5 and < 10, _) => 12.0m,
(>= 10, _) => 15.0m,
_ => 0.0m,
The preceding example uses
relational
and
logical
patterns.
You can use the names of tuple elements and
Deconstruct
parameters in a positional pattern, as the following example shows:
var numbers = new List<int> { 1, 2, 3 };
if (SumAndCount(numbers) is (Sum: var sum, Count: > 0))
Console.WriteLine($"Sum of [{string.Join(" ", numbers)}] is {sum}"); // output: Sum of [1 2 3] is 6
static (double Sum, int Count) SumAndCount(IEnumerable<int> numbers)
int sum = 0;
int count = 0;
foreach (int number in numbers)
sum += number;
count++;
return (sum, count);
You can also extend a positional pattern in any of the following ways:
Add a run-time type check and a variable declaration, as the following example shows:
public record Point2D(int X, int Y);
public record Point3D(int X, int Y, int Z);
static string PrintIfAllCoordinatesArePositive(object point) => point switch
Point2D (> 0, > 0) p => p.ToString(),
Point3D (> 0, > 0, > 0) p => p.ToString(),
_ => string.Empty,
The preceding example uses
positional records
that implicitly provide the
Deconstruct
method.
Use a
property pattern
within a positional pattern, as the following example shows:
public record WeightedPoint(int X, int Y)
public double Weight { get; set; }
static bool IsInDomain(WeightedPoint point) => point is (>= 0, >= 0) { Weight: >= 0.0 };
Combine two preceding usages, as the following example shows:
if (input is WeightedPoint (> 0, > 0) { Weight: > 0.0 } p)
// ..
A positional pattern is a recursive pattern. That is, you can use any pattern as a nested pattern.
For more information, see the
Positional pattern
section of the feature proposal note.
var
pattern
You use a
var
pattern
to match any expression, including
null
, and assign its result to a new local variable, as the following example shows:
static bool IsAcceptable(int id, int absLimit) =>
SimulateDataFetch(id) is var results
&& results.Min() >= -absLimit
&& results.Max() <= absLimit;
static int[] SimulateDataFetch(int id)
var rand = new Random();
return Enumerable
.Range(start: 0, count: 5)
.Select(s => rand.Next(minValue: -10, maxValue: 11))
.ToArray();
A
var
pattern is useful when you need a temporary variable within a Boolean expression to hold the result of intermediate calculations. You can also use a
var
pattern when you need to perform more checks in
when
case guards of a
switch
expression or statement, as the following example shows:
public record Point(int X, int Y);
static Point Transform(Point point) => point switch
var (x, y) when x < y => new Point(-x, y),
var (x, y) when x > y => new Point(x, -y),
var (x, y) => new Point(x, y),
static void TestTransform()
Console.WriteLine(Transform(new Point(1, 2))); // output: Point { X = -1, Y = 2 }
Console.WriteLine(Transform(new Point(5, 2))); // output: Point { X = 5, Y = -2 }
In the preceding example, pattern
var (x, y)
is equivalent to a
positional pattern
(var x, var y)
.
In a
var
pattern, the type of a declared variable is the compile-time type of the expression that is matched against the pattern.
For more information, see the
Var pattern
section of the feature proposal note.
Discard pattern
You use a
discard pattern
_
to match any expression, including
null
, as the following example shows:
Console.WriteLine(GetDiscountInPercent(DayOfWeek.Friday)); // output: 5.0
Console.WriteLine(GetDiscountInPercent(null)); // output: 0.0
Console.WriteLine(GetDiscountInPercent((DayOfWeek)10)); // output: 0.0
static decimal GetDiscountInPercent(DayOfWeek? dayOfWeek) => dayOfWeek switch
DayOfWeek.Monday => 0.5m,
DayOfWeek.Tuesday => 12.5m,
DayOfWeek.Wednesday => 7.5m,
DayOfWeek.Thursday => 12.5m,
DayOfWeek.Friday => 5.0m,
DayOfWeek.Saturday => 2.5m,
DayOfWeek.Sunday => 2.0m,
_ => 0.0m,
In the preceding example, a discard pattern is used to handle
null
and any integer value that doesn't have the corresponding member of the
DayOfWeek
enumeration. That guarantees that a
switch
expression in the example handles all possible input values. If you don't use a discard pattern in a
switch
expression and none of the expression's patterns matches an input, the runtime
throws an exception
. The compiler generates a warning if a
switch
expression doesn't handle all possible input values.
A discard pattern can't be a pattern in an
is
expression or a
switch
statement. In those cases, to match any expression, use a
var
pattern
with a discard:
var _
. A discard pattern can be a pattern in a
switch
expression.
For more information, see the
Discard pattern
section of the feature proposal note.
Parenthesized pattern
You can put parentheses around any pattern. Typically, you do that to emphasize or change the precedence in
logical patterns
, as the following example shows:
if (input is not (float or double))
return;
List patterns
Beginning with C# 11, you can match an array or a list against a
sequence
of patterns, as the following example shows:
int[] numbers = { 1, 2, 3 };
Console.WriteLine(numbers is [1, 2, 3]); // True
Console.WriteLine(numbers is [1, 2, 4]); // False
Console.WriteLine(numbers is [1, 2, 3, 4]); // False
Console.WriteLine(numbers is [0 or 1, <= 2, >= 3]); // True
As the preceding example shows, a list pattern is matched when each nested pattern is matched by the corresponding element of an input sequence. You can use any pattern within a list pattern. To match any element, use the
discard pattern
or, if you also want to capture the element, the
var pattern
, as the following example shows:
List<int> numbers = new() { 1, 2, 3 };
if (numbers is [var first, _, _])
Console.WriteLine($"The first element of a three-item list is {first}.");
// Output:
// The first element of a three-item list is 1.
The preceding examples match a whole input sequence against a list pattern. To match elements only at the start or/and the end of an input sequence, use the
slice pattern
..
, as the following example shows:
Console.WriteLine(new[] { 1, 2, 3, 4, 5 } is [> 0, > 0, ..]); // True
Console.WriteLine(new[] { 1, 1 } is [_, _, ..]); // True
Console.WriteLine(new[] { 0, 1, 2, 3, 4 } is [> 0, > 0, ..]); // False
Console.WriteLine(new[] { 1 } is [1, 2, ..]); // False
Console.WriteLine(new[] { 1, 2, 3, 4 } is [.., > 0, > 0]); // True
Console.WriteLine(new[] { 2, 4 } is [.., > 0, 2, 4]); // False
Console.WriteLine(new[] { 2, 4 } is [.., 2, 4]); // True
Console.WriteLine(new[] { 1, 2, 3, 4 } is [>= 0, .., 2 or 4]); // True
Console.WriteLine(new[] { 1, 0, 0, 1 } is [1, 0, .., 0, 1]); // True
Console.WriteLine(new[] { 1, 0, 1 } is [1, 0, .., 0, 1]); // False
A slice pattern matches zero or more elements. You can use at most one slice pattern in a list pattern. The slice pattern can only appear in a list pattern.
You can also nest a subpattern within a slice pattern, as the following example shows:
void MatchMessage(string message)
var result = message is ['a' or 'A', .. var s, 'a' or 'A']
? $"Message {message} matches; inner part is {s}."
: $"Message {message} doesn't match.";
Console.WriteLine(result);
MatchMessage("aBBA"); // output: Message aBBA matches; inner part is BB.
MatchMessage("apron"); // output: Message apron doesn't match.
void Validate(int[] numbers)
var result = numbers is [< 0, .. { Length: 2 or 4 }, > 0] ? "valid" : "not valid";
Console.WriteLine(result);
Validate(new[] { -1, 0, 1 }); // output: not valid
Validate(new[] { -1, 0, 0, 1 }); // output: valid
For more information, see the
List patterns
feature proposal note.
C# language specification
For more information, see the
Patterns and pattern matching
section of the
C# language specification
.
For information about features added in C# 8 and later, see the following feature proposal notes:
Recursive pattern matching
Pattern-matching updates
C# 10 - Extended property patterns
C# 11 - List patterns
C# 11 - Pattern match
Span<char>
on string literal
See also
C# operators and expressions
Pattern matching overview
Tutorial: Use pattern matching to build type-driven and data-driven algorithms
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