switch ExamplesBranch on a constant in an efficient way with switch. Match value types, strings and patterns.
This page was last reviewed on Oct 22, 2022.
Switch. Similar to an if-statement, a C# switch statement receives a value, and branches on that value. It executes a block of code based on the value.
Statement notes. Performance of switch can be better, or worse, than if—testing is required for a sure gain. Pattern matching (with the "when" keyword) is available in switch.
String Switch
This first program uses a switch contained in the Transform method. With a switch, it tests the int against several constants: 0, 1 and 2.
Part 1 We always return a value from Transform(), so we do not need to have a return statement after the switch.
Part 2 We invoke Transform() with the arguments 0, 1 and 2—in the switch, we can see where these are matched.
using System; class Program { static int Transform(int argument) { // Part 1: use switch to return a value. switch (argument) { case 0: return 9; case 1: return 8; case 2: default: return 0; } } static void Main() { // Part 2: call Transform 3 times. Console.WriteLine(Transform(0)); Console.WriteLine(Transform(1)); Console.WriteLine(Transform(2)); } }
9 8 0
String example. Here we use switch() on a string value containing "turnip." The C# compiler detects a string switch and can optimize it with a Dictionary lookup.
String Switch
Info Small string switches, like this one with just 3 cases, are often not compiled into Dictionaries. Performance is better this way.
using System; class Program { static void Main() { string value = "turnip"; // ... Switch on the string. switch (value) { case "lettuce": Console.WriteLine("LETTUCE"); break; case "squash": Console.WriteLine("SQUASH"); break; case "turnip": Console.WriteLine("TURNIP"); break; } } }
Goto, case default. We can use goto statements in switches. These are different from other gotos. With goto we can run multiple cases for a single expression.
Detail This method switches on the code int, and then tests the name string. Goto and break are used to redirect control flow.
Detail We use this keyword to specify a matching constant. Default does not use this keyword. We end a case with break, return or continue.
Detail Break and continue can be used in any switch statement. These 2 keywords are used also within loops—this can be confusing.
Detail Are you breaking out of a switch, or out of the enclosing loop? Scope is important. The deepest construct is broken first.
using System; class Program { static void TestNameAndCode(string name, int code) { switch (code) { case 200: case 300: case 400: if (name == "bird") { Console.WriteLine("Bird 200, 300, or 400"); break; } goto default; default: Console.WriteLine("Unknown"); break; } } static void Main() { // These will enter the if-statement. TestNameAndCode("bird", 200); TestNameAndCode("bird", 400); // This will go to the default case. TestNameAndCode("cat", 400); } }
Bird 200, 300, or 400 Bird 200, 300, or 400 Unknown
Curly brackets. This example introduces the default case, and a more verbose syntax. The program accepts an int from the user (with ReadLine). It then tests it for 6 values.
Here We see how the curly brackets are used in the switch cases. And we combine some of the case statements.
using System; class Program { static void Main() { while (true) { Console.WriteLine("Type number and press Return"); try { int i = int.Parse(Console.ReadLine()); switch (i) { case 0: case 1: case 2: { Console.WriteLine("Low number"); break; } case 3: case 4: case 5: { Console.WriteLine("Medium number"); break; } default: { Console.WriteLine("Other number"); break; } } } catch { } } } }
Type number and press Return 5 Medium number Type number and press Return 2 Low number Type number and press Return 500 Other number
Nested switch. Sometimes one switch is not enough. But we can nest a switch within another switch, successively testing values. This approach is sometimes helpful.
But Our code logic, with nested switches, quickly turns into a mess. With comments, this approach may succeed.
Here I test the first two elements in an int array with switches. The second element is tested if the first is 4.
using System; class Program { static void Main() { int[] array = { 4, 10, 14 }; switch (array[0]) { case 3: Console.WriteLine(3); // Not reached. break; case 4: Console.WriteLine(4); // ... Use nested switch. switch (array[1]) { case 10: Console.WriteLine(10); break; } break; } } }
4 10
Pattern matching, types. We can use pattern matching on types in a switch. We switch on a variable. In each case, we can match its type. A local variable (cast to that type) can be used.
Here We introduce a class hierarchy—the Bird and Cat classes inherit from Animal. We then create some class instances.
And We match the types of the Animal class. The most derived class is matched first—in this switch form, order matters.
using System; class Animal { public int size; } class Bird : Animal { public int color; } class Cat : Animal { public bool wild; } class Program { static void Test(Animal animal) { // Switch on a class type with pattern matching. switch (animal) { case Cat c: Console.WriteLine($"CAT wild = {c.wild}"); break; case Bird b: Console.WriteLine($"BIRD color = {b.color}"); break; case Animal a: Console.WriteLine($"ANIMAL size = {a.size}"); break; } } static void Main() { // Create some class instances. Cat cat = new Cat(); cat.wild = true; Bird bird = new Bird(); bird.color = 5; Animal animal = new Animal(); animal.size = 10; // Test class instances. Test(cat); Test(bird); Test(animal); } }
CAT wild = True BIRD color = 5 ANIMAL size = 10
Pattern matching, when. We can place a condition on each case statement. This can test another variable. Here we test a local variable called secondValue on the first 2 cases.
Tip We have repeat "case 200" statements. They are different only because of their "when" clauses.
Tip 2 With the "when pattern-matching" syntax, order matters in a switch. This is an enhanced syntax form.
using System; class Program { static void Main() { int value = 200; int secondValue = 300; // Use switch with pattern matching. switch (value) { case 200 when secondValue == 0: Console.WriteLine("Y"); break; case 200 when secondValue == 300: Console.WriteLine("Value is 200, secondValue is 300"); break; case 400: Console.WriteLine("Z"); break; } } }
Value is 200, secondValue is 300
Double. Some values may not be switched upon. Floating-point values, like doubles, will cause an error in a switch expression. Most classes and custom types will also fail.
Detail A bool may be used in the expression of a switch. But this is somewhat strange as only true and false are allowed.
Detail A nullable type can be used, but only if the nullable type "wraps" a valid switch type like an enum.
class Program { static void Main() { double value = 1.4; switch (value) { case 1: break; } } }
Error 1 A switch expression or case label must be a bool, char, string, integral, enum, or corresponding nullable type....
Fall through. Every case must have a break, continue, goto, return or throw at its end. In C# we cannot have cases with statements fall through to the following case.
Detail We can use the goto statement, as in "goto case 1," to run both cases on a 0 value. As shown, the program does not compile.
using System; class Program { static void Main() { int value = 0; // ... Every switch statement must be terminated. switch (value) { case 0: Console.WriteLine("Zero"); case 1: Console.WriteLine("One"); break; } } }
Error 1 Control cannot fall through from one case label ('case 0:') to another
Duplicate cases. A switch can only have unique case labels—each constant value must be distinct. This program will not compile. But it shows us what happens when we have duplicate cases.
using System; class Program { static void Main() { short number = 0; // ... Cases may not be duplicated. switch (number) { case 0: case 0: Console.WriteLine("ZERO"); return; case 1: Console.WriteLine("ONE"); return; } } }
Error 1 The label 'case 0:' already occurs in this switch statement
Constants. We must use only constants for case statements. This is a limitation, but it is part of the language specification. The C# compiler is not even tempted.
using System; class Program { static void Main() { int number = 0; int test = 10; // ... Constant values are required. switch (number) { case test + 1: Console.WriteLine(100); return; case 0: Console.WriteLine(0); return; } } }
Error 1 A constant value is expected
Benchmark, switch. A switch statement helps optimize some programs. There are many considerations here, and switch is not always an improvement over if.
Version 1 This version of the code uses a switch statement. It returns an int based on the argument int "v."
Version 2 This code does the same thing as Method 1, but instead uses a series of if-statements.
Result Tested on .NET 7 in 2022, the benchmark shows that the 2 versions of the code are both optimized well and the performance is the same.
using System; using System.Diagnostics; class Program { static int Method1(int v) { switch (v) { case 0: return 10; case 1: return -1; case 2: return 20; default: return 0; } } static int Method2(int v) { if (v == 0) return 10; if (v == 1) return -1; if (v == 2) return 20; return 0; } static void Main() { Method1(0); Method2(0); const int max = 100000000; var s1 = Stopwatch.StartNew(); // Version 1: use switch. for (int i = 0; i < max; i++) { Method1(3); Method1(3); Method1(3); Method1(3); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use if-else. for (int i = 0; i < max; i++) { Method2(3); Method2(3); Method2(3); Method2(3); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / max).ToString("0.00 ns")); } }
0.32 ns switch 0.31 ns if
Types. It is possible to switch on integers or other value types, such as enums or chars. Strings are a reference type, but the C# compiler can handle switches on strings as well.
Switch Char
Switch Enum
String Switch
Performance notes. Switch can be translated by the compiler into a jump table. Large switches can be much faster than long series of if-else statements.
If vs. Switch
Note Switch() sometimes outperforms if. But more importantly, it can help with code design—switch enforces all tested values are constants.
Usage. Should we use switches everywhere? This is probably a bad idea. With polymorphism, we abstract selections with virtual methods and inheritance. This leads to cleaner code.
A summary. This construct imparts a greater sense of symmetry. Switches test value types and strings. They speed up selections—and with them, we write clearer code.
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Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Oct 22, 2022 (simplify).
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