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C# switch ExamplesUse switch to branch on a constant in an efficient way. Match value types, strings and patterns.
Switch. Similar to an if-statement, a 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
First example. This 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.
C# program that uses switch
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.
C# program that switches on string
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; } } }
TURNIP
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.
Goto
TestNameAndCode This method switches on the code int, and then tests the name string. Goto and break are used to redirect control flow.
Case We use this keyword to specify a matching constant. Default does not use this keyword. We end a case with break, return or continue.
Case
Break Break and continue can be used in any switch statement. These 2 keywords are used also within loops—this can be confusing.
Break
Continue
Question Are you breaking out of a switch, or out of the enclosing loop? Scope is important. The deepest construct is broken first.
C# program that uses case, goto, default in switch
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.
Console.ReadLine
Here We see how the curly brackets are used in the switch cases. And we combine some of the case statements.
C# program that uses int switch
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.
C# program that uses nested switch
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.
C# program that uses switch, matches types
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.
C# program that uses pattern matching, when keyword
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.
Bool A bool may be used in the expression of a switch. But this is somewhat strange as only true and false are allowed.
Nullable A nullable type can be used, but only if the nullable type "wraps" a valid switch type like an enum.
C# program that causes switch double error
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.
Goto 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.
C# program that has fall-through error
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.
C# program that has duplicate case
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.
C# program that lacks constant case
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 5 on Linux (in 2021), the benchmark shows that the switch statement version is slightly faster.
C# program that benchmarks switch
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")); } }
7.63 ns switch 10.67 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
Regex
Note Switch() often outperforms if. But more importantly, it can help with code design—switch enforces all tested values are constants.
And Open source development in .NET in 2021 has improved performance. But switch still can yield a performance boost over if.
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.
Virtual
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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