break const goto return String switch switch Unreachable var when

Case

The C# case-keyword is part of switch. We use this keyword to match constant values in switches. Case specifies a constant to be matched in the switch selection statement.

Cases can be stacked and combined

We can target a case with a goto-statement. And "default" is a special kind of case—it is matched when nothing else does.

Simple example

Cases specify constants that match the selection in a switch statement. The blocks following a specific case statement are only executed when the case constants are matched.

Info Default matches all values that are not matched by the specified case-statements. It is like "else" in an if-else chain.

using System;

int test = 5;
string result = null;
switch (test)
{
    case 5:
        result = "Five";
        break;
    default:
        result = "Not five";
        break;
}
Console.WriteLine(result);
Five

Complex example

Here is an example that stacks cases. This code demonstrates the case keyword used in different ways. A string switch statement is shown.

Note The first 3 cases are stacked on top of each other. This syntax can match multiple cases to a single executable code block.

Note 2 At the end of each case-statement block, you must have a break, return or goto statement for the program to compile.

Important The default case does not use the case-keyword. It is the case that is matched when no other cases are matched.

using System;

class Program
{
    static string TestCase(string value)
    {
        const string _special = "constant";

        // Begin the switch.
        switch (value)
        {
            case "100":
            case "1000":
            case "10000":
                {
                    // You can use the parentheses in a case body.
                    return "Multiple of ten";
                }
            case "500":
            case "5000":
            case "50000":
                // You can omit the parentheses and stack the cases.
                return "Multiple of fifty";
            case _special:
                // You can use a constant identifier in the case.
                return "*";
            default:
                // You can use the default case.
                return "Invalid";
        }
    }

    static void Main()
    {
        // Test the method.
        Console.WriteLine(TestCase("100"));
        Console.WriteLine(TestCase("1000"));
        Console.WriteLine(TestCase("5000"));
        Console.WriteLine(TestCase("constant"));
        Console.WriteLine(TestCase(null));
    }
}Multiple of ten
Multiple of ten
Multiple of fifty
*
Invalid

Case expressions

We can use case with expressions in newer versions of the C# language. These are compiled in a similar way to the equivalent if-statements.

However The C# compiler detects unreachable case expressions. Try adding a default here—it will be detected as unreachable.

using System;

int z = 999;

// Test the value with complex case statements.
switch (z) {
    case > 2000:
        Console.WriteLine("> 2000");
        break;
    case < 2000 and not 999:
        Console.WriteLine("< 2000 and not 999");
        break;
    case > 100:
        Console.WriteLine("> 100");
        break;
}
> 100

Type cases

We can test the type of an object with a switch-statement. We use switch on an object (or other type) and then specify possible derived types in cases.

Part 1 We have an int local variable, and pass it as an argument to the Test method, which checks its type in a switch.

Part 2 We have a string, which is implicitly cast to an object. We pass this an argument to Test().

Part 3 We use a switch-statement on the object argument, and each case is a possible derived type of the object.

using System;

class Program
{
    public static void Main()
    {
        // Part 1: use int as argument.
        int a = 999;
        object z = a;
        Test(z);

        // Part 2: use string as argument.
        object z2 = "x";
        Test(z2);
    }

    static void Test(object z)
    {
        // Part 3: check types in case statements.
        switch (z) {
            case int:
                Console.WriteLine("Type is int");
                break;
            case string:
                Console.WriteLine("Type is string");
                break;
        }
    }
}Type is int
Type is string

Var keyword

It is possible to capture local variables from part of a match in a case-statement. We use the var keyword, with a "case var" statement.

Here We have a tuple called "data" and we match its 2 items and then reference those 2 items elsewhere in the code.

Info We use a when-clause to test the second item in the tuple, and print out the value of the first item.

using System;

var data = (1000, 'x');
// Match the tuple with case var.
switch (data)
{
    case var (id, c) when c == 'x':
        Console.WriteLine($"Second item is x, {id}");
        break;
    case var (id, _):
        Console.WriteLine($"Second item is not x, {id}");
        break;
}
Second item is x, 1000

Constant expected

It is important to know what values are constant before using them in a switch. We cannot have a case which is a local variable, as it is not a constant.

int value = 100;
int value2 = 100;
switch (value)
{
    case value2:
        // This won't compile because the case is not constant.
        break;
}
error CS9135: A constant value of type 'int' is expected [...]

Summary

Case is used in switch-statements. We also find this keyword in certain goto-statements. The case-statement is specified with a constant, which may be defined elsewhere.