What are switch expressions in C# 8.0?

Switch expressions are used extensively to solve pattern matching problems, and are very similar to switch statements. They were introduced in C# 8.0.

Semantics

A switch expression is identified by the switch keyword, and its return value depends on the case label matched. If no case is matched, the value in the default case is returned.

A switch expression can evaluate variables that contain multiple values, such as a tuple, and single-valued variables, such as numerical values of type int.

It must be noted that a switch expression:

Must not contain duplicate cases.
Must not have more than one default case.
May or may not have a default case, as it is optional.
Supports the break statement.

A switch expression evaluates such an input based on a pre-defined template because the data type of the input must match the data type of the switch case labels. The _ operator indicates that any value is acceptable.

The => operator identifies the return value of a particular case and is placed before the definition of the input template, as demonstrated by the program below.

Case guards

Switch expressions also support optional case guards, which are used to specify conditions in the case expression:

("Cricket", "Football", "Swimming",_) when num > 1 => "I like Cricket, Football, and Swimming.",

The case above will only execute if the value of num is greater than 1, as specified by the when keyword.

Since the patterns specified in a switch expression are exhaustive, an exception is thrown at run-time if no case is matched. A warning is generated if the switch expression fails to cater to all possible inputs.

Example

The following program declares a function named sports_i_like, which takes in one tuple as its parameter. It compares this tuple with tuple templates defined as switch expression cases, and returns the corresponding string if a case label is matched. If no case is matched, the default case is executed.

Using the Controle.WriteLine function, we output the return value of the function:

using System;
public class Program {
  
    // Main function
    public static void Main()
    {
        // define function containng a switch expression
		static string sports_i_like(string sport1, string sport2, string sport3, int num)
        //denoting input to the switch expression, which is a tuple of three strings
        => (sport1, sport2, sport3, num) switch
        {
            //matches if Cricket, Football, and Swimming given as input
            ("Cricket", "Football", "Swimming",_) when num > 1 => "I like Cricket, Football, and Swimming.",
            //matches if Cricket, Football, and Baseball given as input
            ("Cricket", "Football", "Baseball",_)  when num > 1 => "I like Cricket, Football, and Baseball.",
            //matches if Hockey, Football, and Swimming given as input
            ("Hockey", "Football", "Swimming",_)  when num > 1 => "I like Hockey, Football, and Swimming.",
		    //matches if Table Tennis, Football, and Swimming given as input
            ("Table Tennis", "Football", "Swimming",_)  when num > 1 => "I like Table Tennis, Football and Swimming.",
            //Default case
            (_, _, _, _) => "Invalid input!"
        };
		Console.WriteLine(sports_i_like("Cricket", "Football", "Swimming",2));
		Console.WriteLine(sports_i_like("Cricket", "Football", "Racing",2));
						  
	}
		
}

The input of the program is as follows, as the first call matches the first case label and the second call results in the default case:

I like Cricket, Football, and Swimming.
Invalid input.

Note that Racing is not part of any case label.

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