C# 8.0 – the SWITCH expression

C# 8 introduces switch expressions, which enable the following: terser syntax, returns a value since it is an expression, and fully integrated with pattern matching. The switch keyword is “infix”, meaning the keyword sits between the tested value (here, that’s o) and the list of cases, much like expression lambdas. The following examples use the lambda syntax for methods, which integrates well with switch expressions but isn’t required.

You can see the syntax for switch expressions in the following example:

static string Display(object o) => o switch
Point { X: 0, Y: 0 } => "origin",
Point { X: var x, Y: var y } => $"({x}, {y})",
_ => "unknown"

There are two patterns at play in this example. o first matches with the Point type pattern and then with the property pattern inside the {curly braces}. The _ describes the discard pattern, which is the same as default for switch statements.

You can go one step further, and rely on tuple deconstruction and parameter position, as you can see in the following example:

static State ChangeState(State current, Transition transition, bool hasKey) =>
(current, transition) switch
(Opened, Close) => Closed,
(Closed, Open) => Opened,
(Closed, Lock) when hasKey => Locked,
(Locked, Unlock) when hasKey => Closed,
_ => throw new InvalidOperationException($"Invalid transition")

In this example, you can see you do not need to define a variable or explicit type for each of the cases. Instead, the compiler can match the tuple being testing with the tuples defined for each of the cases.

All of these patterns enable you to write declarative code that captures your intent instead of procedural code that implements tests for it. The compiler becomes responsible for implementing that boring procedural code and is guaranteed to always do it correctly.

There will still be cases where switch statements will be a better choice than switch expressions and patterns can be used with both syntax styles.

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