Records and pattern matching

Records

1var a = new Money(10m, "EUR");2var b = new Money(10m, "EUR");3Console.WriteLine(a == b);                 // True: same values4Console.WriteLine(ReferenceEquals(a, b));  // False: two objects5var c = a with { Amount = 25m };6Console.WriteLine(c);                      // Money { Amount = 25, Currency = EUR }7 8record Money(decimal Amount, string Currency);

The positional properties are init-only, so a record is immutable unless you add settable properties yourself. That makes records good keys in dictionaries and good messages, DTOs and value objects. Two caveats: equality compares each property with its own Equals, so a record holding a List<T> compares the lists by reference; and a mutable property that changes after the record is used as a dictionary key breaks the lookup.

record struct gives the same features to a value type. A plain record struct has settable properties; readonly record struct is the immutable version.

Switch expressions and patterns

1static string Describe(Shape s) => s switch2{3    Circle { Radius: <= 0 } => "invalid circle",4    Circle c => $"circle, area {Math.PI * c.Radius * c.Radius:F2}",5    Rect(var w, var h) when w == h => $"square {w}",6    Rect(var w, var h) => $"rect {w}x{h}",7    _ => "unknown",8};9 10static string Classify(int[] xs) => xs switch11{12    [] => "empty",13    [var only] => $"one: {only}",14    [var first, .., var last] => $"from {first} to {last}",15};16 17abstract record Shape;18record Circle(double Radius) : Shape;19record Rect(double W, double H) : Shape;

With Circle(1), Rect(2, 2), Rect(2, 3) and [1, 2, 3], this printed circle, area 3.14, square 2, rect 2x3 and from 1 to 3 on .NET 10. The patterns used:

• type pattern Circle c: matches and binds a typed variable;
• property pattern { Radius: <= 0 } with a relational pattern inside;
• positional pattern Rect(var w, var h), which calls the generated Deconstruct;
• a case guard with when;
• list patterns (C# 11) with .. for "any number of elements";
• logical patterns and, or and not, as in is not null or >= 0 and < 10.

Arms are tried in order, and the compiler reports an error if an arm can never be reached because an earlier one covers it, and a warning (CS8509) if some input matches no arm. That warning is the reason to prefer a switch expression over a chain of if statements: add a new kind of input and the compiler shows you every place that doesn't handle it yet. C# 14 can't declare that a hierarchy is closed, so a _ arm is still needed for an abstract base like Shape.