Records & Pattern Matching

A with expression produces a new instance — a shallow copy of the source with the listed members replaced; the source itself is untouched. The compiler calls a synthesized copy constructor, so objects the record references are shared, not cloned.

A record is a reference type, but the compiler synthesizes Equals, GetHashCode and == comparing its type and every field, so two records with equal contents are equal. A plain class keeps reference equality: two instances are never equal.

The is type pattern checks a value's runtime type and, on a match, assigns it to a new typed variable — if (o is string s) — replacing a test-then-cast pair. null never matches a type pattern, so a match also guarantees the value is not null.

A positional record Point(int X, int Y) synthesizes public init-only properties, the matching constructor and a Deconstruct, so it fits positional patterns. Hand-written properties give none of that, but let you pick accessibility or required.

A switch expression produces a value: each arm is pattern => expression, with no case, break or fall-through, so it can be assigned or returned. A switch statement runs statements. An unmatched value throws SwitchExpressionException.

An init accessor allows assignment only while the object is initialized — constructor, object initializer, or with expression. A readonly field is assignable only in its declaration or a constructor, so an object initializer can never touch it.

A type pattern tests the runtime type; a property pattern inspects members ({ Age: > 18 }); a positional one deconstructs it; a relational one compares to a constant, with and/or; a list pattern matches a sequence's shape ([1, .., 9]).

On a class the primary-constructor parameters are constructor parameters captured into scope for the whole body — usable in field initializers and members. They do NOT become public properties; only a positional record does that.

required forces callers to assign the member in the object initializer, or it will not compile — the compiler checks, not the runtime. It is orthogonal to init: the member may stay mutable. A constructor setting it needs [SetsRequiredMembers].

A record's synthesized equality compares a hidden EqualityContract property — the runtime type — before the fields. The derived record overrides it, so the contracts differ and equality fails both ways. Comparing the type keeps == symmetric.

A constructor does not satisfy required by default: callers still assign the members in an initializer. [SetsRequiredMembers] says it sets them all, lifting that. A class's primary constructor does not — its parameters are not properties.

The compiler proves coverage from the arms' patterns; when it cannot, it warns (CS8509) and an unmatched value throws SwitchExpressionException. A type pattern never matches null, so a maybe-null input needs a null arm or a _ arm.