modern-csharp

Modern C# (12–14)

.NET 10 ships C# 14 as the default language version — no <LangVersion> needed. The syntax of these features is assumed known; this file carries the traps.

Primary Constructors (C# 12)

• Parameters are captured as mutable hidden fields, not readonly. Assigning to one inside a method mutates class state. When that matters, copy into private readonly T _x = x; and use only the field.
• Using a parameter both to initialise a field and directly in a member body captures it twice (warning CS9124) — two copies that can diverge. Pick one.
• On records, primary-constructor parameters become public properties; on classes and structs they do not.

Collection Expressions (C# 12)

• The concrete type behind IEnumerable<T> x = [..] / IReadOnlyList<T> x = [..] is compiler-chosen. Don't downcast it to List<T> or T[].
• [..a, ..b] allocates a new collection every time. On a hot path that is a real allocation, not a view.

Records

• Equality is member-wise, but collection members compare by reference: two records holding equal List<T> contents are not equal. Records used as EF keys, cache keys or dedupe keys need scalar members or a custom Equals.
• with is a shallow copy — a mutated list inside the copy is the original's list.
• record struct is a value type; it is not guaranteed to be stack-allocated (boxed in interfaces, on the heap as a field of a class). Unlike readonly record struct, it is mutable.
• required members are bypassed by a constructor marked [SetsRequiredMembers] — the compiler then trusts that constructor to set them all.

Pattern Matching

• is not null or 42 parses as (is not null) or 42; the compiler warns that 42 is redundant. Write is not (null or 42).
• A switch expression without a discard arm throws SwitchExpressionException at runtime on an unmatched value. Add _ => throw new UnreachableException() so the failure is explicit.

Nullable Reference Types