C# Language Patterns

C# Language Patterns

Priority: P0 (CRITICAL)

Modern C# standards for type-safe, performant, maintainable code.

Implementation Guidelines

• Nullable Reference Types: Enable <Nullable>enable</Nullable>. Use ? for nullable, avoid ! except when compiler can't infer.
• Records: record for immutable DTOs, record struct for stack-allocated value types.
• Pattern Matching: is patterns, switch expressions, property/positional patterns.
• Async/Await: Always use CancellationToken. ValueTask for hot paths. ConfigureAwait(false) in libraries.
• LINQ: Prefer method syntax. Avoid multiple enumerations (ToList() once). Use AsNoTracking() for read-only EF queries.
• Generics: Constraints (where T : class, new()), covariance (out T), contravariance (in T).
• Spans: Span<T>, ReadOnlySpan<T> for zero-allocation slicing.
• Primary Constructors: C# 12+ class Foo(int x) for concise DI.
• Collection Expressions: C# 12+ [1, 2, 3] syntax.
• Raw String Literals: """multi-line""" for SQL, JSON templates.

Anti-Patterns

• No async void: Use async Task. Exception: event handlers.
• No Task.Result/.Wait(): Deadlock risk. Always await.
• No DateTime.Now: Use DateTimeOffset.UtcNow for timezone safety.
• No string concat in loops: Use StringBuilder or string.Join.
• No ! abuse: Prefer null checks or ?? over null-forgiving.

Code

// Record with primary constructor
public record UserDto(string Name, string Email);

// Pattern matching with switch expression
string GetStatus(Order order) => order switch
{
    { Status: OrderStatus.Pending } => "Waiting",
    { Status: OrderStatus.Shipped, TrackingNumber: not null } => "In Transit",
    { IsCancelled: true } => "Cancelled",
    _ => "Unknown"
};

// Async with cancellation token
async Task<User?> GetUserAsync(int id, CancellationToken ct = default)
{
    return await _db.Users
        .AsNoTracking()
        .FirstOrDefaultAsync(u => u.Id == id, ct);
}

// Span for zero-allocation parsing
ReadOnlySpan<char> GetFirstWord(ReadOnlySpan<char> text)
{
    int idx = text.IndexOf(' ');
    return idx < 0 ? text : text[..idx];
}

// Primary constructor (C# 12)
public class UserService(IUserRepository repo, ILogger<UserService> logger)
{
    public async Task<User?> GetAsync(int id) => await repo.GetByIdAsync(id);
}

Reference & Examples

For advanced patterns, spans, and nullable annotations: See references/REFERENCE.md.

Related Topics

best-practices | security | tooling