modern-csharp

Modern C# (12–14)

Reference for language features available in .NET 8+. Used by dnp-planner and dnp-tdd-developer-hard.

Primary Constructors (C# 12)

Eliminates constructor boilerplate. Parameters are in scope for the entire class body.

// Before
public class OrderService
{
    private readonly IOrderRepository _repo;
    private readonly ILogger<OrderService> _logger;

    public OrderService(IOrderRepository repo, ILogger<OrderService> logger)
    {
        _repo = repo;
        _logger = logger;
    }
}

// After (C# 12)
public class OrderService(IOrderRepository repo, ILogger<OrderService> logger)
{
    public async Task<Order?> GetByIdAsync(int id, CancellationToken ct)
    {
        logger.LogInformation("Fetching order {Id}", id);
        return await repo.GetByIdAsync(id, ct);
    }
}

Caveat: primary constructor parameters are captured by reference — if you need a private field (e.g., for mutation tracking), assign explicitly: private readonly IOrderRepository _repo = repo;

Collection Expressions (C# 12)

Unified syntax for arrays, lists, and spans. Supports spread (..) operator.

// Array
int[] ids = [1, 2, 3];

// List<T>
List<string> tags = ["dotnet", "csharp"];

// Spread — merge sequences
var basePermissions = new[] { "read", "list" };
string[] adminPermissions = [..basePermissions, "write", "delete"];

// Empty collection
IReadOnlyList<string> empty = [];

// In method calls
ProcessItems([1, 2, 3]);

Records and With-Expressions

Records are reference types with value semantics. Use for DTOs, domain events, and immutable data shapes.

// Positional record (primary constructor + deconstruct)
public record Order(int Id, string CustomerId, string Status, decimal Total);

// Non-destructive mutation
var order = new Order(1, "cust-42", "Pending", 99.99m);
var shipped = order with { Status = "Shipped" };

// Record struct (stack-allocated, value semantics)
public record struct Money(decimal Amount, string Currency);

// Class-style record with validation
public record CreateOrderRequest
{
    public required string CustomerId { get; init; }
    public required List<OrderLine> Lines { get; init; }
}