csharp-idioms

name	csharp-idioms
description	Use when applying modern C# idioms — records, pattern matching, primary constructors, collection expressions.
metadata	{"category":"core","agent":"dotnet-architect","alwaysApply":true}

Modern C# Idioms

Core Principles

• Prefer the latest stable C# features supported by the project's target framework
• Use file-scoped namespaces in all new files
• Prefer records for immutable data transfer objects and value objects
• Use pattern matching over chains of if-else or type checks
• Apply primary constructors to reduce boilerplate in DI-heavy classes
• Gate features by target framework: do not emit C# 14 syntax for .NET 8 projects

Version Feature Matrix

Feature	Minimum	C# Version
File-scoped namespaces	.NET 6	C# 10
required modifier	.NET 7	C# 11
Primary constructors	.NET 8	C# 12
Collection expressions	.NET 8	C# 12
field keyword	.NET 10	C# 14
Extension types	.NET 10	C# 14

Patterns

File-Scoped Namespaces

// PREFER — file-scoped (one less indent level)
namespace {Company}.{Domain}.Features.Orders;

public sealed class OrderService { }

Records for DTOs and Value Objects

// Immutable DTO
public sealed record OrderResponse(Guid Id, string CustomerName, decimal Total);

// Value object with behavior
public sealed record Money(decimal Amount, string Currency)
{
    public Money Add(Money other)
    {
        if (Currency != other.Currency)
            throw new InvalidOperationException("Cannot add different currencies");
        return this with { Amount = Amount + other.Amount };
    }
}

// Record struct for lightweight value types
public readonly record struct OrderId(Guid Value)
{
    public static OrderId New() => new(Guid.NewGuid());
}

Primary Constructors (.NET 8+ / C# 12)

// Service with DI — no field declarations needed
public sealed class OrderService(
    IOrderRepository repository,
    ILogger<OrderService> logger)
{
    public async Task<Order?> GetAsync(Guid id, CancellationToken ct)
    {
        logger.LogInformation("Fetching order {OrderId}", id);
        return await repository.FindAsync(id, ct);
    }
}

Collection Expressions (C# 12)

int[] numbers = [1, 2, 3];
List<string> merged = [..existing, ..additional, "extra"];
ReadOnlySpan<byte> bytes = [0x01, 0x02, 0x03];

// Empty collection
List<Order> orders = [];

Pattern Matching

// Switch expression
string label = order.State switch
{
    OrderState.Pending when order.Total > 1000 => "Requires approval",
    OrderState.Pending => "Awaiting processing",
    OrderState.Shipped => "In transit",
    OrderState.Delivered => "Complete",
    _ => "Unknown"
};

// Property pattern
if (order is { Status: OrderStatus.Shipped, Total: > 500 })
    ApplyDiscount(order);

// List pattern (C# 12)
var result = args switch
{
    [var cmd, ..var rest] => ProcessCommand(cmd, rest),
    [] => ShowHelp()
};

// is null / is not null (prefer over == null)
if (order is not null)
    Process(order);

Sealed Classes and Expression Bodies

// Seal classes not designed for inheritance
public sealed class OrderValidator
{
    // Expression body for single-line methods
    public bool IsValid(Order order) => order.Items.Count > 0 && order.Total > 0;

    // Expression body for properties
    public string Name => nameof(OrderValidator);
}

Naming Conventions

namespace {Company}.{Domain}.Features.Orders;

public sealed class OrderService
{
    // PascalCase: types, methods, properties, constants
    private const string DefaultCurrency = "USD";

    // _camelCase: private fields
    private readonly IOrderRepository _repository;

    // camelCase: parameters and locals
    public async Task<OrderResponse?> GetOrderAsync(Guid orderId)
    {
        var order = await _repository.FindAsync(orderId);
        return order is null ? null : MapToResponse(order);
    }

    // Async suffix on async methods
    private static OrderResponse MapToResponse(Order order) => new(
        Id: order.Id,
        CustomerName: order.CustomerName,
        Total: order.Total);
}

field Keyword (C# 14 / .NET 10)

// Auto-property with validation — no backing field declaration
public string Name
{
    get => field;
    set => field = value?.Trim() ?? throw new ArgumentNullException(nameof(value));
}

Anti-Patterns

• Using block-scoped namespaces in new files (wastes indentation)
• Mutable classes where a record would suffice
• Long if-else chains instead of switch expressions
• Using == null instead of is null
• String interpolation in log messages (loses structure)
• Emitting C# 14 syntax for projects targeting .NET 8

Detect Existing Patterns

1. Check <LangVersion> in .csproj or Directory.Build.props
2. Check <TargetFramework> for net8.0, net9.0, net10.0
3. Scan .cs files for primary constructor usage
4. Look for record keyword usage in model/DTO files
5. Check .editorconfig for naming and style rules
6. Detect file-scoped vs block-scoped namespaces in existing files
7. Check private field naming convention (_camelCase vs camelCase)

Adding to Existing Project

1. Match existing style first — if the project uses block-scoped namespaces, continue until a team-wide migration is agreed
2. Adopt incrementally — introduce records for new DTOs, primary constructors for new services
3. Do not mix conventions — all files in a project should follow the same naming rules
4. Add .editorconfig if missing to enforce style rules:

[*.cs]
csharp_style_namespace_declarations = file_scoped:warning
csharp_style_var_for_built_in_types = false:suggestion
csharp_style_prefer_pattern_matching = true:suggestion
dotnet_naming_rule.private_fields_should_be_camel_case.severity = warning

Decision Guide

Scenario	Recommendation
New DTO / response type	Use record
Service class with DI	Use primary constructor (.NET 8+)
Multiple type checks	Use switch expression with patterns
Null check	Use is null / is not null
New file	Use file-scoped namespace
Lightweight ID type	Use readonly record struct

References

• https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/
• https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/coding-style/coding-conventions