modern-csharp-coding-standards

name	modern-csharp-coding-standards
description	Write modern, high-performance C# code using records, pattern matching, value objects, async/await, Span<T>/Memory<T>, and best-practice API design patterns. Emphasizes functional-style programming with C# 12+ features.

Modern C# Coding Standards

When to Use This Skill

Use this skill when:

• Writing new C# code or refactoring existing code
• Designing public APIs for libraries or services
• Optimizing performance-critical code paths
• Implementing domain models with strong typing
• Building async/await-heavy applications
• Working with binary data, buffers, or high-throughput scenarios

Core Principles

1. Immutability by Default - Use record types and init-only properties
2. Type Safety - Leverage nullable reference types and value objects
3. Modern Pattern Matching - Use switch expressions and patterns extensively
4. Async Everywhere - Prefer async APIs with proper cancellation support
5. Zero-Allocation Patterns - Use Span<T> and Memory<T> for performance-critical code
6. API Design - Accept abstractions, return appropriately specific types
7. Composition Over Inheritance - Avoid abstract base classes, prefer composition
8. Value Objects as Structs - Use readonly record struct for value objects

---

Language Patterns

Records for Immutable Data (C# 9+)

Use record types for DTOs, messages, events, and domain entities.

// Simple immutable DTO
public record CustomerDto(string Id, string Name, string Email);

// Record with validation in constructor
public record EmailAddress
{
    public string Value { get; init; }

    public EmailAddress(string value)
    {
        if (string.IsNullOrWhiteSpace(value) || !value.Contains('@'))
            throw new ArgumentException("Invalid email address", nameof(value));

        Value = value;
    }
}

// Record with computed properties
public record Order(string Id, decimal Subtotal, decimal Tax)
{
    public decimal Total => Subtotal + Tax;
}

// Records with collections - use IReadOnlyList
public record ShoppingCart(
    string CartId,
    string CustomerId,
    IReadOnlyList<CartItem> Items
)
{
    public decimal Total => Items.Sum(item => item.Price * item.Quantity);
}

When to use record class vs record struct:

• record class (default): Reference types, use for entities, aggregates, DTOs with multiple properties
• record struct: Value types, use for value objects (see next section)

---

Value Objects as readonly record struct

Value objects should always be readonly record struct for performance and value semantics.

// Single-value object
public readonly record struct OrderId(string Value)
{
    public OrderId(string value) : this(
        !string.IsNullOrWhiteSpace(value)
            ? value
            : throw new ArgumentException("OrderId cannot be empty", nameof(value)))
    {
    }

    public override string ToString() => Value;

    // Implicit conversion for convenience
    public static implicit operator string(OrderId orderId) => orderId.Value;
}

// Multi-value object
public readonly record struct Money(decimal Amount, string Currency)
{
    public Money(decimal amount, string currency) : this(
        amount >= 0 ? amount : throw new ArgumentException("Amount cannot be negative", nameof(amount)),
        ValidateCurrency(currency))
    {
    }

    private static string ValidateCurrency(string currency)
    {
        if (string.IsNullOrWhiteSpace(currency) || currency.Length != 3)
            throw new ArgumentException("Currency must be a 3-letter code", nameof(currency));
        return currency.ToUpperInvariant();
    }

    public Money Add(Money other)
    {
        if (Currency != other.Currency)
            throw new InvalidOperationException($"Cannot add {Currency} to {other.Currency}");

        return new Money(Amount + other.Amount, Currency);
    }

    public override string ToString() => $"{Amount:N2} {Currency}";
}

// Complex value object with factory pattern
public readonly record struct PhoneNumber
{
    public string Value { get; }

    private PhoneNumber(string value) => Value = value;

    public static Result<PhoneNumber, string> Create(string input)
    {
        if (string.IsNullOrWhiteSpace(input))
            return Result<PhoneNumber, string>.Failure("Phone number cannot be empty");

        // Normalize: remove all non-digits
        var digits = new string(input.Where(char.IsDigit).ToArray());

        if (digits.Length < 10 || digits.Length > 15)
            return Result<PhoneNumber, string>.Failure("Phone number must be 10-15 digits");

        return Result<PhoneNumber, string>.Success(new PhoneNumber(digits));
    }

    public override string ToString() => Value;
}

// Percentage value object with range validation
public readonly record struct Percentage
{
    private readonly decimal _value;

    public decimal Value => _value;

    public Percentage(decimal value)
    {
        if (value < 0 || value > 100)
            throw new ArgumentOutOfRangeException(nameof(value), "Percentage must be between 0 and 100");
        _value = value;
    }

    public decimal AsDecimal() => _value / 100m;

    public static Percentage FromDecimal(decimal decimalValue)
    {
        if (decimalValue < 0 || decimalValue > 1)
            throw new ArgumentOutOfRangeException(nameof(decimalValue), "Decimal must be between 0 and 1");
        return new Percentage(decimalValue * 100);
    }

    public override string ToString() => $"{_value}%";
}

// Strongly-typed ID
public readonly record struct CustomerId(Guid Value)
{
    public static CustomerId New() => new(Guid.NewGuid());
    public override string ToString() => Value.ToString();
}

// Quantity with units
public readonly record struct Quantity(int Value, string Unit)
{
    public Quantity(int value, string unit) : this(
        value >= 0 ? value : throw new ArgumentException("Quantity cannot be negative"),
        !string.IsNullOrWhiteSpace(unit) ? unit : throw new ArgumentException("Unit cannot be empty"))
    {
    }

    public override string ToString() => $"{Value} {Unit}";
}

Why readonly record struct for value objects:

• Value semantics: Equality based on content, not reference
• Stack allocation: Better performance, no GC pressure
• Immutability: readonly prevents accidental mutation
• Pattern matching: Works seamlessly with switch expressions

---

Pattern Matching (C# 8-12)

Leverage modern pattern matching for cleaner, more expressive code.

// Switch expressions with value objects
public string GetPaymentMethodDescription(PaymentMethod payment) => payment switch
{
    { Type: PaymentType.CreditCard, Last4: var last4 } => $"Credit card ending in {last4}",
    { Type: PaymentType.BankTransfer, AccountNumber: var account } => $"Bank transfer from {account}",
    { Type: PaymentType.Cash } => "Cash payment",
    _ => "Unknown payment method"
};

// Property patterns
public decimal CalculateDiscount(Order order) => order switch
{
    { Total: > 1000m } => order.Total * 0.15m,
    { Total: > 500m } => order.Total * 0.10m,
    { Total: > 100m } => order.Total * 0.05m,
    _ => 0m
};

// Relational and logical patterns
public string ClassifyTemperature(int temp) => temp switch
{
    < 0 => "Freezing",
    >= 0 and < 10 => "Cold",
    >= 10 and < 20 => "Cool",
    >= 20 and < 30 => "Warm",
    >= 30 => "Hot",
    _ => throw new ArgumentOutOfRangeException(nameof(temp))
};

// List patterns (C# 11+)
public bool IsValidSequence(int[] numbers) => numbers switch
{
    [] => false,                                      // Empty
    [_] => true,                                      // Single element
    [var first, .., var last] when first < last => true,  // First < last
    _ => false
};

// Type patterns with null checks
public string FormatValue(object? value) => value switch
{
    null => "null",
    string s => $"\"{s}\"",
    int i => i.ToString(),
    double d => d.ToString("F2"),
    DateTime dt => dt.ToString("yyyy-MM-dd"),
    Money m => m.ToString(),
    IEnumerable<object> collection => $"[{string.Join(", ", collection)}]",
    _ => value.ToString() ?? "unknown"
};

// Combining patterns for complex logic
public record OrderState(bool IsPaid, bool IsShipped, bool IsCancelled);

public string GetOrderStatus(OrderState state) => state switch
{
    { IsCancelled: true } => "Cancelled",
    { IsPaid: true, IsShipped: true } => "Delivered",
    { IsPaid: true, IsShipped: false } => "Processing",
    { IsPaid: false } => "Awaiting Payment",
    _ => "Unknown"
};

// Pattern matching with value objects
public decimal CalculateShipping(Money total, Country destination) => (total, destination) switch
{
    ({ Amount: > 100m }, _) => 0m,                    // Free shipping over $100
    (_, { Code: "US" or "CA" }) => 5m,                // North America
    (_, { Code: "GB" or "FR" or "DE" }) => 10m,       // Europe
    _ => 25m                                           // International
};

---

Nullable Reference Types (C# 8+)

Enable nullable reference types in your project and handle nulls explicitly.

// In .csproj
<PropertyGroup>
    <Nullable>enable</Nullable>
</PropertyGroup>

// Explicit nullability
public class UserService
{
    // Non-nullable by default
    public string GetUserName(User user) => user.Name;

    // Explicitly nullable return
    public string? FindUserName(string userId)
    {
        var user = _repository.Find(userId);
        return user?.Name;  // Returns null if user not found
    }

    // Null-forgiving operator (use sparingly!)
    public string GetRequiredConfigValue(string key)
    {
        var value = Configuration[key];
        return value!;  // Only if you're CERTAIN it's not null
    }

    // Nullable value objects
    public Money? GetAccountBalance(string accountId)
    {
        var account = _repository.Find(accountId);
        return account?.Balance;
    }
}

// Pattern matching with null checks
public decimal GetDiscount(Customer? customer) => customer switch
{
    null => 0m,
    { IsVip: true } => 0.20m,
    { OrderCount: > 10 } => 0.10m,
    _ => 0.05m
};

// Null-coalescing patterns
public string GetDisplayName(User? user) =>
    user?.PreferredName ?? user?.Email ?? "Guest";

// Guard clauses with ArgumentNullException.ThrowIfNull (C# 11+)
public void ProcessOrder(Order? order)
{
    ArgumentNullException.ThrowIfNull(order);

    // order is now non-nullable in this scope
    Console.WriteLine(order.Id);
}

---

Composition Over Inheritance

Avoid abstract base classes and inheritance hierarchies. Use composition and interfaces instead.

// ❌ BAD: Abstract base class hierarchy
public abstract class PaymentProcessor
{
    public abstract Task<PaymentResult> ProcessAsync(Money amount);

    protected async Task<bool> ValidateAsync(Money amount)
    {
        // Shared validation logic
        return amount.Amount > 0;
    }
}

public class CreditCardProcessor : PaymentProcessor
{
    public override async Task<PaymentResult> ProcessAsync(Money amount)
    {
        await ValidateAsync(amount);
        // Process credit card...
    }
}

// ✅ GOOD: Composition with interfaces
public interface IPaymentProcessor
{
    Task<PaymentResult> ProcessAsync(Money amount, CancellationToken cancellationToken);
}

public interface IPaymentValidator
{
    Task<ValidationResult> ValidateAsync(Money amount, CancellationToken cancellationToken);
}

// Concrete implementations compose validators
public sealed class CreditCardProcessor : IPaymentProcessor
{
    private readonly IPaymentValidator _validator;
    private readonly ICreditCardGateway _gateway;

    public CreditCardProcessor(IPaymentValidator validator, ICreditCardGateway gateway)
    {
        _validator = validator;
        _gateway = gateway;
    }

    public async Task<PaymentResult> ProcessAsync(Money amount, CancellationToken cancellationToken)
    {
        var validation = await _validator.ValidateAsync(amount, cancellationToken);
        if (!validation.IsValid)
            return PaymentResult.Failed(validation.Error);

        return await _gateway.ChargeAsync(amount, cancellationToken);
    }
}

// ✅ GOOD: Static helper classes for shared logic (no inheritance)
public static class PaymentValidation
{
    public static ValidationResult ValidateAmount(Money amount)
    {
        if (amount.Amount <= 0)
            return ValidationResult.Invalid("Amount must be positive");

        if (amount.Amount > 10000m)
            return ValidationResult.Invalid("Amount exceeds maximum");

        return ValidationResult.Valid();
    }
}

// ✅ GOOD: Records for modeling variants (not inheritance)
public enum PaymentType { CreditCard, BankTransfer, Cash }

public record PaymentMethod
{
    public PaymentType Type { get; init; }
    public string? Last4 { get; init; }           // For credit cards
    public string? AccountNumber { get; init; }    // For bank transfers

    public static PaymentMethod CreditCard(string last4) => new()
    {
        Type = PaymentType.CreditCard,
        Last4 = last4
    };

    public static PaymentMethod BankTransfer(string accountNumber) => new()
    {
        Type = PaymentType.BankTransfer,
        AccountNumber = accountNumber
    };

    public static PaymentMethod Cash() => new() { Type = PaymentType.Cash };
}

When inheritance is acceptable:

• Framework requirements (e.g., ControllerBase in ASP.NET Core)
• Library integration (e.g., custom exceptions inheriting from Exception)
• These should be rare cases in your application code

---

Performance Patterns

Async/Await Best Practices

Always use async for I/O-bound operations:

// ✅ GOOD: Async all the way
public async Task<Order> GetOrderAsync(string orderId, CancellationToken cancellationToken)
{
    var order = await _repository.GetAsync(orderId, cancellationToken);
    var customer = await _customerService.GetCustomerAsync(order.CustomerId, cancellationToken);
    return order;
}

// ❌ BAD: Blocking on async code
public Order GetOrder(string orderId)
{
    return _repository.GetAsync(orderId).Result;  // DEADLOCK RISK!
}

// ✅ GOOD: ValueTask for frequently-called, often-synchronous methods
public ValueTask<Order?> GetCachedOrderAsync(string orderId, CancellationToken cancellationToken)
{
    if (_cache.TryGetValue(orderId, out var order))
        return ValueTask.FromResult<Order?>(order);  // Synchronous path, no allocation

    return GetFromDatabaseAsync(orderId, cancellationToken);  // Async path
}

private async ValueTask<Order?> GetFromDatabaseAsync(string orderId, CancellationToken cancellationToken)
{
    var order = await _repository.GetAsync(orderId, cancellationToken);
    if (order is not null)
        _cache[orderId] = order;
    return order;
}

// ✅ GOOD: IAsyncEnumerable for streaming
public async IAsyncEnumerable<Order> StreamOrdersAsync(
    string customerId,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    await foreach (var order in _repository.StreamAllAsync(cancellationToken))
    {
        if (order.CustomerId == customerId)
            yield return order;
    }
}

// ✅ GOOD: ConfigureAwait(false) in library code (not application code)
public async Task<string> ProcessDataAsync(string input, CancellationToken cancellationToken)
{
    var data = await FetchDataAsync(cancellationToken).ConfigureAwait(false);
    var result = await TransformDataAsync(data, cancellationToken).ConfigureAwait(false);
    return result;
}

Always accept CancellationToken:

// ✅ GOOD: CancellationToken parameter with default
public async Task<List<Order>> GetOrdersAsync(
    string customerId,
    CancellationToken cancellationToken = default)
{
    var orders = await _repository.GetOrdersByCustomerAsync(customerId, cancellationToken);
    return orders;
}

// Pass cancellation through the call stack
public async Task<OrderSummary> GetOrderSummaryAsync(
    string customerId,
    CancellationToken cancellationToken = default)
{
    var orders = await GetOrdersAsync(customerId, cancellationToken);
    var total = orders.Sum(o => o.Total);
    return new OrderSummary(customerId, orders.Count, total);
}

// Link cancellation tokens when composing operations
public async Task<ProcessResult> ProcessWithTimeoutAsync(
    string data,
    TimeSpan timeout,
    CancellationToken cancellationToken = default)
{
    using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
    cts.CancelAfter(timeout);

    return await ProcessAsync(data, cts.Token);
}

---

Span and Memory for Zero-Allocation Code

Use Span<T> and Memory<T> instead of byte[] or string for performance-critical code.

// ✅ GOOD: Span<T> for synchronous, zero-allocation operations
public int ParseOrderId(ReadOnlySpan<char> input)
{
    // Work with data without allocations
    if (!input.StartsWith("ORD-"))
        throw new FormatException("Invalid order ID format");

    var numberPart = input.Slice(4);
    return int.Parse(numberPart);
}

// stackalloc with Span<T>
public void FormatMessage()
{
    Span<char> buffer = stackalloc char[256];
    var written = FormatInto(buffer);
    var message = new string(buffer.Slice(0, written));
}

// ✅ GOOD: Memory<T> for async operations (Span can't cross await)
public async Task<int> ReadDataAsync(
    Memory<byte> buffer,
    CancellationToken cancellationToken)
{
    return await _stream.ReadAsync(buffer, cancellationToken);
}

// ✅ GOOD: String manipulation with Span to avoid allocations
public bool TryParseKeyValue(ReadOnlySpan<char> line, out string key, out string value)
{
    key = string.Empty;
    value = string.Empty;

    int colonIndex = line.IndexOf(':');
    if (colonIndex == -1)
        return false;

    // Only allocate strings once we know the format is valid
    key = new string(line.Slice(0, colonIndex).Trim());
    value = new string(line.Slice(colonIndex + 1).Trim());
    return true;
}

// ✅ GOOD: ArrayPool for temporary large buffers
public async Task ProcessLargeFileAsync(
    Stream stream,
    CancellationToken cancellationToken)
{
    var buffer = ArrayPool<byte>.Shared.Rent(8192);
    try
    {
        int bytesRead;
        while ((bytesRead = await stream.ReadAsync(buffer.AsMemory(), cancellationToken)) > 0)
        {
            ProcessChunk(buffer.AsSpan(0, bytesRead));
        }
    }
    finally
    {
        ArrayPool<byte>.Shared.Return(buffer);
    }
}

// ✅ GOOD: Span-based parsing without substring allocations
public static (string Protocol, string Host, int Port) ParseUrl(ReadOnlySpan<char> url)
{
    var protocolEnd = url.IndexOf("://");
    var protocol = new string(url.Slice(0, protocolEnd));

    var afterProtocol = url.Slice(protocolEnd + 3);
    var portStart = afterProtocol.IndexOf(':');

    var host = new string(afterProtocol.Slice(0, portStart));
    var portSpan = afterProtocol.Slice(portStart + 1);
    var port = int.Parse(portSpan);

    return (protocol, host, port);
}

// ✅ GOOD: Writing data to Span
public bool TryFormatOrderId(int orderId, Span<char> destination, out int charsWritten)
{
    const string prefix = "ORD-";

    if (destination.Length < prefix.Length + 10)
    {
        charsWritten = 0;
        return false;
    }

    prefix.AsSpan().CopyTo(destination);
    var numberWritten = orderId.TryFormat(
        destination.Slice(prefix.Length),
        out var numberChars);

    charsWritten = prefix.Length + numberChars;
    return numberWritten;
}

When to use what:

Type	Use Case
Span<T>	Synchronous operations, stack-allocated buffers, slicing without allocation
ReadOnlySpan<T>	Read-only views, method parameters for data you won't modify
Memory<T>	Async operations (Span can't cross await boundaries)
ReadOnlyMemory<T>	Read-only async operations
byte[]	When you need to store data long-term or pass to APIs requiring arrays
ArrayPool<T>	Large temporary buffers (>1KB) to avoid GC pressure

---

API Design Principles

Accept Abstractions, Return Appropriately Specific

For Parameters (Accept):

// ✅ GOOD: Accept IEnumerable<T> if you only iterate once
public decimal CalculateTotal(IEnumerable<OrderItem> items)
{
    return items.Sum(item => item.Price * item.Quantity);
}

// ✅ GOOD: Accept IReadOnlyCollection<T> if you need Count
public bool HasMinimumItems(IReadOnlyCollection<OrderItem> items, int minimum)
{
    return items.Count >= minimum;
}

// ✅ GOOD: Accept IReadOnlyList<T> if you need indexing
public OrderItem GetMiddleItem(IReadOnlyList<OrderItem> items)
{
    if (items.Count == 0)
        throw new ArgumentException("List cannot be empty");

    return items[items.Count / 2];  // Indexed access
}

// ✅ GOOD: Accept ReadOnlySpan<T> for high-performance, zero-allocation APIs
public int Sum(ReadOnlySpan<int> numbers)
{
    int total = 0;
    foreach (var num in numbers)
        total += num;
    return total;
}

// ✅ GOOD: Accept IAsyncEnumerable<T> for async streaming
public async Task<int> CountItemsAsync(
    IAsyncEnumerable<Order> orders,
    CancellationToken cancellationToken)
{
    int count = 0;
    await foreach (var order in orders.WithCancellation(cancellationToken))
        count++;
    return count;
}

For Return Types:

// ✅ GOOD: Return IEnumerable<T> for lazy/deferred execution
public IEnumerable<Order> GetOrdersLazy(string customerId)
{
    foreach (var order in _repository.Query())
    {
        if (order.CustomerId == customerId)
            yield return order;  // Lazy evaluation
    }
}

// ✅ GOOD: Return IReadOnlyList<T> for materialized, immutable collections
public IReadOnlyList<Order> GetOrders(string customerId)
{
    return _repository
        .Query()
        .Where(o => o.CustomerId == customerId)
        .ToList();  // Materialized
}

// ✅ GOOD: Return concrete types when callers need mutation
public List<Order> GetMutableOrders(string customerId)
{
    // Explicitly allow mutation by returning List<T>
    return _repository
        .Query()
        .Where(o => o.CustomerId == customerId)
        .ToList();
}

// ✅ GOOD: Return IAsyncEnumerable<T> for async streaming
public async IAsyncEnumerable<Order> StreamOrdersAsync(
    string customerId,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    await foreach (var order in _repository.StreamAllAsync(cancellationToken))
    {
        if (order.CustomerId == customerId)
            yield return order;
    }
}

// ✅ GOOD: Return arrays for interop or when caller expects array
public byte[] SerializeOrder(Order order)
{
    // Binary serialization - byte[] is appropriate here
    return MessagePackSerializer.Serialize(order);
}

Summary Table:

Scenario	Accept	Return
Only iterate once	IEnumerable<T>	IEnumerable<T> (if lazy)
Need count	IReadOnlyCollection<T>	IReadOnlyCollection<T>
Need indexing	IReadOnlyList<T>	IReadOnlyList<T>
High-performance, sync	ReadOnlySpan<T>	Span<T> (rarely)
Async streaming	IAsyncEnumerable<T>	IAsyncEnumerable<T>
Caller needs mutation	-	List<T>, T[]

---

Method Signatures Best Practices

// ✅ GOOD: Complete async method signature
public async Task<Result<Order, OrderError>> CreateOrderAsync(
    CreateOrderRequest request,
    CancellationToken cancellationToken = default)
{
    // Implementation
}

// ✅ GOOD: Optional parameters at the end
public async Task<List<Order>> GetOrdersAsync(
    string customerId,
    DateTime? startDate = null,
    DateTime? endDate = null,
    CancellationToken cancellationToken = default)
{
    // Implementation
}

// ✅ GOOD: Use record for multiple related parameters
public record SearchOrdersRequest(
    string? CustomerId,
    DateTime? StartDate,
    DateTime? EndDate,
    OrderStatus? Status,
    int PageSize = 20,
    int PageNumber = 1
);

public async Task<PagedResult<Order>> SearchOrdersAsync(
    SearchOrdersRequest request,
    CancellationToken cancellationToken = default)
{
    // Implementation
}

// ✅ GOOD: Primary constructors (C# 12+) for simple classes
public sealed class OrderService(IOrderRepository repository, ILogger<OrderService> logger)
{
    public async Task<Order> GetOrderAsync(OrderId orderId, CancellationToken cancellationToken)
    {
        logger.LogInformation("Fetching order {OrderId}", orderId);
        return await repository.GetAsync(orderId, cancellationToken);
    }
}

// ✅ GOOD: Options pattern for complex configuration
public sealed class EmailServiceOptions
{
    public required string SmtpHost { get; init; }
    public int SmtpPort { get; init; } = 587;
    public bool UseSsl { get; init; } = true;
    public TimeSpan Timeout { get; init; } = TimeSpan.FromSeconds(30);
}

public sealed class EmailService(IOptions<EmailServiceOptions> options)
{
    private readonly EmailServiceOptions _options = options.Value;
}

---

Error Handling

Result Type Pattern (Railway-Oriented Programming)

For expected errors, use a Result<T, TError> type instead of exceptions.

// Simple Result type as readonly record struct
public readonly record struct Result<TValue, TError>
{
    private readonly TValue? _value;
    private readonly TError? _error;
    private readonly bool _isSuccess;

    private Result(TValue value)
    {
        _value = value;
        _error = default;
        _isSuccess = true;
    }

    private Result(TError error)
    {
        _value = default;
        _error = error;
        _isSuccess = false;
    }

    public bool IsSuccess => _isSuccess;
    public bool IsFailure => !_isSuccess;

    public TValue Value => _isSuccess
        ? _value!
        : throw new InvalidOperationException("Cannot access Value of a failed result");

    public TError Error => !_isSuccess
        ? _error!
        : throw new InvalidOperationException("Cannot access Error of a successful result");

    public static Result<TValue, TError> Success(TValue value) => new(value);
    public static Result<TValue, TError> Failure(TError error) => new(error);

    public Result<TOut, TError> Map<TOut>(Func<TValue, TOut> mapper)
        => _isSuccess
            ? Result<TOut, TError>.Success(mapper(_value!))
            : Result<TOut, TError>.Failure(_error!);

    public Result<TOut, TError> Bind<TOut>(Func<TValue, Result<TOut, TError>> binder)
        => _isSuccess ? binder(_value!) : Result<TOut, TError>.Failure(_error!);

    public TValue GetValueOr(TValue defaultValue)
        => _isSuccess ? _value! : defaultValue;

    public TResult Match<TResult>(
        Func<TValue, TResult> onSuccess,
        Func<TError, TResult> onFailure)
        => _isSuccess ? onSuccess(_value!) : onFailure(_error!);
}

// Error type as readonly record struct
public readonly record struct OrderError(string Code, string Message);

// Usage example
public sealed class OrderService(IOrderRepository repository)
{
    public async Task<Result<Order, OrderError>> CreateOrderAsync(
        CreateOrderRequest request,
        CancellationToken cancellationToken)
    {
        // Validate
        var validationResult = ValidateRequest(request);
        if (validationResult.IsFailure)
            return Result<Order, OrderError>.Failure(validationResult.Error);

        // Check inventory
        var inventoryResult = await CheckInventoryAsync(request.Items, cancellationToken);
        if (inventoryResult.IsFailure)
            return Result<Order, OrderError>.Failure(inventoryResult.Error);

        // Create order
        var order = new Order(
            OrderId.New(),
            new CustomerId(request.CustomerId),
            request.Items);

        await repository.SaveAsync(order, cancellationToken);

        return Result<Order, OrderError>.Success(order);
    }

    // Pattern matching on Result
    public IActionResult MapToActionResult(Result<Order, OrderError> result)
    {
        return result.Match(
            onSuccess: order => new OkObjectResult(order),
            onFailure: error => error.Code switch
            {
                "VALIDATION_ERROR" => new BadRequestObjectResult(error.Message),
                "INSUFFICIENT_INVENTORY" => new ConflictObjectResult(error.Message),
                "NOT_FOUND" => new NotFoundObjectResult(error.Message),
                _ => new ObjectResult(error.Message) { StatusCode = 500 }
            }
        );
    }
}

When to use Result vs Exceptions:

• Use Result: Expected errors (validation, business rules, not found)
• Use Exceptions: Unexpected errors (network failures, system errors, programming bugs)

---

Testing Patterns

// Use record for test data builders
public record OrderBuilder
{
    public OrderId Id { get; init; } = OrderId.New();
    public CustomerId CustomerId { get; init; } = CustomerId.New();
    public Money Total { get; init; } = new Money(100m, "USD");
    public IReadOnlyList<OrderItem> Items { get; init; } = Array.Empty<OrderItem>();

    public Order Build() => new(Id, CustomerId, Total, Items);
}

// Use 'with' expression for test variations
[Fact]
public void CalculateDiscount_LargeOrder_AppliesCorrectDiscount()
{
    // Arrange
    var baseOrder = new OrderBuilder().Build();
    var largeOrder = baseOrder with
    {
        Total = new Money(1500m, "USD")
    };

    // Act
    var discount = _service.CalculateDiscount(largeOrder);

    // Assert
    discount.Should().Be(new Money(225m, "USD")); // 15% of 1500
}

// Span-based testing
[Theory]
[InlineData("ORD-12345", true)]
[InlineData("INVALID", false)]
public void TryParseOrderId_VariousInputs_ReturnsExpectedResult(
    string input,
    bool expected)
{
    // Act
    var result = OrderIdParser.TryParse(input.AsSpan(), out var orderId);

    // Assert
    result.Should().Be(expected);
}

// Testing with value objects
[Fact]
public void Money_Add_SameCurrency_ReturnsSum()
{
    // Arrange
    var money1 = new Money(100m, "USD");
    var money2 = new Money(50m, "USD");

    // Act
    var result = money1.Add(money2);

    // Assert
    result.Should().Be(new Money(150m, "USD"));
}

[Fact]
public void Money_Add_DifferentCurrency_ThrowsException()
{
    // Arrange
    var usd = new Money(100m, "USD");
    var eur = new Money(50m, "EUR");

    // Act & Assert
    var act = () => usd.Add(eur);
    act.Should().Throw<InvalidOperationException>()
        .WithMessage("*different currencies*");
}

---

Anti-Patterns to Avoid

❌ DON'T: Use mutable DTOs

// BAD: Mutable DTO
public class CustomerDto
{
    public string Id { get; set; }
    public string Name { get; set; }
}

// GOOD: Immutable record
public record CustomerDto(string Id, string Name);

❌ DON'T: Use classes for value objects

// BAD: Value object as class
public class OrderId
{
    public string Value { get; }
    public OrderId(string value) => Value = value;
}

// GOOD: Value object as readonly record struct
public readonly record struct OrderId(string Value);

❌ DON'T: Create deep inheritance hierarchies

// BAD: Deep inheritance
public abstract class Entity { }
public abstract class AggregateRoot : Entity { }
public abstract class Order : AggregateRoot { }
public class CustomerOrder : Order { }

// GOOD: Flat structure with composition
public interface IEntity
{
    Guid Id { get; }
}

public record Order(OrderId Id, CustomerId CustomerId, Money Total) : IEntity
{
    Guid IEntity.Id => Id.Value;
}

❌ DON'T: Return List when you mean IReadOnlyList

// BAD: Exposes internal list for modification
public List<Order> GetOrders() => _orders;

// GOOD: Returns read-only view
public IReadOnlyList<Order> GetOrders() => _orders;

❌ DON'T: Use byte[] when ReadOnlySpan works

// BAD: Allocates array on every call
public byte[] GetHeader()
{
    var header = new byte[64];
    // Fill header
    return header;
}

// GOOD: Zero allocation with Span
public void GetHeader(Span<byte> destination)
{
    if (destination.Length < 64)
        throw new ArgumentException("Buffer too small");

    // Fill header directly into caller's buffer
}

❌ DON'T: Forget CancellationToken in async methods

// BAD: No cancellation support
public async Task<Order> GetOrderAsync(OrderId id)
{
    return await _repository.GetAsync(id);
}

// GOOD: Cancellation support
public async Task<Order> GetOrderAsync(
    OrderId id,
    CancellationToken cancellationToken = default)
{
    return await _repository.GetAsync(id, cancellationToken);
}

❌ DON'T: Block on async code

// BAD: Deadlock risk!
public Order GetOrder(OrderId id)
{
    return GetOrderAsync(id).Result;
}

// BAD: Also deadlock risk!
public Order GetOrder(OrderId id)
{
    return GetOrderAsync(id).GetAwaiter().GetResult();
}

// GOOD: Async all the way
public async Task<Order> GetOrderAsync(
    OrderId id,
    CancellationToken cancellationToken)
{
    return await _repository.GetAsync(id, cancellationToken);
}

---

Code Organization

// File: Domain/Orders/Order.cs

namespace MyApp.Domain.Orders;

// 1. Primary domain type
public record Order(
    OrderId Id,
    CustomerId CustomerId,
    Money Total,
    OrderStatus Status,
    IReadOnlyList<OrderItem> Items
)
{
    // Computed properties
    public bool IsCompleted => Status is OrderStatus.Completed;

    // Domain methods returning Result for expected errors
    public Result<Order, OrderError> AddItem(OrderItem item)
    {
        if (Status is not OrderStatus.Draft)
            return Result<Order, OrderError>.Failure(
                new OrderError("ORDER_NOT_DRAFT", "Can only add items to draft orders"));

        var newItems = Items.Append(item).ToList();
        var newTotal = new Money(
            Items.Sum(i => i.Total.Amount) + item.Total.Amount,
            Total.Currency);

        return Result<Order, OrderError>.Success(
            this with { Items = newItems, Total = newTotal });
    }
}

// 2. Enums for state
public enum OrderStatus
{
    Draft,
    Submitted,
    Processing,
    Completed,
    Cancelled
}

// 3. Related types
public record OrderItem(
    ProductId ProductId,
    Quantity Quantity,
    Money UnitPrice
)
{
    public Money Total => new(
        UnitPrice.Amount * Quantity.Value,
        UnitPrice.Currency);
}

// 4. Value objects
public readonly record struct OrderId(Guid Value)
{
    public static OrderId New() => new(Guid.NewGuid());
}

// 5. Errors
public readonly record struct OrderError(string Code, string Message);

---

Best Practices Summary

DO's ✅

• Use record for DTOs, messages, and domain entities
• Use readonly record struct for value objects
• Leverage pattern matching with switch expressions
• Enable and respect nullable reference types
• Use async/await for all I/O operations
• Accept CancellationToken in all async methods
• Use Span<T> and Memory<T> for high-performance scenarios
• Accept abstractions (IEnumerable<T>, IReadOnlyList<T>)
• Return appropriate interfaces or concrete types
• Use Result<T, TError> for expected errors
• Use ConfigureAwait(false) in library code
• Pool buffers with ArrayPool<T> for large allocations
• Prefer composition over inheritance
• Avoid abstract base classes in application code

DON'Ts ❌

• Don't use mutable classes when records work
• Don't use classes for value objects (use readonly record struct)
• Don't create deep inheritance hierarchies
• Don't ignore nullable reference type warnings
• Don't block on async code (.Result, .Wait())
• Don't use byte[] when Span<byte> suffices
• Don't forget CancellationToken parameters
• Don't return mutable collections from APIs
• Don't throw exceptions for expected business errors
• Don't use string concatenation in loops
• Don't allocate large arrays repeatedly (use ArrayPool)

---

Additional Resources

• C# Language Specification: https://learn.microsoft.com/en-us/dotnet/csharp/
• Pattern Matching: https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/pattern-matching
• Span and Memory: https://learn.microsoft.com/en-us/dotnet/standard/memory-and-spans/
• Async Best Practices: https://learn.microsoft.com/en-us/archive/msdn-magazine/2013/march/async-await-best-practices-in-asynchronous-programming
• .NET Performance Tips: https://learn.microsoft.com/en-us/dotnet/framework/performance/