afonsoft/dotnet-best-practices

.NET Best Practices Skill

When to Use This Skill

Use this skill when you need to:

• Review .NET code for adherence to best practices
• Implement new features following established patterns
• Refactor existing code to improve quality
• Ensure production-ready standards are met
• Validate architectural decisions

Prerequisites

• .NET 6+ project
• Familiarity with C# modern features
• Understanding of SOLID principles
• Basic knowledge of Entity Framework Core

Step-by-Step Workflows

1. Code Review and Validation

Input: C# code files or project structure
Output: Detailed analysis with recommendations

Please review this .NET code for best practices compliance:

1. Check SOLID principles adherence
2. Verify modern C# feature usage
3. Validate error handling patterns
4. Assess performance considerations
5. Review security practices
6. Evaluate testability

2. New Feature Implementation

Input: Feature requirements and project context
Output: Complete implementation following best practices

Implement [feature name] following .NET best practices:

1. Use appropriate design patterns
2. Apply SOLID principles
3. Include proper error handling
4. Add comprehensive documentation
5. Consider performance implications
6. Ensure testability

3. Refactoring Guidance

Input: Existing code that needs improvement
Output: Refactored code with explanations

Refactor this code to follow .NET best practices:

1. Identify code smells and anti-patterns
2. Apply appropriate design patterns
3. Improve readability and maintainability
4. Optimize performance where applicable
5. Enhance error handling
6. Add missing documentation

Core Principles

SOLID Principles

1. Single Responsibility Principle (SRP)

   • Each class should have one reason to change
   • Methods should do one thing well
   • Keep classes focused and cohesive

2. Open/Closed Principle (OCP)

   • Open for extension, closed for modification
   • Use interfaces and abstractions
   • Leverage dependency injection

3. Liskov Substitution Principle (LSP)

   • Derived classes must be substitutable for base classes
   • Don't violate base class contracts
   • Maintain behavioral compatibility

4. Interface Segregation Principle (ISP)

   • Clients shouldn't depend on unused interfaces
   • Create focused, role-specific interfaces
   • Avoid fat interfaces

5. Dependency Inversion Principle (DIP)

   • Depend on abstractions, not concretions
   • Use dependency injection containers
   • Invert dependencies using interfaces

Modern C# Features

1. Null Safety

   // Use nullable reference types
   #nullable enable
   
   // Null checks
   ArgumentNullException.ThrowIfNull(value);
   
   // Null-coalescing operators
   string result = input ?? defaultValue;
   string? nullableResult = input?.ToString();
   

2. Pattern Matching

   // Switch expressions
   string result = obj switch
   {
       string s => $"String: {s}",
       int i => $"Integer: {i}",
       null => "Null",
       _ => "Unknown"
   };
   
   // Property patterns
   if (person is { Age: >= 18 })
   {
       // Adult logic
   }
   

3. Records and Init-Only Properties

   // Immutable records
   public record Person(string Name, int Age);
   
   // Init-only properties
   public class Product
   {
       public string Name { get; init; }
       public decimal Price { get; init; }
   }
   

4. Async/Await Best Practices

   // ConfigureAwait for library code
   var result = await SomeAsyncMethod().ConfigureAwait(false);
   
   // Async all the way down
   public async Task<string> GetDataAsync()
   {
       var data = await _service.GetDataAsync();
       return ProcessData(data);
   }
   

Code Quality Standards

Naming Conventions

// Classes: PascalCase
public class CustomerService { }

// Methods: PascalCase
public async Task<Customer> GetCustomerAsync(int id) { }

// Properties: PascalCase
public string CustomerName { get; set; }

// Fields: camelCase (private), _camelCase (private readonly)
private readonly ILogger<CustomerService> _logger;
private string _customerName;

// Constants: PascalCase
public const int MaxRetries = 3;

// Interfaces: PascalCase with I prefix
public interface IRepository<T> { }

// Enums: PascalCase
public enum OrderStatus { Pending, Processing, Shipped }

Error Handling

// Specific exceptions
throw new InvalidOperationException("Invalid operation state");

// Guard clauses
public void ProcessOrder(Order order)
{
    ArgumentNullException.ThrowIfNull(order);
    
    if (order.Items.Count == 0)
        throw new ArgumentException("Order cannot be empty");
    
    // Process order
}

// Try-catch with specific exceptions
try
{
    await _orderProcessor.ProcessAsync(order);
}
catch (OrderValidationException ex)
{
    _logger.LogError(ex, "Order validation failed");
    throw;
}
catch (Exception ex)
{
    _logger.LogError(ex, "Unexpected error processing order");
    throw new OrderProcessingException("Failed to process order", ex);
}

Dependency Injection

// Constructor injection
public class OrderService
{
    private readonly IOrderRepository _repository;
    private readonly ILogger<OrderService> _logger;
    
    public OrderService(IOrderRepository repository, ILogger<OrderService> logger)
    {
        _repository = repository;
        _logger = logger;
    }
}

// Service registration
public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddOrderServices(this IServiceCollection services)
    {
        services.AddScoped<IOrderRepository, OrderRepository>();
        services.AddScoped<IOrderService, OrderService>();
        
        return services;
    }
}

Performance Guidelines

Entity Framework Core

// Use AsNoTracking for read-only queries
var customers = await _context.Customers
    .AsNoTracking()
    .Where(c => c.IsActive)
    .ToListAsync();

// Project only needed columns
var customerDtos = await _context.Customers
    .Where(c => c.IsActive)
    .Select(c => new CustomerDto
    {
        Id = c.Id,
        Name = c.Name,
        Email = c.Email
    })
    .ToListAsync();

// Use proper indexing
public class Customer
{
    [Key]
    public int Id { get; set; }
    
    [Indexed]
    public string Email { get; set; }
    
    public string Name { get; set; }
}

Memory Management

// Use using statements for IDisposable
using var scope = _serviceProvider.CreateScope();
var service = scope.ServiceProvider.GetRequiredService<IMyService>();

// Avoid memory leaks
public class ImageProcessor : IDisposable
{
    private readonly MemoryStream _stream;
    
    public ImageProcessor()
    {
        _stream = new MemoryStream();
    }
    
    public void Dispose()
    {
        _stream?.Dispose();
    }
}

Security Best Practices

Input Validation

// Validate input parameters
public class CreateOrderRequest
{
    [Required]
    [StringLength(100, MinimumLength = 3)]
    public string CustomerName { get; set; }
    
    [Range(0.01, 10000.00)]
    public decimal Amount { get; set; }
    
    [EmailAddress]
    public string Email { get; set; }
}

Authentication and Authorization

// Use claims-based authorization
[Authorize]
[ApiController]
public class OrdersController : ControllerBase
{
    [HttpGet("{id}")]
    [Authorize(Policy = "CanReadOrders")]
    public async Task<ActionResult<Order>> GetOrder(int id)
    {
        var userId = User.FindFirstValue(ClaimTypes.NameIdentifier);
        var order = await _orderService.GetOrderAsync(id, userId);
        
        if (order == null)
            return NotFound();
            
        return Ok(order);
    }
}

Testing Guidelines

Unit Testing with xUnit

public class OrderServiceTests
{
    private readonly Mock<IOrderRepository> _repositoryMock;
    private readonly Mock<ILogger<OrderService>> _loggerMock;
    private readonly OrderService _service;
    
    public OrderServiceTests()
    {
        _repositoryMock = new Mock<IOrderRepository>();
        _loggerMock = new Mock<ILogger<OrderService>>();
        _service = new OrderService(_repositoryMock.Object, _loggerMock.Object);
    }
    
    [Fact]
    public async Task CreateOrder_ValidOrder_ReturnsOrderId()
    {
        // Arrange
        var request = new CreateOrderRequest
        {
            CustomerName = "John Doe",
            Amount = 100.00m
        };
        
        var expectedOrder = new Order { Id = 1, CustomerName = request.CustomerName };
        _repositoryMock.Setup(r => r.AddAsync(It.IsAny<Order>()))
                      .ReturnsAsync(expectedOrder);
        
        // Act
        var result = await _service.CreateOrderAsync(request);
        
        // Assert
        result.Should().Be(1);
        _repositoryMock.Verify(r => r.AddAsync(It.IsAny<Order>()), Times.Once);
    }
}

Troubleshooting

Common Issues and Solutions

1. Null Reference Exceptions

   • Enable nullable reference types
   • Add proper null checks
   • Use null-coalescing operators

2. Performance Issues

   • Profile with Application Insights
   • Check for N+1 queries in EF Core
   • Optimize LINQ queries

3. Memory Leaks

   • Ensure IDisposable objects are properly disposed
   • Check for event handler subscriptions
   • Monitor with memory profilers

4. Dependency Injection Issues

   • Verify service registration
   • Check for circular dependencies
   • Use proper lifetime scopes

References

• .NET Coding Conventions
• Entity Framework Core Performance
• Dependency Injection in .NET
• SOLID Principles

Examples

Complete Service Example

public class OrderService : IOrderService
{
    private readonly IOrderRepository _repository;
    private readonly ILogger<OrderService> _logger;
    private readonly IOrderValidator _validator;
    
    public OrderService(
        IOrderRepository repository,
        ILogger<OrderService> logger,
        IOrderValidator validator)
    {
        _repository = repository ?? throw new ArgumentNullException(nameof(repository));
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
        _validator = validator ?? throw new ArgumentNullException(nameof(validator));
    }
    
    public async Task<int> CreateOrderAsync(CreateOrderRequest request)
    {
        ArgumentNullException.ThrowIfNull(request);
        
        // Validate request
        var validationResult = await _validator.ValidateAsync(request);
        if (!validationResult.IsValid)
        {
            throw new ValidationException(validationResult.Errors);
        }
        
        // Create order
        var order = new Order
        {
            CustomerName = request.CustomerName,
            Amount = request.Amount,
            CreatedAt = DateTime.UtcNow,
            Status = OrderStatus.Pending
        };
        
        try
        {
            var orderId = await _repository.AddAsync(order);
            _logger.LogInformation("Order {OrderId} created successfully", orderId);
            return orderId;
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Failed to create order");
            throw new OrderCreationException("Failed to create order", ex);
        }
    }
    
    public async Task<Order?> GetOrderAsync(int id, string userId)
    {
        if (id <= 0)
            throw new ArgumentException("Invalid order ID", nameof(id));
            
        if (string.IsNullOrWhiteSpace(userId))
            throw new ArgumentException("User ID is required", nameof(userId));
        
        var order = await _repository.GetByIdAsync(id);
        
        if (order == null || order.UserId != userId)
            return null;
            
        return order;
    }
}

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This skill provides comprehensive guidance for .NET development best practices. Use it to ensure your code meets industry standards and follows modern patterns.