// ✅ Correct
public async Task<User> GetUserAsync(int id)
{
    return await _repository.GetByIdAsync(id);
}

// ❌ Wrong - blocking
public User GetUser(int id)
{
    return _repository.GetByIdAsync(id).Result;
}

// Enable in .csproj
<Nullable>enable</Nullable>

// Usage
public User? GetUser(int id)  // Can be null
public User GetRequiredUser(int id)  // Never null

// Immutable data objects
public record UserDto(int Id, string Name, string Email);

// With modification
var updated = user with { Name = "New Name" };

var result = user switch
{
    { Role: "Admin" } => "Full access",
    { Role: "User" } => "Limited access",
    null => "No user",
    _ => "Unknown"
};

// ✅ Correct - efficient
var activeUsers = users
    .Where(u => u.IsActive)
    .Select(u => u.Name)
    .ToList();

// ❌ Wrong - N+1
foreach (var user in users)
{
    var orders = await GetOrdersAsync(user.Id);
}

try
{
    await ProcessAsync();
}
catch (NotFoundException ex)
{
    _logger.LogWarning(ex, "Item not found: {Id}", id);
    throw;
}
catch (Exception ex)
{
    _logger.LogError(ex, "Unexpected error");
    throw;
}

// ✅ Correct: Use enums for types
public enum UserStatus
{
    Active,
    Inactive,
    Suspended
}

// ✅ Correct: Use constants for reusable strings
public static class AppConstants
{
    public const string DefaultLanguage = "en-US";
    public const string ApiVersion = "v1";
}

// ✅ Correct: Use IConfiguration or environment variables
public class MyService
{
    private readonly string _apiKey;
    
    public MyService(IConfiguration config)
    {
        _apiKey = config["ApiSettings:Key"] ?? throw new ArgumentNullException("API Key is missing");
    }
}

// ❌ Wrong: Multiple classes in one file
public class User { ... }
public class Order { ... }

// ✅ Correct: Separate files
// User.cs
public class User { ... }

// Order.cs
public class Order { ... }

// ❌ Wrong: Nested classes
public class User 
{
    // Don't do this unless it's a private DTO used ONLY here
    public class UserConfig { ... }
}