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C#, despite being a well-designed language with strong typing and modern features, still has common anti-patterns that can lead to bugs, performance issues, and maintenance problems. Here are the most important anti-patterns to avoid when writing C# code.
// Anti-pattern: Using exceptions for control flow
public bool IsValidNumber(string input)
{
    try
    {
        int.Parse(input);
        return true;
    }
    catch (FormatException)
    {
        return false;
    }
}

// Better approach: Use TryParse
public bool IsValidNumber(string input)
{
    return int.TryParse(input, out _);
}
Exceptions should be used for exceptional conditions, not for normal control flow. Use methods like TryParse that are specifically designed for validation.
// Anti-pattern: Not disposing IDisposable objects
public void ReadFile(string path)
{
    FileStream fs = new FileStream(path, FileMode.Open);
    // Read from file
    // fs is never disposed if an exception occurs
}

// Better approach: Use using statement
public void ReadFile(string path)
{
    using (FileStream fs = new FileStream(path, FileMode.Open))
    {
        // Read from file
    } // fs is automatically disposed here
}

// Even better in C# 8+: Using declaration
public void ReadFile(string path)
{
    using FileStream fs = new FileStream(path, FileMode.Open);
    // Read from file
    // fs is disposed when the method exits
}
Always dispose IDisposable objects to release unmanaged resources. The using statement ensures proper disposal even if exceptions occur.
// Anti-pattern: Excessive null checks
public string GetCityName(Person person)
{
    if (person != null)
    {
        if (person.Address != null)
        {
            if (person.Address.City != null)
            {
                return person.Address.City.Name;
            }
        }
    }
    return "Unknown";
}

// Better approach in C# 6+: Null conditional operator
public string GetCityName(Person person)
{
    return person?.Address?.City?.Name ?? "Unknown";
}
Excessive null checks lead to deeply nested code. Use the null conditional operator (?.) and null coalescing operator (??) for cleaner code.
// Anti-pattern: Blocking on async code
public string GetDataSync()
{
    // Blocking the thread, can lead to deadlocks
    return GetDataAsync().Result;
}

// Anti-pattern: async void
public async void ProcessDataFireAndForget() // Can't be awaited, exceptions are lost
{
    await Task.Delay(1000);
    throw new Exception("This exception is lost");
}

// Better approach: Proper async/await
public async Task<string> GetDataAsync()
{
    // Asynchronous code
    return await httpClient.GetStringAsync("https://example.com");
}

// Caller awaits the task
public async Task ProcessDataAsync()
{
    string data = await GetDataAsync();
    // Process data
}
Avoid blocking on async code with .Result or .Wait() as it can lead to deadlocks. Also avoid async void except for event handlers as exceptions can’t be caught.
// Anti-pattern: Public fields
public class Person
{
    public string Name; // Public field
    public int Age;     // Public field
}

// Better approach: Properties
public class Person
{
    public string Name { get; set; } // Property
    public int Age { get; set; }     // Property
    
    // With validation
    private int _age;
    public int ValidatedAge
    {
        get => _age;
        set
        {
            if (value < 0)
                throw new ArgumentException("Age cannot be negative");
            _age = value;
        }
    }
}
Public fields break encapsulation. Use properties to encapsulate fields, allowing for validation, lazy loading, and change notification.
// Anti-pattern: Manual iteration and filtering
public List<Person> GetAdults(List<Person> people)
{
    List<Person> adults = new List<Person>();
    foreach (var person in people)
    {
        if (person.Age >= 18)
        {
            adults.Add(person);
        }
    }
    return adults;
}

// Better approach: Use LINQ
public List<Person> GetAdults(List<Person> people)
{
    return people.Where(p => p.Age >= 18).ToList();
}

// Even more complex operations become simple
public List<string> GetAdultNames(List<Person> people)
{
    return people
        .Where(p => p.Age >= 18)
        .OrderBy(p => p.LastName)
        .ThenBy(p => p.FirstName)
        .Select(p => $"{p.FirstName} {p.LastName}")
        .ToList();
}
LINQ provides a concise, readable way to query and transform data. Use it for filtering, projecting, and aggregating data.
// Anti-pattern: Storing passwords as strings
public class User
{
    public string Username { get; set; }
    public string Password { get; set; } // Plain text password as string
}

// Better approach: Use SecureString and proper hashing
using System.Security;

public class User
{
    public string Username { get; set; }
    private byte[] _passwordHash;
    private byte[] _passwordSalt;
    
    public void SetPassword(SecureString password)
    {
        // Convert SecureString to byte[] safely and generate salt
        _passwordSalt = GenerateSalt();
        _passwordHash = HashPassword(password, _passwordSalt);
    }
    
    public bool VerifyPassword(SecureString password)
    {
        byte[] hash = HashPassword(password, _passwordSalt);
        return CompareHash(_passwordHash, hash);
    }
}
Strings are immutable and can’t be securely cleared from memory. Use SecureString for sensitive data in memory and proper hashing for storage.
// Anti-pattern: Not using modern C# features
public class Person
{
    private string _firstName;
    private string _lastName;
    
    public Person(string firstName, string lastName)
    {
        _firstName = firstName;
        _lastName = lastName;
    }
    
    public string GetFullName()
    {
        return _firstName + " " + _lastName;
    }
    
    public override bool Equals(object obj)
    {
        if (obj == null || GetType() != obj.GetType())
            return false;
        
        Person other = (Person)obj;
        return _firstName == other._firstName && _lastName == other._lastName;
    }
    
    public override int GetHashCode()
    {
        return _firstName.GetHashCode() ^ _lastName.GetHashCode();
    }
}

// Better approach: Use modern C# features
public record Person(string FirstName, string LastName)
{
    public string FullName => $"{FirstName} {LastName}";
}
Modern C# provides many features like records, pattern matching, and expression-bodied members that can make code more concise and readable.
// Anti-pattern: Magic strings and numbers
public void ProcessOrder(Order order)
{
    if (order.Status == "completed")
    {
        // Process completed order
    }
    else if (order.Status == "pending")
    {
        // Process pending order
    }
    
    if (order.Amount > 1000) // Magic number
    {
        ApplyDiscount(order, 0.1); // Another magic number
    }
}

// Better approach: Use constants or enums
public enum OrderStatus
{
    Pending,
    Processing,
    Completed,
    Cancelled
}

public static class Constants
{
    public const decimal LargeOrderThreshold = 1000m;
    public const decimal LargeOrderDiscountRate = 0.1m;
}

public void ProcessOrder(Order order)
{
    if (order.Status == OrderStatus.Completed)
    {
        // Process completed order
    }
    else if (order.Status == OrderStatus.Pending)
    {
        // Process pending order
    }
    
    if (order.Amount > Constants.LargeOrderThreshold)
    {
        ApplyDiscount(order, Constants.LargeOrderDiscountRate);
    }
}
Magic strings and numbers make code hard to maintain and understand. Use constants, enums, or static readonly fields to give meaning to these values.
// Anti-pattern: Hardcoded dependencies
public class OrderService
{
    private readonly CustomerRepository _customerRepository = new CustomerRepository();
    private readonly EmailService _emailService = new EmailService();
    
    public void PlaceOrder(Order order)
    {
        var customer = _customerRepository.GetById(order.CustomerId);
        // Process order
        _emailService.SendOrderConfirmation(customer.Email, order);
    }
}

// Better approach: Dependency injection
public class OrderService
{
    private readonly ICustomerRepository _customerRepository;
    private readonly IEmailService _emailService;
    
    public OrderService(ICustomerRepository customerRepository, IEmailService emailService)
    {
        _customerRepository = customerRepository;
        _emailService = emailService;
    }
    
    public void PlaceOrder(Order order)
    {
        var customer = _customerRepository.GetById(order.CustomerId);
        // Process order
        _emailService.SendOrderConfirmation(customer.Email, order);
    }
}
Hardcoded dependencies make code hard to test and maintain. Use dependency injection to provide dependencies from outside the class.
// Anti-pattern: Returning concrete collection types
public List<Customer> GetCustomers()
{
    // Implementation
    return customers;
}

// Better approach: Return IEnumerable<T> for read-only access
public IEnumerable<Customer> GetCustomers()
{
    // Implementation
    return customers;
}
Returning IEnumerable<T> instead of concrete collection types gives you more flexibility to change the implementation and prevents callers from modifying the collection.
// Anti-pattern: Poor exception handling
public void ProcessFile(string path)
{
    try
    {
        // Process file
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message); // Just writing to console
        // Swallowing the exception
    }
}

// Better approach: Proper exception handling
public void ProcessFile(string path)
{
    try
    {
        // Process file
    }
    catch (FileNotFoundException ex)
    {
        _logger.LogError(ex, $"File not found: {path}");
        throw new BusinessException("The specified file could not be found.", ex);
    }
    catch (UnauthorizedAccessException ex)
    {
        _logger.LogError(ex, $"Access denied to file: {path}");
        throw new BusinessException("You don't have permission to access this file.", ex);
    }
    catch (Exception ex)
    {
        _logger.LogError(ex, $"Unexpected error processing file: {path}");
        throw; // Rethrow to preserve stack trace
    }
}
Proper exception handling includes catching specific exceptions, logging with context, and either handling the exception appropriately or rethrowing it.
// Anti-pattern: Not using object initializers
public Person CreatePerson()
{
    Person person = new Person();
    person.FirstName = "John";
    person.LastName = "Doe";
    person.Age = 30;
    return person;
}

// Better approach: Use object initializers
public Person CreatePerson()
{
    return new Person
    {
        FirstName = "John",
        LastName = "Doe",
        Age = 30
    };
}
Object initializers make code more concise and readable when creating and initializing objects.
// Anti-pattern: Using wrong collection types
public void ProcessItems()
{
    // Using List<T> when you only need to enumerate
    List<int> items = GetItems();
    foreach (var item in items)
    {
        // Process item
    }
    
    // Using List<T> for lookups
    List<Customer> customers = GetCustomers();
    Customer customer = customers.FirstOrDefault(c => c.Id == customerId); // O(n) operation
}

// Better approach: Use appropriate collection types
public void ProcessItems()
{
    // Use IEnumerable<T> when you only need to enumerate
    IEnumerable<int> items = GetItems();
    foreach (var item in items)
    {
        // Process item
    }
    
    // Use Dictionary<TKey, TValue> for lookups
    Dictionary<int, Customer> customerDict = GetCustomers().ToDictionary(c => c.Id);
    Customer customer = customerDict.TryGetValue(customerId, out var c) ? c : null; // O(1) operation
}
Choose the appropriate collection type based on how you’ll use it. Use IEnumerable<T> for simple iteration, List<T> when you need to modify the collection, Dictionary<TKey, TValue> for lookups, etc.
// Anti-pattern: Not using nullable reference types (C# 8+)
// In a project without nullable reference types enabled
public string GetUpperCase(string text)
{
    return text.ToUpper(); // Potential NullReferenceException
}

// Better approach: Enable nullable reference types
// In .csproj: <Nullable>enable</Nullable>

// Non-nullable parameter (compiler enforced)
public string GetUpperCase(string text)
{
    return text.ToUpper(); // Compiler ensures text is not null
}

// Nullable parameter (requires null check)
public string? GetUpperCaseOrNull(string? text)
{
    return text?.ToUpper(); // Null-conditional operator handles null
}
In C# 8+, enable nullable reference types to catch potential null reference exceptions at compile time rather than runtime.
// Anti-pattern: Verbose property and method definitions
public class Person
{
    private string _firstName;
    private string _lastName;
    
    public string FirstName
    {
        get { return _firstName; }
        set { _firstName = value; }
    }
    
    public string FullName
    {
        get
        {
            return $"{_firstName} {_lastName}";
        }
    }
    
    public string GetGreeting()
    {
        return $"Hello, {FullName}!";
    }
}

// Better approach: Use expression-bodied members
public class Person
{
    private string _firstName;
    private string _lastName;
    
    public string FirstName
    {
        get => _firstName;
        set => _firstName = value;
    }
    
    public string FullName => $"{_firstName} {_lastName}";
    
    public string GetGreeting() => $"Hello, {FullName}!";
}
Expression-bodied members make simple properties and methods more concise and readable.
// Anti-pattern: Not using pattern matching
public string Describe(object obj)
{
    if (obj is int)
    {
        int number = (int)obj;
        return $"Integer: {number}";
    }
    else if (obj is string)
    {
        string text = (string)obj;
        return $"String: {text}";
    }
    else if (obj is Person)
    {
        Person person = (Person)obj;
        return $"Person: {person.FullName}";
    }
    else
    {
        return "Unknown";
    }
}

// Better approach: Use pattern matching
public string Describe(object obj)
{
    return obj switch
    {
        int number => $"Integer: {number}",
        string text => $"String: {text}",
        Person { Age: >= 18 } person => $"Adult: {person.FullName}",
        Person person => $"Minor: {person.FullName}",
        _ => "Unknown"
    };
}
Pattern matching (introduced in C# 7 and enhanced in later versions) provides a more concise and powerful way to check types and extract values.
// Anti-pattern: Exposing mutable collections
public class Customer
{
    public List<Order> Orders { get; set; } = new List<Order>();
}

// Client can modify the collection directly
customer.Orders.Clear(); // Unexpected side effect

// Better approach: Return read-only collections
public class Customer
{
    private readonly List<Order> _orders = new List<Order>();
    
    public IReadOnlyCollection<Order> Orders => _orders.AsReadOnly();
    
    public void AddOrder(Order order)
    {
        _orders.Add(order);
    }
    
    public void RemoveOrder(Order order)
    {
        _orders.Remove(order);
    }
}
Exposing mutable collections as public properties breaks encapsulation. Return read-only collections and provide methods to modify the collection.
// Anti-pattern: Not using tuple deconstruction
public (string, int) GetPersonInfo()
{
    return ("John Doe", 30);
}

public void ProcessPerson()
{
    var info = GetPersonInfo();
    string name = info.Item1;
    int age = info.Item2;
    // Process name and age
}

// Better approach: Use tuple deconstruction
public (string Name, int Age) GetPersonInfo()
{
    return ("John Doe", 30);
}

public void ProcessPerson()
{
    var (name, age) = GetPersonInfo();
    // Or
    (string name, int age) = GetPersonInfo();
    // Process name and age
}
Tuple deconstruction makes working with tuples more readable and intuitive.