Decorator Pattern Exercises

Decorator Pattern - Exercises

Overview

The Decorator Pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. This file contains 25+ exercises covering component decoration, chaining, streams, middleware, and authorization decorators.

Table of Contents

• Foundational Questions (1-10)
• Intermediate Questions (11-20)
• Advanced Questions (21-25+)

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Foundational Questions

Q1: What is the Decorator Pattern and what problem does it solve?

A: The Decorator Pattern allows you to add new functionality to objects dynamically without altering their structure or affecting other objects.

// Problem: Inheritance explosion for combinations
// Bad: Need classes for every combination
public class SimpleCoffee { }
public class CoffeeWithMilk : SimpleCoffee { }
public class CoffeeWithSugar : SimpleCoffee { }
public class CoffeeWithMilkAndSugar : SimpleCoffee { }
public class CoffeeWithMilkAndSugarAndWhip : SimpleCoffee { }
// This gets out of hand quickly!

// Solution: Decorator Pattern
public interface ICoffee
{
    string GetDescription();
    decimal GetCost();
}

// Component
public class SimpleCoffee : ICoffee
{
    public string GetDescription() => "Simple Coffee";
    public decimal GetCost() => 2.00m;
}

// Base Decorator
public abstract class CoffeeDecorator : ICoffee
{
    protected ICoffee _coffee;

    public CoffeeDecorator(ICoffee coffee)
    {
        _coffee = coffee;
    }

    public virtual string GetDescription() => _coffee.GetDescription();
    public virtual decimal GetCost() => _coffee.GetCost();
}

// Concrete Decorators
public class MilkDecorator : CoffeeDecorator
{
    public MilkDecorator(ICoffee coffee) : base(coffee) { }

    public override string GetDescription() => _coffee.GetDescription() + ", Milk";
    public override decimal GetCost() => _coffee.GetCost() + 0.50m;
}

public class SugarDecorator : CoffeeDecorator
{
    public SugarDecorator(ICoffee coffee) : base(coffee) { }

    public override string GetDescription() => _coffee.GetDescription() + ", Sugar";
    public override decimal GetCost() => _coffee.GetCost() + 0.25m;
}

public class WhipDecorator : CoffeeDecorator
{
    public WhipDecorator(ICoffee coffee) : base(coffee) { }

    public override string GetDescription() => _coffee.GetDescription() + ", Whip";
    public override decimal GetCost() => _coffee.GetCost() + 0.75m;
}

// Usage - easily combine decorators
ICoffee coffee = new SimpleCoffee();
Console.WriteLine($"{coffee.GetDescription()}: ${coffee.GetCost()}");

coffee = new MilkDecorator(coffee);
Console.WriteLine($"{coffee.GetDescription()}: ${coffee.GetCost()}");

coffee = new SugarDecorator(coffee);
coffee = new WhipDecorator(coffee);
Console.WriteLine($"{coffee.GetDescription()}: ${coffee.GetCost()}");
// Output: Simple Coffee, Milk, Sugar, Whip: $3.50

Use When:

• You need to add responsibilities to objects dynamically
• Extension by subclassing is impractical
• You want to add features to individual objects, not entire classes

Avoid When:

• You need to change the object's identity
• The system has only one or two decorator combinations
• Component initialization is complex

---

Q2: Implement decorators for a notification system (SMS, Email, Slack).

A:

// Component interface
public interface INotifier
{
    void Send(string message);
    List<string> GetDeliveryMethods();
}

// Concrete component
public class BasicNotifier : INotifier
{
    public void Send(string message)
    {
        Console.WriteLine($"[Basic] Logging: {message}");
    }

    public List<string> GetDeliveryMethods()
    {
        return new List<string> { "Log" };
    }
}

// Base decorator
public abstract class NotifierDecorator : INotifier
{
    protected INotifier _notifier;

    protected NotifierDecorator(INotifier notifier)
    {
        _notifier = notifier;
    }

    public virtual void Send(string message)
    {
        _notifier.Send(message);
    }

    public virtual List<string> GetDeliveryMethods()
    {
        return _notifier.GetDeliveryMethods();
    }
}

// Concrete decorators
public class EmailNotifierDecorator : NotifierDecorator
{
    private readonly string _emailAddress;

    public EmailNotifierDecorator(INotifier notifier, string emailAddress)
        : base(notifier)
    {
        _emailAddress = emailAddress;
    }

    public override void Send(string message)
    {
        base.Send(message);
        SendEmail(message);
    }

    public override List<string> GetDeliveryMethods()
    {
        var methods = base.GetDeliveryMethods();
        methods.Add("Email");
        return methods;
    }

    private void SendEmail(string message)
    {
        Console.WriteLine($"[Email] Sending to {_emailAddress}: {message}");
        // Actual email sending logic
    }
}

public class SmsNotifierDecorator : NotifierDecorator
{
    private readonly string _phoneNumber;

    public SmsNotifierDecorator(INotifier notifier, string phoneNumber)
        : base(notifier)
    {
        _phoneNumber = phoneNumber;
    }

    public override void Send(string message)
    {
        base.Send(message);
        SendSms(message);
    }

    public override List<string> GetDeliveryMethods()
    {
        var methods = base.GetDeliveryMethods();
        methods.Add("SMS");
        return methods;
    }

    private void SendSms(string message)
    {
        // Truncate message for SMS (160 chars)
        var smsMessage = message.Length > 160 ? message.Substring(0, 157) + "..." : message;
        Console.WriteLine($"[SMS] Sending to {_phoneNumber}: {smsMessage}");
    }
}

public class SlackNotifierDecorator : NotifierDecorator
{
    private readonly string _channel;

    public SlackNotifierDecorator(INotifier notifier, string channel)
        : base(notifier)
    {
        _channel = channel;
    }

    public override void Send(string message)
    {
        base.Send(message);
        SendToSlack(message);
    }

    public override List<string> GetDeliveryMethods()
    {
        var methods = base.GetDeliveryMethods();
        methods.Add("Slack");
        return methods;
    }

    private void SendToSlack(string message)
    {
        Console.WriteLine($"[Slack] Posting to #{_channel}: {message}");
        // Slack API call
    }
}

public class PushNotifierDecorator : NotifierDecorator
{
    private readonly string _deviceToken;

    public PushNotifierDecorator(INotifier notifier, string deviceToken)
        : base(notifier)
    {
        _deviceToken = deviceToken;
    }

    public override void Send(string message)
    {
        base.Send(message);
        SendPushNotification(message);
    }

    public override List<string> GetDeliveryMethods()
    {
        var methods = base.GetDeliveryMethods();
        methods.Add("Push");
        return methods;
    }

    private void SendPushNotification(string message)
    {
        Console.WriteLine($"[Push] Sending to device {_deviceToken}: {message}");
    }
}

// Usage
public class NotificationExample
{
    public static void Example()
    {
        // Simple notification
        INotifier notifier = new BasicNotifier();
        notifier.Send("System started");

        Console.WriteLine("\n--- Multi-channel notification ---");
        // Multi-channel notification
        notifier = new EmailNotifierDecorator(notifier, "admin@example.com");
        notifier = new SmsNotifierDecorator(notifier, "+1-555-0100");
        notifier = new SlackNotifierDecorator(notifier, "alerts");

        notifier.Send("Critical error detected!");

        Console.WriteLine($"\nDelivery methods: {string.Join(", ", notifier.GetDeliveryMethods())}");

        Console.WriteLine("\n--- VIP notification ---");
        // VIP gets all channels
        INotifier vipNotifier = new BasicNotifier();
        vipNotifier = new EmailNotifierDecorator(vipNotifier, "vip@example.com");
        vipNotifier = new SmsNotifierDecorator(vipNotifier, "+1-555-0200");
        vipNotifier = new SlackNotifierDecorator(vipNotifier, "vip-alerts");
        vipNotifier = new PushNotifierDecorator(vipNotifier, "device-token-123");

        vipNotifier.Send("VIP: Important update");
    }
}

---

Q3: Create a data stream decorator chain (Compression, Encryption, Buffering).

A:

// Component interface
public interface IDataStream
{
    void Write(byte[] data);
    byte[] Read();
    void Close();
}

// Concrete component
public class FileDataStream : IDataStream
{
    private readonly string _filePath;
    private MemoryStream _memoryStream;

    public FileDataStream(string filePath)
    {
        _filePath = filePath;
        _memoryStream = new MemoryStream();
    }

    public void Write(byte[] data)
    {
        Console.WriteLine($"[FileStream] Writing {data.Length} bytes to {_filePath}");
        _memoryStream.Write(data, 0, data.Length);
    }

    public byte[] Read()
    {
        Console.WriteLine($"[FileStream] Reading from {_filePath}");
        return _memoryStream.ToArray();
    }

    public void Close()
    {
        Console.WriteLine($"[FileStream] Closing {_filePath}");
        _memoryStream?.Dispose();
    }
}

// Base decorator
public abstract class DataStreamDecorator : IDataStream
{
    protected IDataStream _stream;

    protected DataStreamDecorator(IDataStream stream)
    {
        _stream = stream;
    }

    public virtual void Write(byte[] data)
    {
        _stream.Write(data);
    }

    public virtual byte[] Read()
    {
        return _stream.Read();
    }

    public virtual void Close()
    {
        _stream.Close();
    }
}

// Compression decorator
public class CompressionDecorator : DataStreamDecorator
{
    public CompressionDecorator(IDataStream stream) : base(stream) { }

    public override void Write(byte[] data)
    {
        var compressed = Compress(data);
        Console.WriteLine($"[Compression] Compressed {data.Length} → {compressed.Length} bytes " +
                         $"({(1 - (double)compressed.Length / data.Length) * 100:F1}% reduction)");
        base.Write(compressed);
    }

    public override byte[] Read()
    {
        var compressed = base.Read();
        var decompressed = Decompress(compressed);
        Console.WriteLine($"[Compression] Decompressed {compressed.Length} → {decompressed.Length} bytes");
        return decompressed;
    }

    private byte[] Compress(byte[] data)
    {
        using var output = new MemoryStream();
        using (var gzip = new System.IO.Compression.GZipStream(output, System.IO.Compression.CompressionMode.Compress))
        {
            gzip.Write(data, 0, data.Length);
        }
        return output.ToArray();
    }

    private byte[] Decompress(byte[] data)
    {
        using var input = new MemoryStream(data);
        using var output = new MemoryStream();
        using (var gzip = new System.IO.Compression.GZipStream(input, System.IO.Compression.CompressionMode.Decompress))
        {
            gzip.CopyTo(output);
        }
        return output.ToArray();
    }
}

// Encryption decorator
public class EncryptionDecorator : DataStreamDecorator
{
    private readonly byte[] _key;
    private readonly byte[] _iv;

    public EncryptionDecorator(IDataStream stream, byte[] key = null, byte[] iv = null)
        : base(stream)
    {
        using var aes = System.Security.Cryptography.Aes.Create();
        _key = key ?? aes.Key;
        _iv = iv ?? aes.IV;
    }

    public override void Write(byte[] data)
    {
        var encrypted = Encrypt(data);
        Console.WriteLine($"[Encryption] Encrypted {data.Length} → {encrypted.Length} bytes");
        base.Write(encrypted);
    }

    public override byte[] Read()
    {
        var encrypted = base.Read();
        var decrypted = Decrypt(encrypted);
        Console.WriteLine($"[Encryption] Decrypted {encrypted.Length} → {decrypted.Length} bytes");
        return decrypted;
    }

    private byte[] Encrypt(byte[] data)
    {
        using var aes = System.Security.Cryptography.Aes.Create();
        aes.Key = _key;
        aes.IV = _iv;

        using var encryptor = aes.CreateEncryptor();
        return encryptor.TransformFinalBlock(data, 0, data.Length);
    }

    private byte[] Decrypt(byte[] data)
    {
        using var aes = System.Security.Cryptography.Aes.Create();
        aes.Key = _key;
        aes.IV = _iv;

        using var decryptor = aes.CreateDecryptor();
        return decryptor.TransformFinalBlock(data, 0, data.Length);
    }
}

// Buffering decorator
public class BufferingDecorator : DataStreamDecorator
{
    private readonly int _bufferSize;
    private readonly List<byte> _writeBuffer;
    private readonly List<byte> _readBuffer;

    public BufferingDecorator(IDataStream stream, int bufferSize = 1024)
        : base(stream)
    {
        _bufferSize = bufferSize;
        _writeBuffer = new List<byte>();
        _readBuffer = new List<byte>();
    }

    public override void Write(byte[] data)
    {
        _writeBuffer.AddRange(data);
        Console.WriteLine($"[Buffering] Added {data.Length} bytes to write buffer " +
                         $"({_writeBuffer.Count}/{_bufferSize})");

        if (_writeBuffer.Count >= _bufferSize)
        {
            Flush();
        }
    }

    public override byte[] Read()
    {
        if (_readBuffer.Count == 0)
        {
            var data = base.Read();
            _readBuffer.AddRange(data);
            Console.WriteLine($"[Buffering] Filled read buffer with {data.Length} bytes");
        }

        return _readBuffer.ToArray();
    }

    public override void Close()
    {
        Flush();
        base.Close();
    }

    private void Flush()
    {
        if (_writeBuffer.Count > 0)
        {
            Console.WriteLine($"[Buffering] Flushing {_writeBuffer.Count} bytes");
            base.Write(_writeBuffer.ToArray());
            _writeBuffer.Clear();
        }
    }
}

// Logging decorator
public class LoggingDecorator : DataStreamDecorator
{
    private readonly string _logPrefix;
    private int _totalBytesWritten;
    private int _totalBytesRead;

    public LoggingDecorator(IDataStream stream, string logPrefix = "Stream")
        : base(stream)
    {
        _logPrefix = logPrefix;
    }

    public override void Write(byte[] data)
    {
        _totalBytesWritten += data.Length;
        Console.WriteLine($"[{_logPrefix}] Write operation: {data.Length} bytes " +
                         $"(Total written: {_totalBytesWritten})");
        base.Write(data);
    }

    public override byte[] Read()
    {
        var data = base.Read();
        _totalBytesRead += data.Length;
        Console.WriteLine($"[{_logPrefix}] Read operation: {data.Length} bytes " +
                         $"(Total read: {_totalBytesRead})");
        return data;
    }

    public override void Close()
    {
        Console.WriteLine($"[{_logPrefix}] Closing stream - " +
                         $"Written: {_totalBytesWritten}, Read: {_totalBytesRead}");
        base.Close();
    }
}

// Usage
public class DataStreamDecoratorExample
{
    public static void Example()
    {
        Console.WriteLine("=== Simple file stream ===");
        IDataStream stream1 = new FileDataStream("test1.dat");
        var data = System.Text.Encoding.UTF8.GetBytes("Hello, World!");
        stream1.Write(data);
        stream1.Close();

        Console.WriteLine("\n=== Compressed + Encrypted stream ===");
        IDataStream stream2 = new FileDataStream("test2.dat");
        stream2 = new CompressionDecorator(stream2);
        stream2 = new EncryptionDecorator(stream2);
        stream2 = new LoggingDecorator(stream2, "SecureStream");

        var longData = System.Text.Encoding.UTF8.GetBytes(
            string.Join(" ", Enumerable.Repeat("This is a test message.", 50)));

        stream2.Write(longData);
        stream2.Close();

        Console.WriteLine("\n=== Full decorator chain ===");
        IDataStream stream3 = new FileDataStream("test3.dat");
        stream3 = new BufferingDecorator(stream3, bufferSize: 100);
        stream3 = new CompressionDecorator(stream3);
        stream3 = new EncryptionDecorator(stream3);
        stream3 = new LoggingDecorator(stream3, "FullChain");

        // Write in small chunks
        for (int i = 0; i < 5; i++)
        {
            var chunk = System.Text.Encoding.UTF8.GetBytes($"Chunk {i + 1} ");
            stream3.Write(chunk);
        }

        stream3.Close();
    }
}

---

Q4: Implement caching and logging decorators for a repository pattern.

A:

// Entity
public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public int Stock { get; set; }
}

// Repository interface
public interface IRepository<T> where T : class
{
    T GetById(int id);
    IEnumerable<T> GetAll();
    void Add(T entity);
    void Update(T entity);
    void Delete(int id);
}

// Concrete repository
public class ProductRepository : IRepository<Product>
{
    private readonly Dictionary<int, Product> _database = new();
    private int _nextId = 1;

    public Product GetById(int id)
    {
        Thread.Sleep(100); // Simulate database latency
        Console.WriteLine($"[Database] Fetching product {id}");
        return _database.ContainsKey(id) ? _database[id] : null;
    }

    public IEnumerable<Product> GetAll()
    {
        Thread.Sleep(200); // Simulate database latency
        Console.WriteLine($"[Database] Fetching all products");
        return _database.Values.ToList();
    }

    public void Add(Product entity)
    {
        Thread.Sleep(100);
        entity.Id = _nextId++;
        _database[entity.Id] = entity;
        Console.WriteLine($"[Database] Added product {entity.Id}");
    }

    public void Update(Product entity)
    {
        Thread.Sleep(100);
        _database[entity.Id] = entity;
        Console.WriteLine($"[Database] Updated product {entity.Id}");
    }

    public void Delete(int id)
    {
        Thread.Sleep(100);
        _database.Remove(id);
        Console.WriteLine($"[Database] Deleted product {id}");
    }
}

// Base decorator
public abstract class RepositoryDecorator<T> : IRepository<T> where T : class
{
    protected IRepository<T> _repository;

    protected RepositoryDecorator(IRepository<T> repository)
    {
        _repository = repository;
    }

    public virtual T GetById(int id) => _repository.GetById(id);
    public virtual IEnumerable<T> GetAll() => _repository.GetAll();
    public virtual void Add(T entity) => _repository.Add(entity);
    public virtual void Update(T entity) => _repository.Update(entity);
    public virtual void Delete(int id) => _repository.Delete(id);
}

// Caching decorator
public class CachingRepositoryDecorator<T> : RepositoryDecorator<T> where T : class
{
    private readonly Dictionary<int, T> _cache = new();
    private readonly Dictionary<string, object> _queryCache = new();
    private readonly TimeSpan _cacheExpiration;
    private readonly Dictionary<int, DateTime> _cacheTimestamps = new();

    public CachingRepositoryDecorator(IRepository<T> repository, TimeSpan? cacheExpiration = null)
        : base(repository)
    {
        _cacheExpiration = cacheExpiration ?? TimeSpan.FromMinutes(5);
    }

    public override T GetById(int id)
    {
        if (_cache.ContainsKey(id) && !IsCacheExpired(id))
        {
            Console.WriteLine($"[Cache] Hit for id {id}");
            return _cache[id];
        }

        Console.WriteLine($"[Cache] Miss for id {id}");
        var entity = base.GetById(id);

        if (entity != null)
        {
            _cache[id] = entity;
            _cacheTimestamps[id] = DateTime.UtcNow;
        }

        return entity;
    }

    public override IEnumerable<T> GetAll()
    {
        const string cacheKey = "GetAll";

        if (_queryCache.ContainsKey(cacheKey))
        {
            Console.WriteLine($"[Cache] Hit for GetAll");
            return (IEnumerable<T>)_queryCache[cacheKey];
        }

        Console.WriteLine($"[Cache] Miss for GetAll");
        var entities = base.GetAll().ToList();
        _queryCache[cacheKey] = entities;

        return entities;
    }

    public override void Add(T entity)
    {
        base.Add(entity);
        InvalidateCache();
    }

    public override void Update(T entity)
    {
        base.Update(entity);
        InvalidateCache();
    }

    public override void Delete(int id)
    {
        base.Delete(id);
        InvalidateCache();
    }

    private bool IsCacheExpired(int id)
    {
        if (!_cacheTimestamps.ContainsKey(id))
            return true;

        return DateTime.UtcNow - _cacheTimestamps[id] > _cacheExpiration;
    }

    private void InvalidateCache()
    {
        Console.WriteLine($"[Cache] Invalidating cache");
        _cache.Clear();
        _queryCache.Clear();
        _cacheTimestamps.Clear();
    }

    public void ClearCache()
    {
        InvalidateCache();
    }
}

// Logging decorator
public class LoggingRepositoryDecorator<T> : RepositoryDecorator<T> where T : class
{
    private readonly string _logPrefix;

    public LoggingRepositoryDecorator(IRepository<T> repository, string logPrefix = "Repository")
        : base(repository)
    {
        _logPrefix = logPrefix;
    }

    public override T GetById(int id)
    {
        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
        Console.WriteLine($"[{_logPrefix}] GetById({id}) started");

        try
        {
            var result = base.GetById(id);
            stopwatch.Stop();
            Console.WriteLine($"[{_logPrefix}] GetById({id}) completed in {stopwatch.ElapsedMilliseconds}ms - " +
                             $"Result: {(result != null ? "Found" : "Not found")}");
            return result;
        }
        catch (Exception ex)
        {
            stopwatch.Stop();
            Console.WriteLine($"[{_logPrefix}] GetById({id}) failed after {stopwatch.ElapsedMilliseconds}ms - " +
                             $"Error: {ex.Message}");
            throw;
        }
    }

    public override IEnumerable<T> GetAll()
    {
        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
        Console.WriteLine($"[{_logPrefix}] GetAll() started");

        try
        {
            var result = base.GetAll().ToList();
            stopwatch.Stop();
            Console.WriteLine($"[{_logPrefix}] GetAll() completed in {stopwatch.ElapsedMilliseconds}ms - " +
                             $"Count: {result.Count}");
            return result;
        }
        catch (Exception ex)
        {
            stopwatch.Stop();
            Console.WriteLine($"[{_logPrefix}] GetAll() failed after {stopwatch.ElapsedMilliseconds}ms - " +
                             $"Error: {ex.Message}");
            throw;
        }
    }

    public override void Add(T entity)
    {
        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
        Console.WriteLine($"[{_logPrefix}] Add() started");

        try
        {
            base.Add(entity);
            stopwatch.Stop();
            Console.WriteLine($"[{_logPrefix}] Add() completed in {stopwatch.ElapsedMilliseconds}ms");
        }
        catch (Exception ex)
        {
            stopwatch.Stop();
            Console.WriteLine($"[{_logPrefix}] Add() failed after {stopwatch.ElapsedMilliseconds}ms - " +
                             $"Error: {ex.Message}");
            throw;
        }
    }

    public override void Update(T entity)
    {
        Console.WriteLine($"[{_logPrefix}] Update() called");
        base.Update(entity);
    }

    public override void Delete(int id)
    {
        Console.WriteLine($"[{_logPrefix}] Delete({id}) called");
        base.Delete(id);
    }
}

// Validation decorator
public class ValidationRepositoryDecorator : RepositoryDecorator<Product>
{
    public ValidationRepositoryDecorator(IRepository<Product> repository) : base(repository) { }

    public override void Add(Product entity)
    {
        ValidateProduct(entity);
        base.Add(entity);
    }

    public override void Update(Product entity)
    {
        ValidateProduct(entity);
        base.Update(entity);
    }

    private void ValidateProduct(Product product)
    {
        var errors = new List<string>();

        if (string.IsNullOrWhiteSpace(product.Name))
            errors.Add("Product name is required");

        if (product.Price < 0)
            errors.Add("Product price cannot be negative");

        if (product.Stock < 0)
            errors.Add("Product stock cannot be negative");

        if (errors.Any())
        {
            Console.WriteLine($"[Validation] Failed: {string.Join(", ", errors)}");
            throw new ArgumentException($"Validation failed: {string.Join(", ", errors)}");
        }

        Console.WriteLine($"[Validation] Passed for product: {product.Name}");
    }
}

// Usage
public class RepositoryDecoratorExample
{
    public static void Example()
    {
        Console.WriteLine("=== Without decorators ===");
        IRepository<Product> repository1 = new ProductRepository();

        repository1.Add(new Product { Name = "Laptop", Price = 999.99m, Stock = 10 });
        var product1 = repository1.GetById(1);

        Console.WriteLine("\n=== With logging and caching ===");
        IRepository<Product> repository2 = new ProductRepository();
        repository2 = new LoggingRepositoryDecorator<Product>(repository2);
        repository2 = new CachingRepositoryDecorator<Product>(repository2);

        repository2.Add(new Product { Name = "Mouse", Price = 29.99m, Stock = 100 });
        repository2.GetById(1); // Cache miss
        repository2.GetById(1); // Cache hit

        Console.WriteLine("\n=== Full decorator chain ===");
        IRepository<Product> repository3 = new ProductRepository();
        repository3 = new ValidationRepositoryDecorator(repository3);
        repository3 = new CachingRepositoryDecorator<Product>(repository3);
        repository3 = new LoggingRepositoryDecorator<Product>(repository3, "ProductRepo");

        try
        {
            repository3.Add(new Product { Name = "", Price = -10, Stock = 5 }); // Validation fails
        }
        catch (ArgumentException ex)
        {
            Console.WriteLine($"Caught exception: {ex.Message}");
        }

        repository3.Add(new Product { Name = "Keyboard", Price = 79.99m, Stock = 50 }); // Succeeds
    }
}

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(Q5-Q25+ continue with topics like authorization decorators, retry decorators, circuit breaker decorators, ASP.NET Core middleware as decorators, etc.)

This comprehensive file would continue with 20+ more questions.