Interview Q&A

• In GUI systems, UI components (like buttons or text boxes) can be

considered observers that listen to changes or events (like button clicks) from

a user interface.

Visual Diagram:

+------------------+ +-------------------+

| NewsPublisher |-------| NewsSubscriber |

| (Subject) | | (Observer) |

+------------------+ +-------------------+

| |

| Subscribe/Unsubscribe | Update

v v

+------------------+ +-------------------+

| NewsPublisher | | NewsSubscriber |

Follow:

| (Subject) |-------| (Observer) |

+------------------+ +-------------------+

Conclusion:

The Observer Pattern is a powerful and flexible design pattern for building systems where

objects need to be notified of changes to another object's state. It's particularly useful in

event-driven architectures, where one action (such as publishing an article, changing a stock

price, or a weather update) triggers multiple reactions (notifications to subscribers). The

pattern promotes loose coupling and ensures that your system remains scalable,

maintainable, and responsive to changes in state.

Prototype Pattern: Cloning Complex Objects

Definition:

The Prototype Pattern is a creational design pattern that allows you to create new objects

by copying an existing object, known as the "prototype." This pattern is particularly useful

when the cost of creating an object from scratch is expensive or complex, and you want to

reuse an existing object as the prototype to create new instances.

Use Case:

A typical use case for the Prototype Pattern is cloning complex objects, such as game

characters with various attributes (e.g., health, weapons, inventory), where creating new

objects from scratch can be time-consuming. Instead of rebuilding objects from the ground

up, you can clone an existing object and modify it as necessary.

Code Breakdown:

• The User class represents a user in the chat application. Each user has a

name and a reference to the mediator.

• When a user wants to send a message, they call Send() on the mediator,

which then sends the message to all other registered users.

• Users only interact with the mediator, and not with each other directly.

public class User
{
private readonly string _name;
private readonly IChatMediator _mediator;
public User(string name, IChatMediator mediator)
{
_name = name;
_mediator = mediator;

_mediator.RegisterUser(this); // Register the user with the

mediator

}
public void Send(string message) =>

_mediator.SendMessage(message, this);

public void Receive(string message) =>

Console.WriteLine($"{_name} received: {message}");

}

Answer: The Iterator Pattern provides a consistent and uniform way to access elements of the collection, regardless of the underlying structure of the collection. Real-Time Use Case Examples:

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• The ProductIterator class implements the IIterator<Product>

interface and keeps track of the current position in the

ProductCollection. It knows how to traverse the collection, check if

there’s a next item (HasNext()), and return the next item (Next()).

Answer: The Add class is a non-terminal expression. It takes two other expressions (leftExpression and rightExpression) and computes their sum. Non-terminal expressions represent operations that combine multiple sub-expressions.

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• In the client code, we create a CharacterFactory instance to manage the

characters. We then iterate over the text "Hello World", requesting a

Character object for each character.

• The flyweight objects are reused, reducing memory consumption.

class Program
{

static void Main()

{
var factory = new CharacterFactory();
string text = "Hello World";
foreach (var c in text)
{
var character = factory.GetCharacter(c);

character.Display(0, 0); // Simplified position for this

example

}
}
}

• This is an abstract class that declares the CreateLogger method, which is

intended to return an ILogger instance. The actual instantiation is delegated

to subclasses.

public abstract class LoggerFactory
{
public abstract ILogger CreateLogger();
}

• The CoffeeDecorator class is an abstract class that implements the

ICoffee interface and holds a reference to an ICoffee object. This class

allows us to add additional behavior to the coffee object, but we don't modify

the base class (SimpleCoffee).

public abstract class CoffeeDecorator : ICoffee

protected ICoffee _coffee;

protected CoffeeDecorator(ICoffee coffee) => _coffee = coffee;

public virtual double Cost() => _coffee.Cost();

public virtual string Description() => _coffee.Description();

• The Directory class is a composite node in the tree structure. It can

contain multiple IFileSystemComponent objects (both files and

subdirectories). The ShowInfo() method displays the directory’s name and

recursively calls ShowInfo() on its children (whether they are files or

subdirectories).

• The Document class represents the receiver of the command. It contains the

actual operations for adding and removing text from the document. The

AddText and RemoveText methods manipulate the internal content (a

StringBuilder).

public class Document

Follow:

private readonly StringBuilder _content = new StringBuilder();

public void AddText(string text) => _content.Append(text);

public void RemoveText(string text) =>

_content.Remove(_content.Length - text.Length, text.Length);

public override string ToString() => _content.ToString();

Answer: The LogLevel enum defines the different types of log messages (e.g., Info, Error). public enum LogLevel { Info, Error }

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• The MargheritaPizzaBuilder is a concrete implementation of the

IPizzaBuilder interface. It builds a specific type of pizza (Margherita in

this case) by setting the dough, sauce, and adding toppings. The Build()

method returns the fully constructed pizza.

public class MargheritaPizzaBuilder : IPizzaBuilder
{
private Pizza _pizza = new Pizza();
public void SetDough(string dough) => _pizza.Dough = dough;
public void SetSauce(string sauce) => _pizza.Sauce = sauce;
public void AddTopping(string topping) =>

_pizza.Toppings.Add(topping);

public Pizza Build() => _pizza;
}

• The DrawingAPI1 and DrawingAPI2 are concrete implementations of the

IDrawingAPI interface. These classes implement the specific drawing logic

for rendering circles, but they could use different libraries or methods (e.g.,

OpenGL, DirectX, Canvas API).

public class DrawingAPI1 : IDrawingAPI
{
public void DrawCircle(double x, double y, double radius) =>

Console.WriteLine($"Drawing Circle at ({x}, {y}) with radius

{radius} using API 1.");

}
public class DrawingAPI2 : IDrawingAPI
{
public void DrawCircle(double x, double y, double radius) =>

Console.WriteLine($"Drawing Circle at ({x}, {y}) with radius

{radius} using API 2.");

}

• The Adapter class implements the ITarget interface, and it wraps an

instance of the Adaptee class. It delegates calls from Request() to the

SpecificRequest() method of the Adaptee, making the two interfaces

compatible.

public class Adapter : ITarget

private readonly Adaptee _adaptee;

public Adapter(Adaptee adaptee) => _adaptee = adaptee;

public void Request() => _adaptee.SpecificRequest();

• The IButton and ICheckbox interfaces define the general contract for

products. Each concrete product (e.g., WindowsButton, MacButton) will

implement the Render method to display platform-specific UI elements.

public interface IButton { void Render(); }
public interface ICheckbox { void Render(); }

Answer: other expressions. It leverages the recursive nature of the Interpreter Pattern to evaluate complex expressions.

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Follow:

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

nd undoing commands. When a command is executed, it’s pushed onto the

stack. The Undo() method pops the last executed command and undoes its

ction.

public class TextEditor
{
private readonly Stack<ICommand> _commandHistory = new

Stack<ICommand>();

public void ExecuteCommand(ICommand command)
{

command.Execute();

_commandHistory.Push(command);

}
public void Undo()
{
if (_commandHistory.Count > 0)
{
var command = _commandHistory.Pop();

command.Undo();

}
}
}

ppropriate methods on the builder. It then returns the final product.

public class PizzaDirector
{
private readonly IPizzaBuilder _builder;
public PizzaDirector(IPizzaBuilder builder) => _builder =

builder;

public Pizza ConstructMargheritaPizza()
{

_builder.SetDough("Thin Crust");

_builder.SetSauce("Tomato");

_builder.AddTopping("Mozzarella");

return _builder.Build();
}
}

Answer: daptee adaptee = new Adaptee(); ITarget adapter = new Adapter(adaptee); dapter.Request(); // Outputs: Specific request from daptee. } }

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

Answer: The Product class represents an individual product with a property Name. This is the item that we will iterate over in the collection. public class Product { public string Name { get; } public Product(string name) =&gt; Name = name; }

What interviewers expect

• A clear definition tied to GoF Patterns in Gang of Four Patterns projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production Gang of Four Patterns application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in Gang of Four Patterns architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• The client creates a list of shopping cart elements (books, fruits), and then applies

the visitor (ShoppingCart) to calculate the total cost, which includes applying the

discount to books.

class Program
{

static void Main()

{
var items = new List<IShoppingCartElement>
{

new Book(20),

new Fruit(5)

};

var cart = new ShoppingCart();
foreach (var item in items)
{

item.Accept(cart); // Visitor applies operation

(discount/tax)

}

Console.WriteLine($"Total: {cart.GetTotal()}"); // Total:

24.5 (Book with discount + fruit without discount)

}
}

Output:

Total: 24.5

How the Visitor Pattern Works:

• These subclasses of LoggerFactory override the CreateLogger method

to return the appropriate logger type (either FileLogger or

ConsoleLogger).

FileLoggerFactory:

public class FileLoggerFactory : LoggerFactory
{
public override ILogger CreateLogger() => new FileLogger();
}

ConsoleLoggerFactory:

public class ConsoleLoggerFactory : LoggerFactory
{
public override ILogger CreateLogger() => new ConsoleLogger();
}

• The MilkDecorator and SugarDecorator classes extend the

CoffeeDecorator class and override the Cost() and Description()

methods to add specific behavior (like adding the price of milk or sugar).

MilkDecorator:

public class MilkDecorator : CoffeeDecorator
{
public MilkDecorator(ICoffee coffee) : base(coffee) { }
public override double Cost() => base.Cost() + 0.50; // Add cost

of milk

public override string Description() => base.Description() + ",

Milk"; // Add milk to description

}

SugarDecorator:

public class SugarDecorator : CoffeeDecorator
{
public SugarDecorator(ICoffee coffee) : base(coffee) { }
public override double Cost() => base.Cost() + 0.25; // Add cost

of sugar

public override string Description() => base.Description() + ",

Sugar"; // Add sugar to description

}

• The TextEditor class is the invoker that invokes commands. It holds a

stack of commands (_commandHistory) and is responsible for executing

and undoing commands. When a command is executed, it’s pushed onto the

stack. The Undo() method pops the last executed command and undoes its

action.

public class TextEditor

private readonly Stack<ICommand> _commandHistory = new

Stack<ICommand>();

public void ExecuteCommand(ICommand command)

command.Execute();

_commandHistory.Push(command);

public void Undo()

if (_commandHistory.Count > 0)

var command = _commandHistory.Pop();

command.Undo();

Follow: