Interview Q&A

Answer: When you want to share common code among related classes. When you need fields or constructors. When future changes may require adding non-breaking methods.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: bstract classes? Yes → Multiple interfaces No → Multiple abstract classes (C# does not support multiple class inheritance)

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Yes → Multiple interfaces No → Multiple abstract classes (C# does not support multiple class inheritance)

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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.

Interface allows multiple inheritance.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Yes, starting from C# 8, interfaces can have default method implementations. interface ILogger { void Log(string message); void LogWarning(string message) => Console.WriteLine("Warning: " + message); }

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Yes, abstract classes can have fully implemented methods along with abstract methods.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Yes, both can define properties. Interface properties are abstract by default; abstract class properties can have implementation. interface ICar { int Speed { get; set; } } bstract class Vehicle { public int Speed { get; set; } }

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: No, you cannot instantiate an interface. You can only use it as a reference type. ICar car = new Car(); // Interface reference // ICar c = new ICar(); // Not allowed

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Yes, abstract classes can be injected as service contracts, but interfaces are preferred for looser coupling.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Both can define contracts for derived classes. Both support polymorphism. Both cannot be instantiated directly. Both can be used with dependency injection.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Interface contracts are pure method signatures. Abstract class contracts can contain shared code and fields.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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.

• Implementing class must provide implementation once.
• Explicit interface implementation can resolve ambiguity.

interface IDriveable { void Start(); }

bstract class Vehicle { public abstract void Start(); }

class Car : Vehicle, IDriveable
{
public override void Start() => Console.WriteLine("Car

started");

}

No, interfaces can only inherit other interfaces.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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.

Yes, abstract classes can implement interfaces partially or fully.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: bstract classes? Abstract classes are slightly faster because they use direct method calls. Interfaces may incur slight overhead due to indirect method calls via vtable. Difference is usually negligible in most applications.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Abstract classes are slightly faster because they use direct method calls. Interfaces may incur slight overhead due to indirect method calls via vtable. Difference is usually negligible in most applications.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Interfaces are preferred for public APIs because they: Allow multiple inheritance Support loose coupling Avoid breaking changes when adding new implementations Abstract classes are better for internal APIs where shared code is required.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Interfaces: Adding new members breaks existing implementations unless using default interface methods (C# 8+). Abstract Classes: Can add new methods with implementation without breaking derived classes.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Interfaces support multiple inheritance. Abstract classes do not. Interview Q&A

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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.

• Define an interface like IPlugin with a Run() method.
• Each plugin implements IPlugin and can be loaded dynamically.

interface IPlugin { void Run(); }
class PluginA : IPlugin { public void Run() =>

Console.WriteLine("Plugin A running"); }

class PluginB : IPlugin { public void Run() =>

Console.WriteLine("Plugin B running"); }

// Usage

List<IPlugin> plugins = new List<IPlugin> { new PluginA(), new

PluginB() };

foreach (var p in plugins) p.Run();

nd interfaces?

• Use an interface for common operations: IPayment.
• Use an abstract class for shared behavior like logging.

interface IPayment { void Pay(decimal amount); }

bstract class PaymentBase : IPayment

{
public abstract void Pay(decimal amount);
public void Log(string message) => Console.WriteLine(message);
}
class CreditCardPayment : PaymentBase
{
public override void Pay(decimal amount) =>

Console.WriteLine($"Paid {amount} by Credit Card");

}

• Use an interface for common operations: IPayment.
• Use an abstract class for shared behavior like logging.

interface IPayment { void Pay(decimal amount); }

abstract class PaymentBase : IPayment

public abstract void Pay(decimal amount);

public void Log(string message) => Console.WriteLine(message);

class CreditCardPayment : PaymentBase

public override void Pay(decimal amount) =>

Console.WriteLine($"Paid {amount} by Credit Card");

• Define a base Notification class or interface.
• Derive classes like EmailNotification, SMSNotification.

bstract class Notification { public abstract void Send(string

message); }

class EmailNotification : Notification { public override void
Send(string msg) => Console.WriteLine("Email: " + msg); }
class SMSNotification : Notification { public override void
Send(string msg) => Console.WriteLine("SMS: " + msg); }
List<Notification> notifications = new List<Notification> { new

EmailNotification(), new SMSNotification() };

foreach (var n in notifications) n.Send("Hello World!");

Answer: Protects internal data by restricting direct access. Ensures sensitive fields are accessed only via methods/properties, preventing misuse.

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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: Use mocking frameworks like Moq or NSubstitute. Provides fake implementations to test dependent classes. var mockLogger = new Mock<ILogger>(); mockLogger.Setup(x => x.Log(It.IsAny<string>()));

What interviewers expect

• A clear definition tied to OOP in C# OOP projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# OOP 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 C# OOP 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.