Design Patterns in .NET (C#) – Categories, Key Concepts & Most Important Patterns with Examples

Introduction

Design Patterns in .NET are standard, proven solutions to recurring software design problems. After learning SOLID principles, design patterns help developers apply those principles practically in real-world applications.

In senior .NET interviews, the focus is no longer on syntax. Instead, interviewers evaluate how you design systems, manage complexity, and prepare applications for future changes — and that is exactly where design patterns play a crucial role.

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What Are Design Patterns?

Design Patterns are conceptual solutions, not frameworks or libraries. They describe how to solve a problem, not exact code to copy.

Why Design Patterns Matter

• Reduce tight coupling
• Improve code readability
• Make systems scalable
• Simplify maintenance
• Improve testability

Design patterns are about thinking like an architect, not just writing code.

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1️⃣ Category-Wise Design Patterns in .NET (Key Overview)

🔹 Creational Design Patterns (Object Creation)

These patterns deal with how objects are created, ensuring flexibility and control over instantiation.

Key Goals

• Hide object creation logic
• Avoid tight coupling
• Control object lifecycle

Patterns

• Singleton
• Factory Method
• Abstract Factory
• Builder
• Prototype

Used when object creation is complex, expensive, or needs strict control.

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🔹 Structural Design Patterns in .NET (Object Structure)

These patterns focus on how classes and objects are composed to form larger structures.

Key Goals

• Simplify structure
• Improve flexibility
• Enable extension without modification

Patterns

• Adapter
• Decorator
• Facade
• Proxy
• Composite
• Bridge
• Flyweight

Used when integrating systems, adding features dynamically, or hiding complexity.

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🔹 Behavioral Design Patterns in .NET (Object Communication)

These patterns manage communication and responsibility between objects.

Key Goals

• Reduce coupling
• Improve interaction clarity
• Allow behavior changes at runtime

Patterns

• Strategy
• Observer
• Command
• State
• Template Method
• Chain of Responsibility
• Mediator
• Memento
• Iterator
• Visitor
• Interpreter

Used when behavior changes frequently or logic becomes conditional-heavy.

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2️⃣ Most Common & Important Design Patterns in .NET (With Examples)

These are the patterns most frequently used in real .NET projects and interviews.

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✅ Singleton Pattern

What it is

Ensures only one instance of a class exists throughout the application lifecycle.

When to use

• Logging
• Configuration settings
• Caching

Benefits

• Controlled access to shared resources
• Memory efficient

Common Mistake

Overusing Singleton breaks unit testing and scalability.

C# Example

public class Logger
{
    private static Logger _instance;
    private static readonly object _lock = new();

    private Logger() { }

    public static Logger Instance
    {
        get
        {
            lock (_lock)
            {
                return _instance ??= new Logger();
            }
        }
    }
}

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✅ Factory Pattern

What it is

Creates objects without exposing creation logic to the client.

When to use

• Multiple implementations
• Runtime object selection

Benefits

• Follows Open/Closed Principle
• Removes tight coupling

C# Example

public interface INotification
{
    void Send();
}

public class EmailNotification : INotification
{
    public void Send() => Console.WriteLine("Email sent");
}

public class NotificationFactory
{
    public static INotification Create(string type)
    {
        return type == "Email" ? new EmailNotification() : null;
    }
}

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✅ Repository Pattern

What it is

Acts as a middle layer between business logic and data access, hiding database details.

When to use

• Any application using a database
• Clean architecture requirements

Benefits

• Centralized data access
• Easier unit testing
• Database abstraction

Common Mistake

Adding business logic inside repositories.

C# Example

public interface IUserRepository
{
    User GetById(int id);
}

public class UserRepository : IUserRepository
{
    public User GetById(int id)
    {
        return new User { Id = id, Name = "John" };
    }
}

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✅ Unit of Work (UOW) Pattern

What it is

Manages multiple repository operations under a single transaction.

When to use

• Multiple DB operations in one request
• Transaction consistency

Benefits

• Single SaveChanges()
• Better transaction control

C# Example

public interface IUnitOfWork
{
    IUserRepository Users { get; }
    void Commit();
}

Repository handles queries, Unit of Work handles transactions.

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✅ Strategy Pattern

What it is

Allows changing behavior at runtime without modifying existing code.

When to use

• Discount logic
• Tax calculation
• Payment rules

Benefits

• Eliminates if-else chains
• Highly extensible

C# Example

public interface IDiscountStrategy
{
    decimal Apply(decimal amount);
}

public class FestivalDiscount : IDiscountStrategy
{
    public decimal Apply(decimal amount) => amount * 0.8m;
}

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✅ Observer Pattern

What it is

Allows objects to subscribe and get notified when state changes.

When to use

• Events
• Notifications
• Real-time updates

Benefits

• Loose coupling
• Scalable event handling

C# Example

public class Order
{
    public event Action OrderPlaced;

    public void Place()
    {
        OrderPlaced?.Invoke();
    }
}

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✅ Decorator Pattern

What it is

Adds new behavior to an object without modifying its code.

When to use

• Logging
• Authorization
• Caching

Benefits

• Follows Open/Closed Principle
• Flexible feature addition

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✅ Facade Pattern

What it is

Provides a simple interface to a complex subsystem.

When to use

• Service layers
• External integrations

Benefits

• Reduced complexity
• Cleaner API usage

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3️⃣ Why These Patterns Matter in Interviews

Interviewers assess:

• Your problem-solving approach
• Pattern selection logic
• Ability to avoid over-engineering

They do NOT expect all 23 patterns — they expect clarity and judgment.

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When NOT to Use Design Patterns

❌ Simple CRUD apps
❌ Small scripts
❌ When readability suffers

Using no pattern is better than using the wrong one.

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Conclusion

Design Patterns in .NET help developers write scalable, maintainable, and professional software. You don’t need to use every pattern, but knowing when and why to use the right one separates an average developer from a senior engineer.

Read our detailed guides on Single Responsibility Principle, Open/Closed Principle, Liskov Substitution Principle, and Interface Segregation Principle to fully understand SOLID principles in C#.

Explore more important concepts here…