Singleton Pattern in C# – Real-Time Example (Shared Instance)

What is Singleton Pattern?

The Singleton Pattern is a creational design pattern that ensures only one instance of a class exists throughout the application and provides a global access point to it.

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Why Use Singleton?

• Share a single instance across the app
• Avoid multiple object creation
• Manage global resources efficiently

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Real-Time Scenario

• Logging service
• Configuration settings
• Cache service
• Application-wide settings

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Basic Implementation (Classic)

public class Logger
{
    private static Logger _instance;

    private Logger() { }

    public static Logger GetInstance()
    {
        if (_instance == null)
            _instance = new Logger();

        return _instance;
    }

    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

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Usage

var logger1 = Logger.GetInstance();
var logger2 = Logger.GetInstance();

Console.WriteLine(logger1 == logger2); // True

logger1.Log("Hello Singleton");

👉 Both variables point to the same instance

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Thread-Safe Singleton (Recommended)

public class Logger
{
    private static readonly Lazy<Logger> _instance =
        new Lazy<Logger>(() => new Logger());

    private Logger() { }

    public static Logger Instance => _instance.Value;

    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

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.NET Core Way (Best Practice)

👉 Don’t manually create singleton
👉 Use Dependency Injection

builder.Services.AddSingleton<ILogService, LogService>();

public class LogService : ILogService
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

👉 Same instance shared across entire app automatically

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Key Concept

Instead of:

var obj1 = new Logger();
var obj2 = new Logger();

We do:

var obj1 = Logger.Instance;
var obj2 = Logger.Instance;

👉 Only one object exists

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Advantages

• Ensures single instance
• Saves memory/resources
• Global access point
• Useful for shared services

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Disadvantages

• Hard to unit test (global state)
• Can create hidden dependencies
• Not suitable for all scenarios
• Tight coupling if overused

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When to Use

• When exactly one instance is required
• When managing shared resources
• When object creation is expensive
• When global access is needed

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Real Project Mapping (.NET + Angular)

Feature	Usage
Logging service	Singleton
Configuration service	Singleton
Cache manager	Singleton
Auth/session service	Singleton

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Pro Tip (Advanced .NET Usage)

• Always prefer DI-based Singleton in ASP.NET Core
• Avoid static singletons unless necessary
• Combine with Options pattern for configuration

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Summary

Singleton Pattern helps you:

• Maintain a single shared instance
• Manage global resources efficiently
• Improve performance and consistency

👉 Perfect for logging, caching, configuration services