🧩 When to Use Singleton and Using Statements in .NET Core Applications

 🧠 Introduction

In every .NET Core application, managing object lifecycles efficiently is essential for performance, maintainability, and scalability. Two important concepts that developers often compare are the Singleton Pattern and the Using Statement.

Although both control object lifetimes, they serve entirely different purposes. This article clearly explains when to use Singleton and when to use Using in your .NET Core applications with real examples and best practices.

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🔹 What Is the Singleton Pattern?

The Singleton Pattern ensures that only one instance of a class exists throughout the entire application lifetime.

This is commonly used for shared services that are:

• Stateless (no per-user data)

• Expensive to create

• Used across multiple requests

✅ Example of Singleton in C#

public class MyLogger
{
    private static readonly MyLogger _instance = new MyLogger();
    private MyLogger() { }
    public static MyLogger Instance => _instance;
    public void Log(string message) => Console.WriteLine(message);
}

Or, using Dependency Injection (preferred in .NET Core):

builder.Services.AddSingleton<ILogger, MyLogger>();

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🔹 What Is the Using Statement?

The Using Statement in C# is used for managing the lifetime of disposable objects that implement the IDisposable interface.

When the code inside a using block finishes executing, the object is automatically disposed, ensuring resources like database connections, streams, or files are properly released.

✅ Example of Using Statement

using (var connection = new SqlConnection(connectionString))
{
    connection.Open();
    // Execute database operations
} // Automatically calls connection.Dispose()

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⚙️ When to Use Singleton in .NET Core

Use the Singleton pattern for services that:

• Are stateless and shared across the entire application.

• Don’t depend on request-scoped services.

• Need to maintain global configuration or cache data.

✅ Examples of Singleton Usage

• Logging services (ILogger)

• Configuration or settings manager

• In-memory caching

• Factory classes that create other stateless objects

⚠️ Avoid Singleton For:

• Services that hold per-user or per-request data

• Classes like DbContext that are not thread-safe

❌ Incorrect Singleton Example

builder.Services.AddSingleton<MyDbContext>(); // Bad Practice

DbContext must not be singleton because it’s not thread-safe and could cause data corruption if shared.

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⚙️ When to Use Using Statement

Use the Using statement for short-lived, disposable objects that you create manually and want to clean up after use.

✅ Examples

• File streams (FileStream, StreamReader)

• Database connections (SqlConnection)

• Network sockets

• Temporary objects implementing IDisposable

using (var stream = new FileStream("data.txt", FileMode.Open))
{
    // Read or write file data
}

In ASP.NET Core applications, you typically don’t need to use using for objects injected by Dependency Injection (DI).
The DI container handles the cleanup automatically at the end of each request scope.

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⚡ Combining Singleton and Using in .NET Core Applications

Here’s a practical example showing both in action:

// In Program.cs
builder.Services.AddSingleton<ILogService, LogService>();
builder.Services.AddScoped<IProductService, ProductService>();
builder.Services.AddScoped<ApplicationDbContext>();

// In ProductService.cs
public class ProductService : IProductService
{
    private readonly ApplicationDbContext _db;
    private readonly ILogService _log;
    
    public ProductService(ApplicationDbContext db, ILogService log)
    {
        _db = db;   // Scoped (per request)
        _log = log; // Singleton (shared)
    }

    public void SaveProduct(Product product)
    {
        _db.Products.Add(product);
        _db.SaveChanges();
        _log.Log("Product saved successfully!");
    }
}

Here’s what happens:

• ILogService → Singleton, shared across all requests.

• ApplicationDbContext → Scoped, created per request.

• No manual using is required; Dependency Injection automatically disposes of scoped services.

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🧩 Singleton vs Using Statement: Quick Comparison

Scenario	Use Singleton	Use Using Statement
Shared, stateless service	✅ Yes	❌ No
Temporary, disposable object	❌ No	✅ Yes
Global configuration or logging	✅ Yes	❌ No
Database or file operation	❌ No	✅ Yes
Managed by Dependency Injection	✅ Yes	❌ No
Manually created short-lived resource	❌ No	✅ Yes

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🧠 Best Practices Summary

Rule	Recommended Approach
Use Singleton for stateless shared services	✔️
Use Using for temporary disposable objects	✔️
Avoid Singleton for DbContext and per-user state	⚠️
Let DI manage the disposal of registered services	💡
Manually dispose only manually created resources	💡

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🚀 Conclusion

The Singleton Pattern and the Using Statement in .NET Core serve completely different roles in application lifecycle management.

• Singleton ensures a single, shared instance for stateless services.

• Using ensures proper disposal of short-lived resources.

By understanding their differences and applying them correctly, you can build high-performance, memory-efficient, and scalable .NET Core applications.