Singleton

Singleton Pattern

Intent

Ensure a class has only one instance and provide a global point of access to it.

Problem

Sometimes you need to ensure that a class has exactly one instance. For example, a single database connection, a configuration manager, or a logging service shared across the application.

Solution

The Singleton pattern ensures that a class has only one instance by:

• Making the constructor private
• Providing a static method that returns the single instance
• Storing the instance as a static variable

Structure

Singleton
├── private static instance
├── private constructor
└── public static GetInstance()

When to Use

• When exactly one instance of a class is needed
• When the single instance should be accessible from a well-known access point
• When the sole instance should be extensible by subclassing

C# Implementation

See the Program.cs file for a thread-safe implementation example.

How to Run

1. Create a .NET console project in this directory:

   dotnet new console -n Singleton -f net10.0

2. Replace the generated Program.cs with the provided one

3. Run the application:

   dotnet run

Pros

• Controlled access to the sole instance
• Reduced namespace pollution
• Permits refinement of operations and representation
• Lazy initialization possible

Cons

• Difficult to unit test
• Violates Single Responsibility Principle (manages its own lifecycle)
• Can hide dependencies
• Requires special care in multi-threaded applications