Creational Design Pattern: Singleton Pattern

Define Singleton Pattern

The Singleton pattern is one of the most basic design patterns. This pattern ensures that a class has only one instance and offers global access to it.

When should you utilize the singleton pattern?

You just need one instance of an object throughout the application. For example:

• Exception Logging
• Database Manager

When Not to Use the Singleton Design Pattern in C#?

• The singleton design complicates unit testing by persisting state between tests, limiting test independence. 
• It also introduces global state, leading to hidden dependencies and system complexity.
• Concurrency difficulties may emerge in multithreaded systems due to the mutable singleton state.
• Limits scalability & flexibility, which could become a bottleneck as the program grows.
• Overuse & misuse of Singletons can lead to design issues and difficulty in later stages of development.

Singleton Design pattern with UML diagram

Methods to implement the Singleton Design Pattern in C#

• In C#, there is no thread-safe singleton design pattern. 
• However, thread-safe singletons are implemented via locks.
• Use Double-Check Locking to implement the Thread-Safety Singleton Design Pattern.
• To provide thread safety in a singleton design pattern, use eager loading. 
• For lazy loading, use the Lazy<T> generic class.

Singleton Class Structure

The singleton pattern requires a class to have the following structure:

• Should include a private or protected constructor. No public or parameterized constructors.
• Should have a static property (with a private underlying field) that returns a class instance. A static method may also be used to return an instance.
• Have at least one non-static public method for a singleton operation.

Advantages of Singleton Pattern in C#

• Thread-safe access: Ensures smooth concurrent access to shared resources.
• Lazy loading: Delays object creation until first needed, saving memory.
• Static initialization: Guarantees a single instance throughout the application.
• Common data sharing: Ideal for caching infrequently changing data.
• Reduced overhead: Avoids repeated creation of expensive objects.
• Single access point: Simplifies maintenance and access control.

Disadvantages of Singleton Design Pattern in C#

• Unit testing is hard since it adds a global state to a program.
• It lowers the ability for parallelism within a program because accessing the singleton instance in a multi-threaded environment necessitates serializing the object with locking.

Real-time scenarios using the Singleton Design Pattern in C#

Here are several real-time instances where the Singleton design pattern can be useful in C#:

• Configuration Management: Singletons ensures consistent access to configuration settings across the application's lifecycle.
• Logging: The Singleton logger centralizes log entries, ensuring consistency and managing access to log files.
• Database Connections: Singleton manages database connections, optimizing performance by using a shared connection pool.
• Hardware Interface Communication: Singleton facilitates centralized management of hardware interactions, avoiding conflicts.
• Caching: Singleton caches expensive data, providing global access and consistency.
• Application Services: The singleton pattern is suitable for implementing application-wide services like licensing or task scheduling.
• State Management: Singletons can manage complex shared states, useful in certain types of applications like game development.