SOLID & Design Patterns Interview Question #33

What are the disadvantages of the Singleton Pattern?

Structural, Creational & Behavioral Patterns Junior Beginner

Quick Interview Answer

A global singleton can hide dependencies, introduce shared mutable state, make tests influence one another, and complicate concurrency. It can also tightly couple application code to a concrete global instance.

Detailed Explanation

If an application genuinely needs one service instance, registering that service with a DI container's singleton lifetime often preserves explicit constructor dependencies and improves testability.

Code Example

// Prefer explicit dependency:
public class AuditService
{
    private readonly IClock _clock;

    public AuditService(IClock clock)
        => _clock = clock;
}

// DI can control the required lifetime.