Singleton
Learn to write a single-instance class with the Singleton pattern.
Singleton pattern
Singleton is a pattern that guarantees a class has only one instance. Three pieces make it work: a private constructor (private Config()) that blocks external new, a static field holding the single instance (private static readonly Config _instance), and a static property exposing it (public static Config Instance).
Marking the class sealed prevents subclasses from creating a second instance. Static fields in C# are lazily initialized on first access, so a basic Singleton needs no extra locking.
sealed and lazy initialization
The sealed keyword seals the class; subclasses cannot create new instances. This protects the single-instance guarantee from being broken through derivation. In C#, static fields are created the first time they are accessed; this is lazy initialization.
An expression-bodied property is a concise way to return the single instance: public static Config Instance => _instance;. The => arrow is a one-line expression that computes the property value.
Exercise Define the Singleton Config class
Define a sealed class Config. Add a private constructor (private Config()), a static readonly _instance field, a public static Config Instance property, and a string Greet() method. In Main, print Config.Instance.Greet().
Show example solution
Console.WriteLine(Config.Instance.Greet());
Example solution:
using System;
sealed class Config
{
private static readonly Config _instance = new Config();
public static Config Instance => _instance;
private Config() { }
public string Greet() => "Merhaba";
}
class Program
{
static void Main()
{
Console.WriteLine(Config.Instance.Greet());
}
}Test yourself
-
What is the main goal of the Singleton pattern?
- Ensure a single instance and provide global access
- Create many instances
- Force inheritance
- Hide static methods
Show answer
Ensure a single instance and provide global access — Correct. Singleton guarantees one instance and exposes it via a static property.
-
Complete the expression-bodied property
public static Config Instance ____ _instance;Show answer
public static Config Instance => _instance; -
Why use a private constructor in Singleton?
- To make the class static
- To make it run faster
- To prevent external new from creating another instance
- To hide inherited methods
Show answer
To prevent external new from creating another instance — Correct. A private constructor prevents external new from creating more instances.