Understanding the Singleton Design Pattern

Introduction to the Singleton Design Pattern

The Singleton Design Pattern is a creational design pattern that ensures a class has only one instance while providing a global point of access to that instance. It involves a single class which is responsible to create an object while making sure that only single object gets created. This class provides a way to access its only object which can be accessed directly without need to instantiate the object of the class.

Key Concepts

• Single Instance: Ensures that only one instance of the Singleton class ever exists.
• Global Access: Provides global access to that instance.

Benefits

• Controlled Access: There is exactly one instance of a class that is accessible from anywhere in the application.
• Lazy Initialization: Often, the Singleton is only created when it is first needed.
• Reduced Global State: Helps in reducing global state usage in an application.

Example in Code

Here’s how you might implement the Singleton Pattern in C# to ensure that there is only one instance of a database connection manager:

public class DatabaseManager
{
    private static DatabaseManager _instance;
    private static readonly object _lock = new object();

    // Constructor is 'protected' to prevent instantiation outside the class
    protected DatabaseManager()
    {
    }

    public static DatabaseManager Instance
    {
        get
        {
            if (_instance == null)
            {
                lock (_lock)
                {
                    if (_instance == null)
                    {
                        _instance = new DatabaseManager();
                    }
                }
            }
            return _instance;
        }
    }

    public void Query(string sql)
    {
        Console.WriteLine("Executing query: " + sql);
    }
}

// Example usage
class Program
{
    static void Main(string[] args)
    {
        DatabaseManager manager = DatabaseManager.Instance;
        manager.Query("SELECT * FROM users");
    }
}