1. Singleton Pattern

Description: The Singleton Pattern ensures a class has only one instance and provides a global point of access to it.

Pros:

• Controlled access to the sole instance.
• Reduced memory footprint by preventing the creation of multiple instances.
• Facilitates lazy initialization.

Cons:

• Can be difficult to unit test due to its global state.
• May introduce a bottleneck if multiple threads access the singleton instance concurrently.

Practical Usage:

• Configuration settings management.
• Logging service.
• Connection pooling.

Sample Implementation:

public sealed class Singleton
{
    private static Singleton instance = null;
    private static readonly object padlock = new object();

    Singleton() { }

    public static Singleton Instance
    {
        get
        {
            lock (padlock)
            {
                if (instance == null)
                {
                    instance = new Singleton();
                }
                return instance;
            }
        }
    }
}

2. Factory Pattern

Description: The Factory Pattern defines an interface for creating an object, but allows subclasses to alter the type of objects that will be created.

Pros:

• Promotes loose coupling by eliminating the need to bind application-specific classes into the code.
• Enhances code readability and maintainability.