What design patterns do you apply in your practice?

In practice, I often use the following design patterns in C#:

• Singleton — for creating a single instance of a class, for example, for a logger or configuration manager.
• Factory Method — for creating objects without specifying the exact class, convenient when working with class hierarchies.
• Repository — for abstracting data access, simplifies testing and maintenance.
• Dependency Injection — for managing dependencies and increasing testability.
• Observer — for implementing an event-driven model, for example, through delegates and events.

Example of Singleton:

public class Logger
{
    private static Logger _instance;
    private Logger() { }

    public static Logger Instance => _instance ??= new Logger();

    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

The choice of pattern depends on the task and the architecture of the project.