What design patterns do you apply when developing the server part of a system?

When developing the server-side, I often use the following design patterns:

• Repository — for abstracting data access, separating database logic from business logic.
• Dependency Injection — for managing dependencies and simplifying testing.
• Singleton — for creating a single instance of services or managers, such as configuration.
• Factory — for creating objects with different configurations or types without tight coupling to specific classes.
• Strategy — for selecting algorithms or behaviors at runtime.

Example of using Repository in C#:

public interface IUserRepository {
    User GetById(int id);
    void Add(User user);
}

public class UserRepository : IUserRepository {
    public User GetById(int id) {
        // implementation of database access
    }
    public void Add(User user) {
        // adding a user
    }
}

This allows changing the data access implementation without modifying the business logic.