The problem: You want to extend the functionality of a class in a flexible way. The solution: Create a "Decorator" class implementing the same interface your original class is.

This way you follow the Single Responsibility Principle because the caller should not know nor care whether the class it is calling is the original instance, or a decorated one.

public interface IService
{
	public void DoSomething()
}

public class Service : IService
{
	private int x = 1;
	public void DoSomething()
	{
		this.x++;
	}
}

public class LoggedService : IService
{
	private IService service;
	private ILogger logger
	
	public LoggedService(IService service, ILogger logger)
	{
		this.service = service;
		this.logger = logger
	}
	
	public void DoSomething()
	{
		this.logger.Log("DoSomething() started");
		
		this.service.DoSomething();
		
		this.logger.Log("DoSomething() ended");
	}
}