godot-demo-projects/mono/monkey_pong/CeilingFloor.cs

using Godot;
using System;

public class CeilingFloor : Area2D
{
    [Export]
    private int yDirection = 1;

    public void OnAreaEntered(Area2D area)
    {
        if (area is Ball ball)
        {
            ball.direction += new Vector2(0, yDirection);
        }
    }
}