Program.cs

namespace DesignPatterns.Composite;

interface IPlaceHolder 
{
    void DoubleClick();
}

class Folder(string name) : IPlaceHolder
{
    readonly List<IPlaceHolder> _files = [];

    public void Add(IPlaceHolder file)=> _files.Add(file);
    
    public void DoubleClick() 
    {
        Console.WriteLine($"{name} folder is opened");
        _files.ForEach(file => file.DoubleClick());
    }
}

class File(string name) : IPlaceHolder
{
    public void DoubleClick() => Console.WriteLine($"{name} file is opened in a program");
}

class App
{
    public static void Main()
    {
        File file1 = new File("File 1");
        File file2 = new File("File 2");
        File file3 = new File("File 3");

        Folder folder1 = new Folder("Folder 1");
        folder1.Add(file1);
        folder1.Add(file2);
        folder1.Add(file3);

        Folder folder2 = new Folder("Folder 2");
        File file4 = new File("File 4");
        File file5 = new File("File 5");

        folder2.Add(file4);
        folder2.Add(file5);

        folder1.Add(folder2);
        folder1.DoubleClick();
    }
}