Composite

Composes objects into tree structures and treats them uniformly as if they were individual objects.

Context

Your model is hierarchical: folders with files and subfolders, groups of figures, nested comments.

Problem

You want to treat leaves and composites with the same code (e.g., CalculateSize()).

Solution

A common interface (IFileSystemEntry) implemented by leaves (File) and composites (Directory) which delegates recursively.

#Example in C#

public interface IFileSystemEntry { long GetSize(); }

public class File : IFileSystemEntry
{
    public long Size { get; init; }
    public long GetSize() => Size;
}

public class Directory : IFileSystemEntry
{
    private readonly List<IFileSystemEntry> _children = new();
    public void Add(IFileSystemEntry e) => _children.Add(e);
    public long GetSize() => _children.Sum(c => c.GetSize());  // recursive and uniform
}

When NOT to use it

When leaves and composites have divergent behaviors that force branches (if (isFolder)).

Tradeoffs

Pro	Con
Uniform client	Difficult to restrict child types in the common interface

#structural #gof #tree