Composite

Context: Compose objects into tree structures to represent part‑whole hierarchies. Composite lets clients treat individual objects and compositions uniformly.

public abstract class Component
{
    public abstract void Operation();
}

public class Leaf : Component
{
    public override void Operation() => Console.WriteLine("Leaf");
}

public class Composite : Component
{
    private List<Component> _children = new();
    public void Add(Component c) => _children.Add(c);
    public override void Operation()
    {
        foreach (var child in _children)
            child.Operation();
    }
}

Real-world usage example

File system: Files (leaf) and directories (composite) both implement IFileSystemNode. Operations like GetSize() or Delete() work on both.

Example: In UI frameworks, Control can be a single button or a panel containing other controls. ASP.NET Core’s ICompositeMetadataDetailsProvider uses composite pattern.

Related sections

• 5.2 Structural patterns
• 5.2.5 Bridge
• 5.2.7 Flyweight