C#

public abstract class BaseClass
{
}

public class Leaf
{
}

public class LeafA : Leaf
{
}

public class LeafB : Leaf
{
}

public class Composite<TChild> where TChild : BaseClass
{
  private List<TChild> _children = new List<TChild>();
  public List<TChild> Children { get { return _children;}}
}

Composite<BaseClass> root = new Composite<BaseClass>();

root.Children.Add(new LeafB());
root.Children.Add(new LeafA());


Composite<LeafA> comp = new Composite<LeafA>();
comp.Children.Add(new LeafA());
comp.Children.Add(new LeafA());

root.Children.Add(comp);

comp = new Composite<LeafB>();
comp.Children.Add(new LeafB());
comp.Children.Add(new LeafB());

root.Children.Add(comp);

foreach (BaseClass b in root.Children)
{
  // test if Leaf
  if (b is Leaf)
  {
  }
  // else it must be Composite
  else
  {
    Composite<BaseClass> child = (b as Composite<BaseClass>); // child is null after this cast
  }
}