What Kind Of Items Expected By List In Below Case? Please See The Code. Its Simple I Think But I Am Confused.

abstract class Element
{

    public abstract void Accept(Visitor visitor);

}



/// <summary>

/// A 'ConcreteElement' class

/// </summary>

class ConcreteElementA : Element
{

    public override void Accept(Visitor visitor)
    {

        visitor.VisitConcreteElementA(this);

    }



    public void OperationA()
    {

    }

}



/// <summary>

/// A 'ConcreteElement' class

/// </summary>

class ConcreteElementB : Element
{

    public override void Accept(Visitor visitor)
    {

        visitor.VisitConcreteElementB(this);

    }



    public void OperationB()
    {

    }

}



/// <summary>

/// The 'ObjectStructure' class

/// </summary>

class ObjectStructure
{

    private List<Element> _elements = new List<Element>();



    public void Attach(Element element)
    {

        _elements.Add(element);

    }



    public void Detach(Element element)
    {

        _elements.Remove(element);

    }

http://www.dofactory.com/net/visitor-design-pattern