Visitor Pattern

The visitor design pattern enables us to create new operations to be
performed on an existing structure. The new operations don’t change the
classes/nodes. With the pattern you define two class hierarchies.
The first for the elements which the operations will operate on.
The second is for the visitors that define the operations on the elements.
The pattern is rarely used but it is common in compilers implementation.
 
Use Cases for the Visitor Pattern
You should use the pattern in the following cases:

• You have a class hierarchy with many distinct operations.
• The classes that defined in the object structure rarely change, but you
  need sometimes to define new operations on the object structure.

UML Diagram    
     

Example in C# The following code is an example of how to implement the pattern:

#region Visitor
public interface IVisitor
{
    #region Methods
    void Visit(Element element);
    #endregion
}
#endregion

#region Visitor Concrete
public class VisitorConcreteA : IVisitor
{
    #region IVisitor Members
    public void Visit(Element element)
    {
        Console.WriteLine("{0} Visited {1}",
            this.GetType().Name, element.GetType().Name);
    }
    #endregion
}
public class VisitorConcreteB : IVisitor
{
    #region IVisitor Members
    public void Visit(Element element)
    {
        Console.WriteLine("{0} Visited {1}",
            this.GetType().Name, element.GetType().Name);
    }
    #endregion
}
#endregion

#region Element
public interface IElement
{
    #region Methods
    void Accept(IVisitor visitor);
    #endregion
}

public abstract class Element : IElement
{
    #region IElement Members
    public void Accept(IVisitor visitor)
    {
        visitor.Visit(this);
    }
    #endregion
}
#endregion

#region Concrete Element
public class ElementConcreteA : Element
{
    // implement whatever you need
}

public class ElementConcreteB : Element
{
    // implement whatever you need
}
#endregion

#region Object Structure
public class ObjectStructure
{
    #region Members
    private List<Element> _elements;
    #endregion
    #region Ctor
    /// <summary>
    /// Construct a new object structure
    /// </summary>
    public ObjectStructure()
    {
        _elements = new List<Element>();
    }
    #endregion

    #region Methods
    /// <summary>
    /// Attach the given element to the object structure
    /// </summary>
    /// <param name="element">The element to attach</param>
    public void AttachElement(Element element)
    {
        _elements.Add(element);
    }

    /// <summary>
    /// Detaches the given element from the object structure
    /// </summary>
    /// <param name="element">The element to detach</param>
    public void DetachElement(Element element)
    {
        _elements.Remove(element);
    }

    /// <summary>
    /// Perform accept operation on all the object structure
    /// with the given visitor
    /// </summary>
    /// <param name="visitor">The given visitor</param>
    public void Accept(IVisitor visitor)
    {
        foreach (Element element in _elements)
        {
            element.Accept(visitor);
        }
    }
    #endregion
}
#endregion

In the example I use an object structure that is implemented with an inner list. You can use whatever object structure you need in order to implement the pattern. As can be seen in the example, I use two main interfaces to implement the pattern – IVisitor and IElement. The IElement dictates the Accept method to the Element abstract class which every concrete element implement (I could drop the IElement or the Element class but I have chosen not to). Every visitor concrete implement the Visit operation to perform the desired operation (in my implementation the Visit method is implement the same in both concrete classes).

The next code shows an example of how to use the classes above:

// Build the structure
ObjectStructure structure = new ObjectStructure();
structure.AttachElement(new ElementConcreteA());
structure.AttachElement(new ElementConcreteB());
// Create visitor objects
VisitorConcreteA visitorA = new VisitorConcreteA();
VisitorConcreteB visitorB = new VisitorConcreteB();
// Structure accepting visitors
structure.Accept(visitorA);
structure.Accept(visitorB);
Console.Read(); 

Summary

To sum up, you were introduced to the visitor design pattern. The pattern is used in order to extend elements of an object structure with new operations without the need to change the elements. As written earlier the pattern is rarely used.