Thursday, April 20, 2006

Cool Generic Visitor Pattern in C# 2.0

I recently discovered that C# 2.0 was more then generic, you also get anonymous methods, partial and static classes, and some others...
The more I work with generic and anonymous methods, the more I feel like hacking with a dynamic language such as Ruby, Python or Boo (Boo is not dynamic I know, but so close).
My following example is somewhat trivial in this case, iterating a simple list is better done with a foreach block. But this pattern could be useful to browse some more complex hierarchy like a tree or a graph.

So here's the implementation of the actual visitor:

public class ListVisitor<T>
{
public delegate void VisitDelegate(T visited);

private IList _list;

public ListVisitor(IList list)
{
_list = list;
}

public void Visit(VisitDelegate callback)
{
foreach (T item in _list)
{
callback.Invoke(item);
}
}
}


And now to call it we specify an anonymous method with a block executed on each iteration. The delegate type is inferred by the compiler.

ArrayList list = new ArrayList("This is a test".Split(' '));

ListVisitor<string> visitor = new ListVisitor<string>(list);

visitor.Visit(delegate(string obj) { Console.WriteLine(obj); });


This is starting to look a lot like closures, don't you think ?

A rather more useful application would be to add a ForEach(delegate) to Castle's ActiveRecord so we could do something like this:

int greatTotal = 0;
Order.ForEach(delegate(Order o) { greatTotal += o.Total; });


Or even cooler (ActiveRecord.Update() would be called after each iteration.):

Order.UpdateEach(delegate(Order o) { if (o.Date > DateTime.Now.Date) o.Closed = true; });

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The visitor pattern remain tight coupled in one direction(visitor needs to know visitable concrete objects so adding new visitable objects can be done only changing all existing iterators). A way to reduce it is to use reflection like here Visitor Pattern

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