I was explaining my dilemma to a Java friend and the e-mail ended up being fairly interesting, so I thought I would share it.

I’m sure Java has extension methods, but I’ll give it a once over just in case.  You can extend existing classes without sub-classing by extension methods.

public static IEnumerable<SelectListItem>

                        AsSelectListItems( this IEnumerable&lt;Person&gt; listOfPeople){


   IList<SelectListItem> selectListItems = new List();
   foreach(var person in listOfPeople){

  selecListItems.Add(new SelectListItem(person.ID, 
                    string.Format(&quot;{0} {1} {2}&quot;, 
                                                      person.First, 
                                                      person.Middle, 
                                                      person.Last));


   }
   return selectListItems;
}

Right. So the this operator tells the compiler to basically create this function:

public static IEnumerable<SelectListItem>

                       AsSelectListItems(IEnumerable&lt;Person&gt; listOfPeople);</pre>


  
And invoke it every time you see this:

  
IEnunerable<Person> p = myService.GetAllPeople();
IEnumerable<SelectListItem> s = p.AsSelectListItems();

  
You probably knew all this, but it explains how we are using them. We end up calling a lot of extension methods off other extension methods. 

  
myViewData.PersonSelectListItems = myService.GetAllPeople()

                                               .AsSelectListItems()
                                               .OrderedByText()
                                               .StartingWithAnEmptyItem();</pre>


  
In this example we have three extension method calls and I think it really makes the code much more readable, but it makes testing this code a little more difficult. Obviously, the myService is a mocked dependency which will return a fake list of people, but the other extension method calls cannot be mocked, and the scope of the test bleeds a bit. 

  
I like the syntax it gives, but now I’m going to test potentially four classes when I should be testing only one. 

This gets pretty pandemic in our code. Especially trying to make Seleniums API and the assertion API of Nunit and xUnit more fluent.