UI: Good Use For Extension Methods

Ok, this is a little weird for me, but I think I just had my epiphany. I think I have finally converted in to an Extnsions Methods believer! I know, first there was C# 3.0 Extension Methods? A Good Idea? and then there was Reserving Judgement, but now I have found where I really, really like them.

How many of you have written a class like this:

public class Person
{
   private string firstName;
   private string lastName;

   public string FirstName
   {
       get { return firstName; }
       set { firstName = value; }
   }

   public string LastName
   {
       get { return lastName; }
       set { lastName = value; }
   }

   public string GetFullName()
   {
       return LastName + ", " + FirstName;
   }
}

I don't know about you, but I feel dirty every time I write something like that. To me this is display logic, and now it's polluting my business objects! This really smells to me. This smells so bad that I prefer to put a method on my page which does the formatting for me. But the problem is that people are used to this sort of syntax. They expect this to be how they format that class.

How do we satisfy both sides? Keep the presentation logic out of the business layer, yet keep the interfaces clean? Enter Extension Methods!

In order to solve this problem I would now create a new static class in my web assembly (UIHelper possibly, or another name that fits). This new class would look like the following:

public static class UIHelper
{
   public static string GetFullName(this Person person)
   {
       return person.LastName + ", " + person.FirstName;
   }
}

Now you can take the GetFullName method out of the Person class and use the extension method in your UI layer instead. The class will still work exactly the same way from a UI perspective, except when working within the Person there will no longer be a GetFullName method.

The Person class now looks like this:

public class Person
{
   private string firstName;
   private string lastName;

   public string FirstName
   {
       get { return firstName; }
       set { firstName = value; }
   }

   public string LastName
   {
       get { return lastName; }
       set { lastName = value; }
   }
}

Just make sure you add the using directive to the namespace which contains the UIHelper class on your page (or add an Import directive to your aspx file). Your code on the page now looks like the following:

This is actually the happiest I've been with any use of extension methods when you control the class which is being extended.

--John Chapman