Special Case It, Part Two

Following my post on the Special Case pattern, I just wanted to give a few more examples in which the pattern proves to be really helpful. In the previous post, we talked about using the Special Case pattern when returning collections, but you can use it in other places as well.

Subclassing

Suppose we have a cached collection of User objects, and we want to fetch a specific User and save it to the database.

User user = cache.Get(”John”);
if (user != null)
{
     user.Save();
}

If we subclass User with a class called MissingUser, and have our cache object return it when it can’t find a user, we can reduce our calling code to:

User user = cache.Get(”John”);
user.Save(); 

MissingUser.Save() overrides User.Save() and does nothing.

Side note: we could definitely go with checking cache.Contains(”John”), and acting on the result, but I think this is too much detail for this level of abstraction, and may be violating Tell, Don’t Ask.

Instantiation

Suppose we want to display a User’s profile. Our User class has a string called Name and a bitmap called Picture.

User user = profiles.Get(”Mark”);
if (user != null)
{
     this.Name = user.Name; // Binding is for the weak.
     this.Picture = user.Picture;
}

When a User is not found in the profiles repository, we could make it return an instance of the regular User class, where User.Name = “Not Found”, and Picture is something like

This way, we can just write:

User user = profiles.Get(”Mark”);
this.Name = user.Name;
this.Picture = user.Picture;

and a profile will always be displayed.

Conditioning on Instantiation

What if we wanted to check if a User exists in a managers repository, and only if it is not found, fetch it from a clerks repository? One might be tempted to write something like:

User user = managers.Get(”Ahmed”);
if (user != null)
{
     // Ahmed is a manager.
     Display(user);
}
else
{
     // Ahmed must be a clerk.
     user = clerks.Get(”Ahmed”);
     Display(user);
}

In the previous examples, we avoided the conditional (null check) by using Special Case. If you look at the requirement for this example, you’ll see that the conditional is built into it, and there is no way to get around it.

But if we use the same technique we used in the second example, we could create an instance of User, that has a boolean property called Exists, which will be populated according to the result. This way, we could express our condition much better:

User user = managers.Get(”Ahmed”);
if (user.Exists)
{
     // Ahmed is a manager.
     Display(user);
}
else
{
     // Ahmed must be a clerk.
     user = clerks.Get(”Ahmed”);
     Display(user);
}

No code saved here, but our intention is much clearer this way.

What the hell is a “if (user != null)”? English, people.