Question

I have a class hierarchy that represents a JSON based API. There is a generic factory that calls and deserializes the api into classes using .NET 4 (no 3rd party libs). I am trying to avoid having to instantiate the class to retrieve a read-only piece of information that is unique to each class.

I had thought (until I started reading this and this, …) I would associate a static URL with a base class/interface and then set it in the derived class’s constructor. Something like (this example will not work):

abstract class url {
  public abstract static string URL; // This is invalid syntax!
}

class b : url {
  static b () { URL = "http://www.example.com/api/x/?y=1"; }
}

class c: url {
  static c () { URL = "http://www.example.com/api/z"; }
}

// ... so the factory can do something like ...
b result = doJSONRequest<b>(b.URL);

This doesn’t work. The static field can’t be abstract, nor can it be uniquely set in b and c as the static variable is stored in the class it is defined in (url in this case).

How can I have a read only item associated with a class such that you can access the item (etc.) without having to instantiate the class?

Answer 1

I’ve implemented a pattern like this to help remind me of constants that I need to setup per derived class that need to be statically accessible:

public abstract class Foo
{
    public abstract string Bar { get; }
}

public class Derived : Foo
{
    public const string Constant = "value";
    public override string Bar
    {
        get { return Derived.Constant; }
    }
}

I’ve even found that after implementing this pattern that the polymorphic use of the constant to be just as helpful.