What is the DRY-est factory pattern to instantiate subtypes based on one of their properties' values?

Question

Take the following class hierarchy:

• Client
  • FacebookClient
  • PinterestClient
  • TwitterClient

Each client must define a value for an enum property named ClientType (string property Url in the original version of this question; hence the accepted answer). ClientFactory should then be able to instantiate by client type.

ClientFactory.Create(ClientType.Facebook);

ClientType could be a static property for each subtype, and then subtypes could be selected by its value in the factory:

// Create array of possible client subtypes
Type[] clients = new[] { typeof(FacebookClient), typeof(PinterestClient), typeof(TwitterClient) };

// Select subtype with URL that matches one passed to factory
Type client = clients.SingleOrDefault(c => clientType == (Client)c.GetProperty("ClientType").GetValue(null));
return (Client)Activator.CreateInstance(client, parameters);

However, since static members can't be abstract, I'm not sure of a way to ensure that ClientType is set, and there are dozens of subtypes.

Is there a way to force ClientType to be set? Would another way of doing this (e.g., a switch statement) be a better option?

Answer 1

It looks like you're combining a factory with a service-locator.

Anyway, you can build a Dictionary<string,Type> instead of the array of types and querying them for which Url they respond to. This is even more flexible, because then you can allow your application to register new Clients at runtime, if that ever becomes necessary.