What's the difference between an abstract class and a static one?

Question

Neither is instantiable. What are the differences, and in what situations might you use one or the other?

Answer 1

Abstract classes are intended to be used as base classes; they cannot have direct instances. Instead, you have to derive subclasses, which provide the what was (usually intentionally) left out in the abstract base class.

Example: consider you have a complex application, where users may log-in to. Various authentication mechanisms should be usable, say, LDAP, NTLM, you name it. One way to model a "user" or "principal" in such a context would be to collect, what is common across all those mechanisms, into an abstract base class, and leave "gaps" (abstract methods) where the actual implementations come into play:

abstract class Authenticator {

   protected Dictionary userCache;

   ...

   public User LoadUser(string name) {
        User user;
        if( userCache.TryGet(name, out user) ) return user;
        else {
            user = LoadFromStore(name);
            userCache.Add(name, user);
            return user;
        }
   }

   protected abstract User LoadFromStore(string name);
}

Here, caching of users is a common concern, modelled in the base case, whereas the actual retreival is left for a subclass to provide.

Static class are a different matter alltogether. They are essentially a place to keep your utility functions:

static class StrUtil {

    public static string TrimWhitespace(string str) {
        ...
    }
}

Think of them as some kind of special namespace, which can only contain static members. Basically, a place to put functions.