code:

public interface PubsubEvent {}

public class PubSub {

    private class SubscriberSet<T> : HashSet<Action<T>> where T : PubsubEvent { }

    private static Dictionary<Type, SubscriberSet<PubsubEvent>> subscribers = new Dictionary<Type, SubscriberSet<PubsubEvent>>();
    public static void Subscribe<T>(Action<T> callback) where T: PubsubEvent {
        var typ = typeof(T);
        if (!subscribers.ContainsKey(typ)) {
            subscribers[typ] = new SubscriberSet<PubsubEvent>();
        }
        subscribers[typ].Add(callback);
    }
    public static void Unsubscribe<T>(Action<T> callback) where T: PubsubEvent {
        var typ = typeof(T);
        var cb = callback as Action<PubsubEvent>;
        if (!subscribers.ContainsKey(typ) || !subscribers[typ].Contains(cb)) {
            throw new System.Exception("Action " + callback + " not subscribed.");
        }
        subscribers[typ].Remove(cb);
    }
    public static void Publish<T>(T e) where T: PubsubEvent {
        if (!subscribers.ContainsKey(typeof(T))) {
            // Nobody to publish to.
            return;
        }
        foreach (var f in subscribers[typeof(T)]) {
            Debug.Log("Invoking " + f + " on " + e);
            f(e);
        }
    }
}