.Net 4.0 Action<in T> not works for me

here is what I'm trying to do

IDictionary<Type, Action<BaseClass>> handlers = new Dictionary<Type, Action<BaseClass>>();

public void Register<T>开发者_JS百科;(Action<T> handler) where T:BaseClass
{
    handlers.Add(typeof(T),handler);
}

Its not compiling

Is it possible in another way?

Covariant parameters work in the other direction.

Action<in T> allows you to write

Action<Animal> baseProcessor = ...;
Action<Cat> catProcessor = baseProcessor;

Since all Cats are also Animals, this is legal.

Had your code been possible, what would happen if I write

Action<Cat> catProcessor = ...;
Action<Animal> baseProcessor = catProcessor;
baseProcessor(new Dog());

We just passed a Dog to a function that takes a Cat.

IDictionary<Type, Action<BaseClass>> handlers = new Dictionary<Type, Action<BaseClass>>();

public void Register<T>(Action<T> handler) where T:BaseClass
{
Action<BaseClass> action = x=>handler((T)x);
    handlers.Add(typeof(T),action);
}