What does Action<Action> mean?

I just saw a brand-new video on the Rx framework, and one particular signature caught my eye:

Scheduler.schedule(this IScheduler, Action<Action>)

At 23:55, Bart de Smet says:

The earliest version wou开发者_如何学Pythonld be Action of Action.

If Action is a parameterized type, how can it appear unparameterized inside the angle brackets again? Wouldn't it have to be Action<Action<Action<...>>> ad infinitum, which is obviously impossible?

Action<T> describes a delegate that takes a single parameter of type T. Action describes a delegate that takes no parameters.

See http://msdn.microsoft.com/en-us/library/system.action.aspx

Action has several overloads. One is non-generic, the others take one, two, three, etc. type parameters. Suppose they had different names, the one-argument version being called Action1, and the zero-argument (non-generic) being called Action0, then the example would be Action1<Action0>.

From MSDN:

• Action
• Action<T>

Action example

Action showMethod = () => { Console.WriteLine("Line"); };

showMethod();

Action<T> example

Action<int> showMethod = (i) => { Console.WriteLine("Line {0}", i); };

showMethod(1);

Action has a non-generic version with the signature:

public delegate void Action();

So it is an Action that takes an Action of type void. Looks funny, but is perfectly valid.

Default parameter seems like the easy solution here.