Is it possible to write syntax like - ()()?

Question

I read in an ebook somewhere (which I'm desperate to find again), that, by using delegates, it is possible to write code which has syntax as follows:

 ()(); // where delegate precedes this.

Can anyone provide any details how this would be possible/in what situation this would occur?

Answer 1

You can do slightly better than the examples given so far, in fact... you can extend it arbitrarily:

class Test
{
    delegate Hofstadter Hofstadter();

    static void Main()
    {
        // Unfortunately I'm clearly not as smart as the real thing
        Hofstadter douglas = () => null;

        douglas()()()()()()();
    }
}

And here's another horrible alternative, for extra ASCII art:

class Test
{
    delegate __ ___();
    delegate ___ __(___ _);

    static void Main()
    {
        ___ _ = () => null;

        _ ()((_))();
    }
}

Please never ever, ever do this.

EDIT: One last one - although it's as much about just replacing things with underscores as anything else, and reusing names wherever possible:

class Test
{
    delegate void _();
    delegate __<_> ___<_>();
    delegate ___<_> __<_>(___<_> ____);

    static ___<_> ____<_>(___<_> ____) { return ____; }
    static __<_> ____<_>() { return ____<_>; }

    static void Main()
    {
        ((__<_>)____)(____<_>)();
    }
}

Answer 2

Here's a sample program that demonstrates this:

using System;

class Program
{
    static Action GetMethod()
    {
        return () => Console.WriteLine("Executing");
    }
    static void Main()
    {
        GetMethod()();
        Console.ReadKey();
    }
}

That being said, I wouldn't ever do this in production code. It's very unexpected.

---

Edit: Just in case you want to see something even uglier... [especially the "()()[()=>{}]()"]:

using System;

class Program
{
    static void Main()
    {
        (new Program()).Confusion();
        Console.ReadKey();
    }

    public Action this[Action index]
    {
        get {
            return () => Console.WriteLine("Executing");
        }
    }

    Func<Program> GetInstance()
    {
        return () => this;
    }

    void Confusion()
    {
        // This is particularly ugly...
        GetInstance()()[()=>{}]();
    }
}

Answer 3

You just need a little self referencing, and you can call it as many times as you like:

delegate Foo Foo();

class Program {
    static void Main(string[] args) {
        Foo f = null;
        f = () => f;
        // Add more "()" as you feel like...
        f()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()()();
    }
}

Answer 4

static void Foo()
{
    Console.WriteLine("Hello World!");
}

static Action Bar()
{
    return new Action(Foo);
}

static void Main()
{
    Func<Action> func = new Func<Action>(Bar);
    func()();

    Bar()();
}

prints

Hello World!
Hello World!

This works, because func() and Bar() return an Action delegate which can be invoked using normal method invocation syntax.

Answer 5

Something like:

delegate void FunctionA();
delegate FunctionA FunctionB();

void TestA() { }
FunctionA TestB() { return TestA; }

void Main()
{
   TestB()();
}