Question

I have next snippet:

public Action<Action<bool>> GetAction()
{
    return m => MyMethod(123, "string", m);
}

private void MyMethod(int someInteger, string someString, Action<bool> boolAction)
{
    // some work with int and string was done
    boolAction(true);
}

Could you please explain me why this work? I see that Action<Action<bool>> need some void method with only one parameter of Action<bool>. So what is wrong here with two first arguments?

Also it’s not clear for me why we pass m into. How this lambda could be called in the boolAction(true). What will happen there?

Any advice on this will be helpfull.

Answer 1

No reason that it shouldn’t work. In line where lambda is created, C# is automatically infers which parameters your lambda will receive, from type of GetAction return value. To understand this code it’s important that you see that you are not returning m, but you are returning

m => MyMethod(123, "string", m);

Therefore, m is of type Action<bool>, and above expression is of type Action<Action<bool>>, where inner Action is actually m.

I.e.

m => MyMethod(123, "string", m);

creates lambda expression which would correspond to method of this signature:

void _no_name(Action<bool> m) {
  MyMethod(123, "string", <delegate_to_no_name>);
}

From this part we see that m is Action<bool>, and _no_name is Action<Action<bool>>.

In the end you would use this code somehow like this probably:

Action<Action<bool>> action = GetAction();
action(x => MessageBox.Show("X of type bool is " + x.ToString()));

Effectively, our message box call delegate becomes the m parameter.