Notify when event from another class is triggered [duplicate]

Answer 1

You need three things (which is marked by comments in code):

1. Declare event in class B
2. Raise event in class B when something happened (in your case - Button_Click event handler executed). Keep in mind that you need to verify if there are any subscribers exists. Otherwise you will get NullReferenceException on raising event.
3. Subscribe to event of class B. You need to have instance of class B, which even you want to subscribe (another option - static events, but those events will be raised by all instances of class B).

Code:

class A
{
    B b;

    public A(B b)
    {
        this.b = b;
        // subscribe to event
        b.SomethingHappened += MyMethod;
    }

    private void MyMethod() { }
}

class B
{
    // declare event
    public event Action SomethingHappened;

    private void Button_Click(object o, EventArgs s)
    {
         // raise event
         if (SomethingHappened != null)
             SomethingHappened();

         SomeMethod();
    }

    public void SomeMethod() { }
}