Massive Confusion about Delegates

[Sonhospa]

Hi all, it's crazy but even after reading for hours I can't use delegates (with callback function) properly. In a small experimental application I only want that a particular class ("textfile", more or less a reader) updates the UI of a form. My simple question: In which of the classes will I have to a) define / b) instantiate / c) invoke the callback function? I guess I've tried all the different ways I know, but either the Delegate can't be accessed in one of the classes, or it is NOTHING at runtime. Thank you in advance for your advice! Regards Mick