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If, for any reason, you want to detach (remove) the code you have written in an event handler, you have to set the event handler to nil.

procedure TForm1.Button1Click(Sender: TObject) ;
begin
  ShowMessage('Who clicked?') ;
  //remove event handler from event
  Button1.OnClick := nil;
end;

To re-assign the procedure Button1Click to the button's OnClick event, you should use:

Button1.OnClick := Button1Click;