Thread-safe updates of a WinForm control from other classes

Question

Could someone please help me with the following problem:

There are two classes MainForm and LWriter. Below is a method from the LWriter that in addition to writing

Answer 1

Interactions with UI controls must be done on the UI thread. You can build the string on a background thread, but you should use Control.Invoke or Control.BeginInvoke to marshal to the UI thread before interacting with it.

Lots of examples of this on the net and stack overflow.