将void转换为string - void to object issue？

I have the following multi threaded code, that take a list of servers and async runs the heavyoperation, the heavy operation returns the results and im trying to the populate a multiline text box with the results ( either as a batch when completed or as each task returns ), i have add some code to hopefully perform this task but the three lines below with comments on the right cause we confusion regarding void to static and void to objects :( any hints greatly received as to what i require changing.... ThanksI have the following multi threaded code, that