如何在c＃中将值连接到逗号分隔值

I am reading text file where I would like to get values based on condition. First I will take FREQ where CELLID = 639 and ISMAINBCCH=YES,that I have done now next task is I have to concatenate FREQ values in a comma separated way where CELLID=639 and ISMAINBCCH=NO, so the output I want is 24,28,67. How to achieve that?I am reading text file where I would like to ge