I am facing one issue with linq query in c# , my linq query as per below
list = (from row in dt.AsEnumerable() select new Perfmon
You can also use Dynamic Linq. The essence is to parse your string and convert it in to expression trees. See here for more details
http://weblogs.asp.net/rajbk/archive/2007/09/18/dynamic-string-based-queries-in-linq.aspx