Google is your friend: Be nice and visit him often. He can answer questions a lot more quickly than posting them here...
A very quick search would have found you loads of examples.
c# evaluate expression - Google Search[
^] gives you 88,000 examples ...
Here's a good example:
State of the Art Expression Evaluation[
^]
In future, please try to do at least basic research yourself, and not waste your time or ours.