Using the compiler to do implies memory leaks as the generated assemblies are loaded and never released. It's also less performant than using a real expression interpreter. For this purpose you can use Ncalc which is an open-source framework with this solely intent. You can also define your own variables and custom functions if the ones already included aren't enough.
Example:
Expression e = new Expression("2 + 3 * 5");
Debug.Assert(17 == e.Evaluate());
