xFunc is a simple and easy to use application that allows you to build mathematical and logical expressions. It's written on C#. This project consists two libraries and an execution file. The libraries include the code that converts strings to expressions.
Install xFunc libraries using NuGet
or download from CodePlex
The source code and the issue tracket are also available on GitHub
Here is a short description: http://e-x-it.blogspot.com/2012/02/c.html