You can...but I wouldn't.
Reflection.Emit can do what you ask for: http://www.c-sharpcorner.com/uploadfile/puranindia/reflection-and-reflection-emit-in-C-Sharp/
However, it is a big security risk - depending on what your users can type, is what they can run:
Process.Start(@"Format c: /Y");
and such like would be rather nasty...
If you want to evaluate expressions they type, they why not use this: Math Parser .NET C#