How to create an expression evaluator?

Question

I am creating a program that will allow the user to create and execute their own expressions.
Looking into it, I found the Compute of DataTable.
Ex:

DataTable dt = new DataTable();
var exp = dt.Compute("10 * (12+4) / 3", "");

With this command the user can type only mathematical formulas, but not for example a code for him (user) create a text box at runtime.
Ex:

DataTable dt = new DataTable();
var exp = dt.Compute(@"MessageBox.Show(""Teste de messagebox"", ""Título"");", "");

How can I do that?
If possible, without any external bookstore.