operators as strings

The way I see it, you have two options - use an expression evaluator or construct, compile and run C# code on the fly.

I would go with an expression evaluator library, as you do not have to worry about any security issues. That is, you might not be able to use code generation in medium trust environments, such as most shared hosting servers.

Here is an example for generating code to evaluate expressions: http://www.vbforums.com/showthread.php?t=397264

All the other answers are possible overkill.

If all you need is simple arithmetic, do this.

        DataTable dummy = new DataTable();
        Console.WriteLine(dummy.Compute("15 / 3",string.Empty));

EDIT: a little more information. Check out the MSDN documentation for the Expression property of the System.Data.DataColumn class. The stuff on "Expression Syntax" outlines a list of commands you can use in addition to the arithmetic operators. (ex. IIF, LEN, etc.). Thanks everyone for voting up my first posted answer!