using System;
using System.Collections.Generic;
using System.Data;

namespace ConsoleApplication6
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            const string testExpression1 = "2 + 3 * 2";
            const string testExpression2 = "123.45-15.00-40.00";

            Console.WriteLine(testExpression1 + " = " + Evaluate(testExpression1));
            Console.WriteLine(testExpression2 + " = " + Evaluate(testExpression2));

            Console.ReadLine();
        }

        public static object Evaluate(String inputExpression)
        {
            var dt = new DataTable();
            return dt.Compute(inputExpression, string.Empty);
        }
    }
}

2 + 3 * 2 = 8
123.45-15.00-40.00 = 68,45