Create a class to stop Bob fiddling with your variables _number1 and _number2.

    // Problem we want to solve is "How do we sum two numbers?"
    class Adder
    {
        // Attributes

        // Begin by defining the values we need to solve this problem
        // Two numbers
        private int _number1;       // Underscore in front of attribute names is "Mikes Convention"
        private int _number2;

        // Constructors
        public Adder(int num1, int num2)
        {
            // Provide initial values for solving the problem
            _number1 = num1;
            _number2 = num2;
        }

        // Method -- a function within a class

        // Solve the problem
        public int AddThem()
        {
            return _number1 + _number2;
        }
    }

Bob uses class like this:

    class Program
    {
        static void Main(string[] args)
        {
            Adder sum1;
            Adder sum2;
            int result;


            sum1 = new Adder(1, 2);     // Add 2 and 3
            sum2 = new Adder(32, 44);   // Add 32 and 44

            result = sum1.AddThem();
            Console.WriteLine("Sum1 is {0}", result);

            result = sum2.AddThem();
            Console.WriteLine("Sum2 is {0}", result);

            Console.ReadLine();
        }
    }