Modularity

 static void Main(string[] args)
        {
            Char menuItem;
            Console.WriteLine("Welcome to ze calculator");
            menuItem = GetMenuItem();
            while (menuItem != 'X')
            {
                ProcessMenuItem(menuItem);
                menuItem = GetMenuItem();
            }// while
            Console.WriteLine("Goodbye, user");
            Console.ReadLine();
        }// Main
        
        //      OUT      IN
        static Char GetMenuItem()
        {
            Char menuItem;
            DisplayMenu();
            menuItem = Char.ToUpper(Char.Parse(Console.ReadLine()));
            while (menuItem != 'F' && menuItem != 'C' && menuItem != 'I' && menuItem != 'X')
            {
                Console.WriteLine("Invalid menu selection");
                DisplayMenu();
                menuItem = Char.ToUpper(Char.Parse(Console.ReadLine()));
            }// while
            return menuItem;
        }// GetMenuItem
    
        //      OUT      IN
        static void DisplayMenu()
        {
            Console.WriteLine(); // Writes a blank line
            Console.WriteLine("Enter one of the following items: ");
            Console.WriteLine("F - Factorial\nC - Combination\nI - Fibbonacci\nX - Exit\n");
        }// DisplayMenu
 
        //      OUT       IN
        static void ProcessMenuItem(Char MenuItem)
        {
            switch (MenuItem)
            {
                case 'F': 
                    ProcessFactorial();
                    break;
                case 'I':
                    ProcessFibonacci();
                    break;
                case 'C':
                    ProcessCombination();
                    break;
             }//switch
            //Console.WriteLine("ProcessMenuItem");
        }//ProcessMenuItem

        //      OUT       IN
        static void ProcessFactorial()
        {
            Int32 n, nFactorial;
            n = GetPositiveInteger("Enter positive n value");
            nFactorial = Factorial(n);
            Console.WriteLine(n + " factorial is " + nFactorial);
        }//ProcessFactorial

        //      OUT       IN
        static void ProcessFibonacci()
        {
            Int32 n, nthFibonacci;
            n = GetPositiveInteger("Enter position in sequence");
            nthFibonacci = Fibonacci(n);
            Console.WriteLine("The " + n + " number in the sequence is " + nthFibonacci);
        }//ProcessFibonacci

        //      OUT       IN
        static void ProcessCombination()
        {
            Int32 n, r, nChooseR;
            n = GetPositiveInteger("Enter number of objects to choose from");
            r = GetPositiveInteger("Enter number of objects to choose");
            while (r > n)
            {
                Console.WriteLine("Error - must be less than or equal to " + n);
            }//while
            nChooseR = Combination(n, r);
            Console.WriteLine("There are " + nChooseR + " ways to choose " + r + " of " + n + " objects");
        }//ProcessCombination

        //      OUT       IN
        static Int32 Factorial(Int32 n)
        {
            Int32 nFactorial = 1;
            for (; n > 1; n--)
                nFactorial *= n;
            return nFactorial;
        }//Factorial

        //      OUT       IN
        static Int32 Fibonacci(Int32 n)
        {
            Int32 count = 0, current = 0, previous = 1, previousPrevious = 0;
            for (; count < n; count++)
            {
                previousPrevious = previous;
                previous = current;
                current = previous + previousPrevious;
            }
            return current;
        }//Fibonacci

        //      OUT       IN
        static Int32 Combination(Int32 n, Int32 r)
        {
            return Factorial(n) / (Factorial(r) * Factorial(n - r));
        }//Combination

        static Int32 GetPositiveInteger(String prompt)
        {
            Int32 n;
            Console.WriteLine(prompt);
            n = Int32.Parse(Console.ReadLine());
            while (n < 0)
            {
                Console.WriteLine("Input Error - Input must be positive");
                Console.WriteLine(prompt);
                n = Int32.Parse(Console.ReadLine());
            }//while
            return n;
        }//GetPositiveInteger