Don't allow different option

Question

I got stuck with a small problem. I create a program where the user must to choose 5 options( 1.input numbers 2. Show smallest 3. Show Greatest 4. Display all numbers 5 Quit.) All the option are working. The problem is that when the user is choosing the option, if the press any letters is giving me error. I want that if the user is pressing any letter ori any symbol to be a warning message like "Unknown option value entered" and to try again. I suppose is about conversion or something similar, but i can't find where is the problem. Here is my code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Assignmen2014_15
{
    class Program
    {
    const int MAXNUMBERS = 3;
        static void Main(string[] args)
        {
        int[] theNumbers = new int[MAXNUMBERS];
        int chosenOption = 0;
        bool quit = false;

        InitialiseNumbers(theNumbers);
        while (quit == false)
            {
            DisplayHeader();
            DisplayMenu();
            chosenOption = ReadNumber("Please choose an option: ");
            quit = ProcessMenu(chosenOption, theNumbers);
            Console.Clear();
            }
        }
        static void InitialiseNumbers(int[] numbers)
        {
            for (int index = 0; index < MAXNUMBERS; index++)
            {
            numbers[index] = 0;
            }
        }
        static void DisplayHeader()
        {
        WriteText("*******************************************************************************", 0, 0); // Top left hand corner of screen is x = 0, y = 0;
        WriteText("* This application is designed to allow you to choose numbers *", 0, 1); // Next line down is x = 0, y = 1, etc WriteText("* and finds the biggest and the smallest value *", 0, 2);
        WriteText("*******************************************************************************", 0, 3);
        }
        static void DisplayMenu()
        {
            WriteText("Select an option", 20, 8); // Display menu at at x = 20, y = 8
            WriteText("1. Enter the numbers", 20, 9);
            WriteText("2. Find the smallest", 20, 10);
            WriteText("3. Find the largest", 20, 11);
            WriteText("4. Display all numbers", 20, 12);
            WriteText("5. Quit", 20, 13);
        }
        static void WriteText(string text, int x, int y)
        {
            Console.CursorLeft = x;
            Console.CursorTop = y;
            Console.Write(text);
        }
        static int ReadNumber(string prompt)
        {
            string text;
            int number;
            WriteText(prompt, 20, 14);
            text = Console.ReadLine();
            number = int.Parse(text);
            ClearText(20, 14, prompt.Length + text.Length); // Clear the text at this line
            return number;

        }
        static void ClearText(int x, int y, int length)
        {
            // Write space ' ' characters starting at x, y for 'length' times
            WriteText(new String(' ', length), x, y);
        }

        static void DisplayNumbers(int[] theNumbers)
        {
            Console.Write("Your numbers are: ");
            for (int i = 0; i < MAXNUMBERS; i++)
            {
                Console.WriteLine(theNumbers[i]);
            }

        }

        static bool ProcessMenu(int option, int[] numbers)
        {
            bool quit = false;

            switch (option)
            {
                case 1:
                    GetNumbers(numbers);
                    break;
                case 2:
                    WriteText(string.Format("The smallest value is {0}", FindSmallest(numbers)), 20, 15);
                    Console.ReadKey(); // Pause
                    break;
                case 3:
                    WriteText(string.Format("The largest value is {0}", FindLargest(numbers)), 20, 15);
                    Console.ReadKey(); // Pause
                    break;
                case 4:
                    DisplayNumbers(numbers);
                    Console.ReadKey();
                    break;
                case 5:
                    quit = IsQuitting();
                    break;
                default:
                    WriteText("Unknown option value entered", 20, 15);
                    Console.ReadKey(); // Pause 
                    break;
            }
            return quit;
        }
        static void GetNumbers(int[] numbers)
        {
            for (int index = 0; index < MAXNUMBERS; index++)
            {
                numbers[index] = ReadNumber("Enter number: ");
            }
        }
        static int FindSmallest(int[] numbers)
        {
            int smallest = numbers[0];
            for (int index = 0; index < MAXNUMBERS - 1; index++) // <-- subtract 1
            {
                if (numbers[index + 1] < smallest)
                {
                    smallest = numbers[index + 1];
                }
            }
            return smallest;
        }
        static int FindLargest(int[] numbers)
        {
            int largest = numbers[0];
            for (int index = 0; index < MAXNUMBERS - 1; index++) // <-- subtract 1
            {
                if (numbers[index + 1] > largest)
                {
                    largest = numbers[index + 1];
                }
            }
            return largest;
        }
        static bool IsQuitting()
        {
            string response;
            bool quit = false;
            WriteText("Do you really wish to quit? ", 20, 13);
            response = Console.ReadLine();
            if (response.Equals("Yes" , StringComparison.InvariantCultureIgnoreCase) || response.Equals("Y", StringComparison.InvariantCultureIgnoreCase))
            {
                quit = true;
            }
            return quit;
        }
    }
}

Answer 1

Use Int32.TryParse(String, Int32) method which tries to parse string into integer and returns boolean value representing if parsing was successful or not. In your case, if parsing fails, you can return -1 from your ReadNumber method which will call default: part of your switch case and an error message will be displayed. Otherwise, if parsing was successful, you can simply return the parsed number, which will be either be one of your desired numbers or it will call default: action.

The following is example from Microsoft documentation

String[] values = { null, "160519", "9432.0", "16,667",
                      "   -322   ", "+4302", "(100);", "01FA" };
foreach (var value in values) 
{
   int number;

   bool success = Int32.TryParse(value, out number);
   if (success)
   {
      Console.WriteLine("Converted '{0}' to {1}.", value, number);         
   }
   else
   {
      Console.WriteLine("Attempted conversion of '{0}' failed.", 
                         value ?? "<null>");
   }
}

In your specific example you need to modify your ReadNumber(string prompt) method:

static int ReadNumber(string prompt)
{
    string text;
    int number;
    WriteText(prompt, 20, 14);
    text = Console.ReadLine();
    bool is_parsing_successful = Int32.TryParse(text, out number);
    ClearText(20, 14, prompt.Length + text.Length); // Clear the text at this line
    if(is_parsing_successful){
        return number;
    } else {
        return -1;
    }
}