C# validation of input

Question

I am creating a short c# console program that will ask 10 addition questions using random numbers from 0-10. Then it tells the user how many correct or incorrect. I am trying to find a way to validate that my user input is a number and not a letter. I am posting the code I have created so far, but could use some help.

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

namespace ConsoleApplication1
{
  class Program
  {
      static void Main(string[] args)
      {            

         int i = 0;
         int input;
         int correct = 0;
         int incorrect = 0;
         int questions = 10;
         int[] solutions = new int[21];
         int[] answers = new int[21];
         int[] number1 = new int[21];
         int[] number2 = new int[21]; 

         Random number = new Random();

         Console.WriteLine("  This is a test of your basic addition skills. ");
         Console.WriteLine("  Please answer the random addition question below ");
         Console.WriteLine("  with a number from 1 - 20 and press enter to get the");
         Console.WriteLine("  next of 10 questions.  At the end of the questions");
         Console.WriteLine("  your results will be calculated and presented to you.");
         Console.WriteLine("");
         Console.WriteLine("");


         while (i < questions)
         {
            number1[i] = number.Next(0, 10);
            number2[i] = number.Next(0,10);
            solutions[i] = (number1[i] + number2[i]);

          //Console.WriteLine("{0} +  {1} =  {2}", number1[i], number2[i],solutions[i]);  


            Console.Write("  {0} +  {1} =  ", number1[i], number2[i]);


            answers[i] = Convert.ToInt32(Console.ReadLine()); // original code      

            //input = Convert.ToInt32(Console.ReadLine());
            //if (input > 0 && input <21)
            //{
            //   Console.WriteLine("YOur answer is: {0}", input);
            //}
            //else
            //Console.WriteLine("YOur answer is not valid");


            if (solutions[i] == answers[i])
            {
               Console.WriteLine("  Correct");
               correct++;
            }
            else
            {
               Console.WriteLine("  Your answer is incorrect, the correct answer is {0}", solutions[i]);
               incorrect++;

            }
            //Console.WriteLine("{0}", answers[i]);


            //int sum = numberone + numbertwo;
            //answers[sum]++;
            i++;
      }
         Console.WriteLine("");
         Console.WriteLine("");
         Console.WriteLine("The number correct is: {0}, The number incorrect is: {1}", correct, incorrect);
      }
      }
   }

Answer 1

Use int.TryParse() like:

bool isNumber=false;
int number;
while (!isNumber)
{
  string txt = Console.ReadLine();
  if (!int.TryParse(txt, out number))
  {
    // not a number, handle it
    Console.WriteLine("This is not a number, enter a number. For real now.");
  }
  else
  {
    // use number
    answers[i] = number;
    isNumber = true;
  }
}

Answer 2

int iResult = int.MinValue;
bool bParsed = int.TryParse("xyz", out iResult);

TryParse will not throw an exception.

However, you can use Convert.ToInt32() as well if needed but that will throw an exception on bad data.

Answer 3

This allows you to fill all positions of your array:

int result;
bool isInt = Int32.TryParse(Console.ReadLine(), out result);
if (!isInt) {
  Console.WriteLine("Your input is not an integer number.");
  continue;
}

answers[i] = result;

Answer 4

Instead of:

answers[i] = Convert.ToInt32(Console.ReadLine()); // original code

Use:

int input;
bool validInput = int.TryParse(Console.ReadLine(), out input);
if (!validInput || input < 0 && input > 20)
    <throw exception or display some error message here...>

EDIT: If you want to recursively ask for a correct input, this is how you can do it:

int input;
bool validInput = false;

while (!validInput)
{
    validInput = int.TryParse(Console.ReadLine(), out input);
    if (!validInput || input < 0 && input > 20)
    {
     validInput = false; // We need to set this again to false if the input is not between 0 & 20!
     Console.WriteLine("Please enter a number between 0 and 20");
    }
}