How to prevent certain forms of input when writing methods?

Question

I'm still a beginner to programming in high-level programming languages, so I don't know if this is an easy solution, but I'm happy to learn anyway. I've programmed a little alarm program in C# that let's the user input in how many seconds the alarm needs to go off. It works perfectly, but the input that the user needs to give has to be a number. When the user inputs any form of text, the program crashes. Now, how can I prevent that users input text, and call a function or do something else when the user does, instead of the program just crashing?

This is the code I have now:

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

namespace Test
{
    class Alarm
    {
        public static void play()
        {
            int sec;
            sec = Convert.ToInt16(Console.ReadLine());

            for (int i = 0; i < seconds; ++i)
            {
                System.Threading.Thread.Sleep(1000);
            }

            for (int i = 0; i < 10; i++)
            {
                Console.Beep();
            }
        }
    }
}

Answer 1

You should complete a check before converting:

int sec;

if (int.TryParse(Console.ReadLine(), out sec)
{
  // this is valid continue
}
else
{
  // show error highlighting that entry must be a number
}

int.TryParse will return a boolean highlighting whether the input is parsable to an int. It will also set your sec variable to the value if successful.

Answer 2

You could loop indefinitely until the user inputs a number:

int number = 0;
while(!Int32.TryParse(Console.ReadLine(), out number))
{
    Console.WriteLine("Please input a number.");
}

Int32.TryParse returns false if the conversion failed, instead of throwing an exception and if successful returns the result in the second out parameter.

Answer 3

Already people answered here. I love to make that as an extension method so that i can call it in so many places.

public static bool IsNumeric(this string theValue)
{
  long retNum;
  return long.TryParse(theValue, System.Globalization.NumberStyles.Integer, 
            System.Globalization.NumberFormatInfo.InvariantInfo, out retNum);
}

Then I will call it like this

if(Console.ReadLine().IsNumeric())
{
    //The value is numeric. You can use it    
}