How to make sure an int doesn't crash an application when answered with a string

Question

I have a console app with an int variable. I need to read input from the user and be sure I can tell the user that they have entered an invalid answer if the input can't convert to an integer value.

How can I do this? Here is what I have so far:

class Program
{
    static void Main(string[] args)
    {
    Start:
        Console.WriteLine("Enter two numbers");
        var input1 = Convert.ToInt32(Console.ReadLine());
        var input2 = Convert.ToInt32(Console.ReadLine());
        if (input1 + input2 == 3)
            Console.WriteLine("");
        else if (input1 + input2 == 10)
        {
            Console.WriteLine("");
        }
        else
            Console.WriteLine("");
        goto Start;
    }
}

Use `int.TryParse(...)` instead of `Convert.ToInt32`, this will allow you to react to the invalid input. — Lasse V. Karlsen, Dec 06 '18 at 19:48
Also... there are times when `goto` is helpful, but a new programmer should NOT use it. — Joel Coehoorn, Dec 06 '18 at 19:51
Thanks for your quick response, wasn't actually excepting an answer. I will try that. Thank you! — kernalMeaas, Dec 06 '18 at 19:51

score 4 · Answer 1 · answered Dec 06 '18 at 19:55

Convert.ToInt32(string) will throw exceptions, just as you mention, when the input cannot be parsed.

Instead, use int.TryParse(string, out var result).

In your code, it would be

if (!int.TryParse(Console.ReadLine(), out var input1 ||
    !int.TryParse(Console.ReadLine(), out var input2))
{
    Console.WriteLine("You did not enter integers.");
    goto Start;
}

You can read more about the method in the .NET documentation

P.s. By the way, you should avoid using goto in your code. But that's another matter.

score 1 · Answer 2 · answered Dec 06 '18 at 19:58

1

I would recommend using int.TryParse (documented here).

It will return false if the input is not an integer.

Ex:

int num1;
string input = Console.ReadLine();
if(!int.TryParse(input, out num1))
{
    Console.WriteLine("that's not a valid integer");
}

answered Dec 06 '18 at 19:58

nick

2,833
3
34
61

score -1 · Answer 3 · answered Dec 06 '18 at 19:54

-1

use regular expression, Regex is best for c# to check number format. look at this:

Regex for numbers only

answered Dec 06 '18 at 19:54

Ali Mardan

147
5

int.TryParse is better since you don't need to write your own logic to validate number formats. – Adam B Dec 06 '18 at 19:58
1

No, Regex is a hammer and not everything is a nail. – Austin T French Dec 06 '18 at 20:22

How to make sure an int doesn't crash an application when answered with a string

3 Answers3