Retrying until the user input is correct

Question

I'm learning C# via Rob Miles Yellow Book and have hit a problem with understanding variable scope.

The class I have developed (at the bottom) asks for a number and then displays it back to user. I would like to enhance it so that if the user enters ten instead of 10 it outputs a message and the user has another attempt.

Here is one of several attempts so far at the change...

...

    static int readInt(string prompt, int low, int high)
    {
        int result;

        do
        {
            try
            {
                string intString = readString(prompt);
                result = int.Parse(intString);
                Console.WriteLine("Thank you");
            }
            catch
            {
                Console.WriteLine("Invalid age value");
            }

        } while ((result < low) || (result > high));

        return result;
    }
...

Unfortunately this has the effect (I think) of pushing result out of scope because with this change I see the following compliation error:

CS0165 Use of unassigned local variable 'result'

What is the C# way to achieve this?

The complete class:

using System;

class Exception1
{
    static string readString(string prompt)
    {
        string result;

        do
        {
            Console.Write(prompt);
            result = Console.ReadLine();
        } while (result == "");

        return result;
    }

    static int readInt(string prompt, int low, int high)
    {
        int result;

        do
        {            
            string intString = readString(prompt);
            result = int.Parse(intString);            
        } while ((result < low) || (result > high));

        return result;
    }


    public static void Main()
    {
        const int SCORE_SIZE = 1;

        int[] scores = new int[SCORE_SIZE];

        for (int i = 0; i < SCORE_SIZE; i++)
        {
            scores[i] = readInt("Score : ", 0, 1000);

        }

        for (int i = 0; i < SCORE_SIZE; i++)
        {
            Console.WriteLine("Score " + i + " is " + scores[i]);
        }

        Console.Read();
    }
}    

Answer 1

The compiler doesn't know that the while loop actually runs (it can throw and exception before assigning result) and sets a value on result. Therefore it complaints about this.

You can circumvent this by assigning a default value to result:

int result = -1;

Or by setting a value in the catch block:

catch
{
    result = -1;
    Console.WriteLine("Invalid age value");
}

I always use -1 in these cases as indicator of 'something wrong with this value'.

Answer 2

Just wanted to add possible solution if you really don't want to set result to any value initially:

static int readInt(string prompt, int low, int high)
{            
    bool validInput;
    int result;

    do
    {
        if ((validInput = int.TryParse(readString(prompt), out result)))
            Console.WriteLine("Thank you");
        else
            Console.WriteLine("Invalid age value");

    } while (!validInput || (result < low) || (result > high));
}

If TryParse fails, result will be set to integer default value (0) and validInput variable will be set to false so loop will go through once again.