Handling exceptions and executing code if one was found

Question

I have this code:

try
{
 Console.WriteLine("Input number {0}", i + 1);
 int number = ReadNumber(start, end);
}
catch (Exception ex)
{
 if (ex is ArgumentException)
 {
 Console.WriteLine("The input number wasn't in the alowed range. Please try again.");
 }
 else if (ex is FormatException)
 {
 Console.WriteLine("The input isn't an integer number. Please try again.");
 }
 else
 {
 Console.WriteLine("An unknown error occured. Please try again.");
 }
 i--;
}

I am wondering if there is a better way to do this - catch all exceptions and handle them and then, if any exception was caught, I want to reduce i by 1.

• Selman Genç: you can use different catch blocks for different type of exceptions
• frostblooded: I don't think you understood, I want to be able to catch all exceptions and handle them, BUT then have a code that is executed after all catch-es and only if one of them was entered.
• eddie_cat: So you want i-- to happen only if an exception is caught, or you want it to happen regardless?
• frostblooded: Only if an exception was caught :) And it works the way I did it, but I was wondering if there was a better way to do it. If there is a special operator to do it or something like that.
• eddie_cat: This looks fine to me.

Answer 1

So I'm assuming that all of this code is in a for loop, and that for loop is incrementing i as it's loop statement.

What you can do here is not unconditionally increment i in each iteration of the loop, and instead only increment i if you're able to read in the number successfully:

for (int i = 0; i < 10; )
{
 try
 {
 Console.WriteLine("Input number {0}", i + 1);
 int number = ReadNumber(start, end);
 i++;
 }
 catch (ArgumentException)
 {
 Console.WriteLine("The input number wasn't in the alowed range. Please try again.");
 }
 catch (FormatException)
 {
 Console.WriteLine("The input isn't an integer number. Please try again.");
 }
 catch (Exception)
 {
 Console.WriteLine("An unknown error occured. Please try again.");
 }
}

However, for this case I'd say that there's an option that's better still.

What you should really do is, rather than having a method try to get another value from the user and possibly failing and possibly succeeding and needing to be called again, create a method that keeps asking the user for a value until it gets a valid one. This logic is far easier to implement at that scope than it is to try to continually add more iterations to the loop.

Once you create a method that will always get a valid value, trying as hard as it needs to:

public static int ReadNumber(int start, int end, int iteration)
{
 while (true)
 {
 Console.WriteLine("Input number {0}", iteration + 1);
 int n;
 if (!int.TryParse(Console.ReadLine(), out n))
 Console.WriteLine("The input isn't an integer number. Please try again.");
 else if (n < start || n > end)
 Console.WriteLine("The input number wasn't in the alowed range. Please try again.");
 else
 return n;
 }
}

(Also note the use of conditional checks to determine if the user input is valid, rather than using exceptions for non-exceptional control flow.)

Now our loop is dead easy:

for (int i = 0; i < 10; i++)
{
 int number = ReadNumber(start, end, i);
}

• frostblooded: Your answer makes a lot of sense and I like it. Thanks!

Answer 2

You could set a bool indicating if an exception was caught. I see no superiority in this way, but it exists. You might also consider catching these in seperate catch statements:

var wasCaught = false;
try
{
 Console.WriteLine("Input number {0}", i + 1);
 int number = ReadNumber(start, end);
}
catch (Exception ex)
{
 wasCaught = true;
 if (ex is ArgumentException)
 {
 Console.WriteLine("The input number wasn't in the alowed range. Please try again.");
 }
 else if (ex is FormatException)
 {
 Console.WriteLine("The input isn't an integer number. Please try again.");
 }
 else
 {
 Console.WriteLine("An unknown error occured. Please try again.");
 }
}
if (wasCaught)
{
 i--;
}