Should try-catch be avoided for known cases

问题

I have a case which I know will happen but very scarce. For example in every 10 thousand times the code runs, this might happen once.

I can check for this case by a simple if but this if will run many times with no use.

On the other hand I can place the code in try-catch block and when that special case happens I do what is needed to recover.

The question is which one is better? I know that generally speaking try-catch should not be used for known cases because of the overhead issue and also the application logic should not rely on catch code, but running an if multiple times will have more performance issue. I have tested this using this small test code:

static void Main(string[] args)
{
    Stopwatch sc = new Stopwatch();
    var list = new List<int>();
    var rnd = new Random();
    for (int i = 0; i < 100000000; i++)
    {
        list.Add(rnd.Next());
    }

    sc.Start();
    DoWithIf(list);
    sc.Stop();
    Console.WriteLine($"Done with IFs in {sc.ElapsedMilliseconds} milliseconds");
    sc.Restart();
    DoWithTryCatch(list);
    sc.Stop();
    Console.WriteLine($"Done with TRY-CATCH in {sc.ElapsedMilliseconds} milliseconds");
    Console.ReadKey();
}

private static int[] DoWithTryCatch(List<int> list)
{
    var res = new int[list.Count ];
    try
    {
        for (int i = 0; i < list.Count; i++)
        {
            res[i] = list[i];
        }
        return res;
    }
    catch
    {
        return res;
    }
}

private static int[] DoWithIf(List<int> list)
{
    var res = new int[list.Count - 1];
    for (int i = 0; i < list.Count; i++)
    {
        if (i < res.Length)
            res[i] = list[i];
    }
    return res;
}

This code simply copies a lot of numbers to an array with not enough size. In my machine checking array bounds each time takes around 210 milliseconds to run while using try-catch that will hit catch once runs in around 190 milliseconds.

Also if you think it depends on the case my case is that I get push notifications in an app and will check if I have the topic of the message. If not I will get and store the topic information for next messages. There are many messages in few topics.

回答1:

In your case, you have somehow missed the obvious optimization: if you worry that calling an if 100.000 times is too much... don't?

private static int[] DoWithIf(List<int> list)
{
    var res = new int[list.Count - 1];

    var bounds = Math.Min(res.Length, list.Count)

    for (int i = 0; i < bounds; i++)
    {        
        res[i] = list[i];
    }
    return res;
}

So I know this is only a test case, but the answer is: optimize if you need it and for what you need it. If you have something in a loop that's supposedly costly, then try to move it out of the loop. Optimize based on logic, not based on compiler constructs. If you are down to optimizing compiler constructs, you should not be coding in a managed and/or high level language anyway.

回答2:

So, it would be accurate to say that in your test, the if option was slower than the try...catch option by 20 milliseconds, for a loop of 100000000 times.
That translates to 20 / 100,000,000 - that's 0.0000002 milliseconds for each iteration.

Do you really think that kind of nano-optimization is worth writing code that goes goes against proper design standards?

Exceptions are for exceptional cases, the things that you can't control or can't test in advance - for instance, when you are reading data from a database and the connection terminates in the middle - stuff like that. Using exceptions for things that can be easily tested with simple code - well, that's just plain wrong.

If, for instance, you would have demonstrated a meaningful performance difference between these two options then perhaps you could justify using try...catch instead of if - but that's clearly not the case here.

So, to summarize - use if, not try...catch.

You should design your code for clarity, not for performance.
Write code that conveys the algorithm it is implementing in the clearest way possible.
Set performance goals and measure your code's performance against them.
If your code doesn't measure to your performance goals, Find the bottle necks and treat them.
Don't go wasting your time on nano-optimizations when you design the code.

来源：https://stackoverflow.com/questions/47451566/should-try-catch-be-avoided-for-known-cases

标签

performance

if-statement

try-catch