Convert.ToInt32 versus TryParse

Question

We all know the effects that lots of thrown exceptions can have over the performance of our applications, thus, we should stay away from things like using exceptions for control flow. After this statement I must confess that when coding I didn't care that much about this. I've been working mostly on Java platform but lately I was doing it on .NET platform and just found out this handy method: public static bool TryParse(string s,out int result) ,which allows you to transform a String into int whithout raise an exception. From that moment on, I'm keeping on using it. I just wanted to ask you about your preferences regarding the use of public static bool TryParse(string s,out int result) or public static int ToInt32(string value).

And from the point of view of Java, just pointing that it's missing such a similar method, despite we could get it through things like:

boolean isInteger = Pattern.matches("^\d*$", myString);

Thanks.

Answer 1

Yes, Java is missing a similar method, although without out parameters it's actually pretty difficult to express (while wanting to return a primitive). Generally, though, in C# you should use TryParse if you expect the value to not be an integer sometimes, and ToInt32 otherwise; this way the "exceptional" situation is treated as such.

In particular if performance is your main reason for wanting TryParse, the regex matches method you post is considerably worse. The performance "expense" of Exceptions (which is, in reality, very minimal) is dwarfed by how much using them wrongly can fuzz easy understanding of control flow.

Answer 2

I don't know about C#, but in Java exceptions are only expensive when they're actually thrown, but then they're very expensive indeed. If you expect a significant fraction of the strings to be invalid, it's worth your while to validate them first, even if you use a regex.

But don't use String.matches() or Pattern.matches() to apply the regex; those methods recompile the regex every time you call them. Instead, compile the regex ahead of time and save it as a Pattern object, then do your validating with that. In my tests, parsing a list of 10,000 strings of which 20% were invalid, pre-validating with a Pattern is almost twice as fast as using Integer.parseInt() alone and catching the exceptions.

However, this discussion only applies if you're doing a lot of conversions in a tight loop. If you're only doing them once in a while, like when you accept user input, letting Integer.parseInt() do the validating is fine. And if you do choose to validate with a regex, you'll need a much better regex than ^\d*$ - that regex will match the empty string as well as "numbers" larger than Integer.MAX_VALUE, and it won't match negative numbers at all.