A clever alternative in LINQ for iterating over HashSet<string>

Question

I've got a whitelist of URLs I'm using, inside a HashSet<string>. I'm trying to find if the url starts with any of the items in the white list (it has to be that way round).

Edit: The previous example was a bit misleading and had a typo - I already have a base url like yahoo.com, the whitelist is just the path.

HashSet<string> whiteList = new HashSet<string>();

string path = "/sport/baseball/";
bool validUrl = false;

foreach (string item in whiteList)
{
    if (path.StartsWith(item))
    {
        validUrl = true;
        break;
    }
}

Is there a more elegant way of doing this lookup with LINQ (to objects)? The list isn't huge so performance isn't an issue.

Answer 1

bool validUrl = whiteList.Any(item => linkUrl.StartsWith(item));

By the way, in general, hash tables are not good data structures for these kind of problems (where you don't have the key and are matching the key based on a function) as you'll have to enumerate the whole table all the time. You can use a simple List<string> to hold the items instead and you'll get better performance.

Answer 2

The issue here is with the lookup. Do you have any regularity in the whitelist? i.e will it always be a domain you're after, not neccessarily the pages within or a specific subdomain?

If so you could use a string.split to grab the first URL part from your string, then use the .Contains() method of your hashset to get the item. This would remove the string.StartsWith() command which is run once for every element in the list, and an expensive string compare, and replace it with a one off string.split and a O(1) lookup of your hashset.

HashSet<string> whiteList = new HashSet<string>();
//add items

string urlStartsWith = "http://www.yahoo.com";
bool validURL = whiteList.Contains(url);