Checking a list with null values for duplicates in C#

If you’re worried about performance, the following code will stop as soon as it finds the first duplicate item – all the other solutions so far require the whole input to be iterated at least once.

var hashset = new HashSet<string>();
if (myList.Where(s => s != null).Any(s => !hashset.Add(s)))
{
    // there are duplicates
}

hashset.Add returns false if the item already exists in the set, and Any returns true as soon as the first true value occurs, so this will only search the input as far as the first duplicate.