How FirstOrDefault function works?

[Selun]

Hi all,
Im new in coding and i need to change something a code that i barely understand. And the problem is a FirstOrDefalut function.
Below is the code.

public static string GetArticlePageUrl(Article article, int categoryId)
	{
		
		if (categoryId < 0)
		{
            ArticleCategoryLink acl = article.ArticleCategoryLinks.FirstOrDefault();

			if (acl != null && acl.Category != null)
				categoryId = acl.Category.ID;
		}

		if (categoryId > 0)
			retVal = retVal + string.Format("&cid={0}", categoryId);

		return retVal;
	}

I just want to change

ArticleCategoryLink acl = article.ArticleCategoryLinks.FirstOrDefault();

whitch now get the first or default categoryid among the categories that the article belongs, to get the second value, or the first among some certain values that i can give, or the first among all excluding some values that i ll give.

Im sorry if its Code is messy, or if i missed to give any info. i just dont know what i should post to help you giving me an answer.

Thank you in advance.

[Markus]

You haven't explained your question very well. Give it another try. Also, how can we help if we cannot see the method-in-question's code? Post the code for FirstOrDefault( ).