IndexOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. What did I do wrong?

Question

I'm trying to create a Word Occurrence Calculator that takes in a string List and returns a WordOccurrence object List. The WordOccurrence object takes in a string word and has a count attached to it. I want each WordOccurrence object in the List to have an accurate count of how many times a word is in the string List.

I have tried a lot of different approaches. Here is the one I am working with so far. However, I get an error message about the index being out of bounds when I run the program.

public static List<WordOccurrence> CalculateOccurrences(List<string> input)
        {
            List<WordOccurrence> occurrences = new List<WordOccurrence>();
            HashSet<string> wordHash = new HashSet<string>();
            int index = 0;

            try
            {
                if (input == null)
                {
                    throw new ArgumentNullException("Invalid input");
                }

                foreach (string word in input)
                {
                    if(!wordHash.Add(word))
                    {
                         occurrences.Add(new WordOccurrence(word));
                    }
                    else
                    {     
                         index = occurrences.FindIndex(i => i.Word == word);
                         occurrences[index].Count++;
                    }
                }
             }
            
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);

            }
        return occurrences;
    }
    }
} 

What is the best way to go about this? Is there a simpler method that I'm not aware of? Thanks in advance.

Answer 1

Thank you all for the comments, they helped me reach a solution that worked! I went with a foreach loop and used the Find() and Exists() method.

foreach (string word in input)
                {
                    if (!occurrences.Exists(i => i.Word == word))
                    {
                        occurrences.Add(new WordOccurrence(word));
                    }
                    else
                    {
                        occurrences.Find(i => i.Word == word).Count++;

                    }
                }

Thanks again everyone!