Problems changing color to a selected text in a richtextbox

Question

I have this code and it's applied to a form. For every word in the richtextbox it looks if it's present in a txt file that i'm using as a dictionary and if not it changes the color of that word to red. I know the code opens and closes the stream for every word and I'm fixing that soon.

    private void sottolinea_errori_Click(object sender, EventArgs e)
    {
        string line;
        string[] linea = TextBox_stampa_contenuto.Lines;
        if (TextBox_stampa_contenuto.Text != "")
        {
            foreach (string k in linea)
            {
                string[] parole = k.Split(new Char[] { ' ', ',', '.', ':', '\t' });
                foreach (string s in parole)
                {
                    Regex rgx = new Regex(@"\d");
                    if (!rgx.IsMatch(s))
                    {
                        if (s.Trim() != "")
                        {
                            s.Trim();
                            string path = @"280000parole.txt";
                            bool esito = true;
                            StreamReader file = new StreamReader(path);
                            while ((line = file.ReadLine()) != null && esito == true)
                                if (string.Compare(line, s) == 0)
                                    esito = false; // i put this false when I find the word in the dictionary file
                            file.Close();
                            if (esito)  //if true means that the word wasn't found
                            {
                                int inizioParola=0, indice;  //indice means index, inizio parola is for selectionStart
                                while ((indice = TextBox_stampa_contenuto.Text.IndexOf(s, inizioParola)) != -1)
                                {
                                    TextBox_stampa_contenuto.Select(indice, s.Length);
                                    inizioParola = indice + s.Length;
                                }
                                TextBox_stampa_contenuto.SelectionStart = inizioParola - s.Length;
                                TextBox_stampa_contenuto.SelectionLength = s.Length;
                                TextBox_stampa_contenuto.SelectionColor = Color.Red;
                            }
                            TextBox_stampa_contenuto.SelectionLength = 0;
                        }
                    }
                }
            }
        }
    }

The problems are:

1. If the first words of the file are the same it changes the color of the last one only
2. If the last word it's wrong the new text that you input it's going to be red
3. How can I underline the wrong words like bad spelling from Word?

If you can help me I'll really appreciate!

Answer 1

Here is a working example, added caching of the words so you don't read the file each time, trimmed all the words, put all the words to lower case and optimized the logic. I have added the comments to make the code easy to read and understand.

WordList.txt Contains

Apple 
Banana 
Computer

private void sottolinea_errori_Click(object sender, EventArgs e)
{
        // Get all the lines in the rich text box
        string[] textBoxLines = this.TextBox_stampa_contenuto.Lines;

        // Check that there is some text
        if (this.TextBox_stampa_contenuto.Text != string.Empty)
        {
            // Create a regular expression match
            Regex rgx = new Regex(@"\d");

            // Create a new dictionary to hold all the words
            Dictionary<string, int> wordDictionary = new Dictionary<string, int>();

            // Path to the list of words
            const string WordListPath = @"WordList.txt";

            // Open the file and read all the words
            StreamReader file = new StreamReader(WordListPath);

            // Read each file into the dictionary
            int i = 0;
            while (!file.EndOfStream)
            {
                // Read each word, one word per line
                string line = file.ReadLine();

                // Check if the line is empty or null and not in the dictionary
                if (!string.IsNullOrEmpty(line) && !wordDictionary.ContainsKey(line))
                {
                    // Add the word to the dictionary for easy lookup add the word to lower case
                    wordDictionary.Add(line.ToLower(), i);

                    // Incrament the counter
                    i++;
                }
            }

            // Close the file
            file.Close();

            // For each line in the text box loop over the logic
            foreach (string textLine in textBoxLines)
            {
                // Split the text line so we get individual words, remove empty entries and trim all the words
                string[] words = textLine.Split(new char[] { ' ', ',', '.', ':', '\t' }, StringSplitOptions.RemoveEmptyEntries).Select(p => p.Trim().ToLower()).ToArray();

                // For each word that does not contain a digit
                foreach (string word in words.Where(x => !rgx.IsMatch(x)))
                {
                        // Check if the word is found, returns true if found
                        if (!wordDictionary.ContainsKey(word))
                        {
                            // Initialize the text modification variables
                            int wordStartPosition, seachIndex = 0;

                            // Find all instances of the current word
                            while ((wordStartPosition = this.TextBox_stampa_contenuto.Text.IndexOf(word, seachIndex, StringComparison.InvariantCultureIgnoreCase)) != -1)
                            {
                                // Select the word in the text box
                                this.TextBox_stampa_contenuto.Select(wordStartPosition, word.Length);

                                // Set the selection color
                                this.TextBox_stampa_contenuto.SelectionColor = Color.Red;

                                // Increase the search index after the word
                                seachIndex = wordStartPosition + word.Length;
                            }
                    }
                }
            }
        }
}