i'm wondering is there any way to optimize this code? It must be able to work with big files, for example, formatting raw 140kb .txt file (12.5k words) takes 2 seconds (measured with Stopwatch class). Text example http://pastebin.com/2p88v8EN Maybe i used here some bad techniques or there is some part to simplify? Maybe multithreading? I'm not familiar with this yet. Would be grateful for help!

Code below:

class TextManipulations                                                         
{
    public string[] wordsDist;                                                  // main array, contains words in alphabetic order and output lines 

    public void TextFormat(string sourcePath)                                   // creating method that will format our source text according to task
    {
        string textInput = System.IO.File.ReadAllText(sourcePath).ToLower();    // reading text from file, lowercased at start for precise search
        MatchCollection m = Regex.Matches(textInput, @"\b[\w']+\b");            // exact search of all alphanumeric "words" including words with apostrophe
        List<string> words = new List<string>();                                // creating List<T> for containing unknown amount of words
        
        foreach (Match match in m)                                              // assigning all matches to List<string>
        {
            words.Add(match.ToString());
        }
        words.Sort();                                                           // sorting words in alphabetic order
        wordsDist = words.Distinct().ToArray();                                 // assigning words to main array without duplicates
        System.IO.File.WriteAllLines(@"D:\output.txt", wordsDist);      // writing words into txt file to edit in setLineNumbers method
    }

    public void setLineNumbers(string sourcePath)                               // creating method for adding line numbers
    {
        string[] linesOutput = new string[wordsDist.Count()];                   // creating array that will contain line numbers
        string[] lines = System.IO.File.ReadAllLines(sourcePath);               // assigning source text by lines
        
        for (int j = 0; j < wordsDist.Count(); j++)                             // main cycle checking each word for presence in each line 
        {
            for (int i = 0; i < lines.Count(); i++)
            {
                if (Regex.IsMatch(lines[i].ToLower(), "\\b" + wordsDist[j] + "\\b"))   // using ToLower() here, because we can't use it in line 33
                {
                    linesOutput[j] += (i + 1).ToString() + ", ";                // adding line numbers according to word
                }
                
            }
        }

        for (int i = 0; i < wordsDist.Count(); i++)                             // connection of two relative arrays
        {
            wordsDist[i] += "_______________________________" + linesOutput[i];
            wordsDist[i] = wordsDist[i].Remove(wordsDist[i].Length - 2);        // removing last ',' char
        }
        
        System.IO.File.WriteAllLines(@"D:\output.txt", wordsDist);      // writing final output result into txt file

    }

}