Change your code as following--
List <string> matches = new List<string>();
string sourceText = System.IO.File.ReadAllText(fileName);
string[] words = sourceText.Split(new char[] { '.', '?', '!', ',', ' ',
';', '\'', ':', '"', '-' }, StringSplitOptions.RemoveEmptyEntries);
List<string> wordsToMatch = new List<string>(new string[] { "this", "is", "a", "test" });
matches = (from word in words
where wordsToMatch.Contains(word.ToLowerInvariant())
select word).ToList();
foreach (string s in matches)
{
listBox1.Items.Add(s);
}