private void buttonAddBlacklist_Click(object sender, EventArgs e) { string path = textBoxBlacklist.Text.Trim(); //not contains case insensitive if (Blacklist.FindIndex(x => x.Equals(path, StringComparison.InvariantCultureIgnoreCase)) == -1) { listBoxBlacklist.Items.Add(path); textBoxBlacklist.Text = ""; } }