Searching a lot of text files for a specific text written in textbox and display them

Question

I am trying to search many text files from a particular directory and then textchanged event to find text in all files and display on screen only lines which contain that text.

Currently it is working but it is too slow. I am posting a function which searches text and display in listbox. what could be most efficient way to make it work little speedy.

listBox2.Items.Clear();
ArrayList lines = new ArrayList();

if (txtfile.Count > 0)
{
    for (int i = 0; i < txtfile.Count; i++)
    {
        lines.AddRange((File.ReadAllLines(Path.Combine(path, txtfile[i].ToString()))));
    }
    for (int i = 0; i < lines.Count; i++)
    {
        if(lines[i].ToString().IndexOf(txt,StringComparison.InvariantCultureIgnoreCase)>=0)

        {
                listBox2.Items.Add(lines[i].ToString());
        }       
    }

}

Answer 1

How many files are you searching? You could always index them , store the contents in a SQL database, and of course use Parallel.For

Parallel.For(1, 1000, i =>
    {
        //do something here.
    }
);

Answer 2

I would use Directory.EnumerateFiles and File.ReadLines since they are less memory hungry:

var matchingLines = Directory.EnumerateFiles(path, ".txt", SearchOption.TopDirectoryOnly)
    .SelectMany(fn => File.ReadLines(fn))
    .Where(l => l.IndexOf(txt, StringComparison.InvariantCultureIgnoreCase) >= 0);
foreach (var line in matchingLines)
    listBox2.Items.Add(line);

I would also search only when the user triggers it explicitely, so on button-click and not on text-changed.