im a newbie at C# and would like input from more experienced programmers please. I have the following static class that enumerates directories in a folder, then searches each file in the folder (it seems to only work with text files even thought i dont explicitly specify that) for a given string and returns an IEnumerable that holds the results.

It takes about 15 seconds to go through 40 text files that are about 250kb in size and i think it could be faster. Perhaps i could use a better algorithm or is there a better method of achieving this?

All help is appreciated. Thanks

public static class LogFileReader
{
    public static IEnumerable<string> GetLines(string path, string searchterm)
    {

        var dirs = Directory.EnumerateDirectories(path);
        List<string> thelines = new List<string>();

        foreach (var dir in dirs)
        {

            var files = Directory.EnumerateFiles(dir);


            foreach (var file in files)
            {
                using (StreamReader sr = new StreamReader(file))
                {
                    string line = string.Empty;
                    while ((line = sr.ReadLine()) != null)
                    {
                        if (line.IndexOf(searchterm, StringComparison.CurrentCultureIgnoreCase) >= 0)
                        {
                            thelines.Add(line);
                        }
                    }

                }
            }

        }

        return thelines;
    }
}