How does String.Contains work? [duplicate]

Answer 1

To my surprise, a compiled Regex is actually much, much faster than Contains. Of course, the cost of compiling is only worth it if you search for the same string multiple times. But if so, it's more than two times faster. Try it yourself:

static void Test()
{
  var random = new Random(10);

  var alphabet = "abcdefghijklmnopqrstuvwxyz";
  var content = new String((from x in Enumerable.Range(0, 10000000)
                            select a[random.Next(0, a.Length)]).ToArray());

  var searchString = content.Substring(5000000, 4096);

  var regex = new Regex(searchString);

  var sw = Stopwatch.StartNew();
  for (int i = 0; i < 1000; i++)
  {
    if (!regex.IsMatch(content))
    {
      throw new Exception();
    }
  }

  sw.Stop();
  Console.WriteLine("Regex: " + sw.Elapsed);
  sw.Restart();

  for (int i = 0; i < 1000; i++)
  {
    if (!content.Contains(searchString))
    {
      throw new Exception();
    }
  }
  sw.Stop();
  Console.WriteLine("String.Contains: " + sw.Elapsed);

}

I still haven't figured out how it can be this fast, looking at the compiled assembly, it's an obfuscated mess of switch statements. But it's fast, really fast.