Why is function isprefix faster than Startswith in C#?

.net performance string startswith
Frantisek picture Frantisek · Apr 4, 2009 · Viewed 8.5k times · Source

Does anyone know why C# (.NET)'s StartsWith function is considerably slower than IsPrefix?

Answer

Jon Skeet picture Jon Skeet · Apr 4, 2009

I think it's mostly fetching the thread's current culture.

If you change Marc's test to use this form of String.StartsWith:

    Stopwatch watch = Stopwatch.StartNew();
    CultureInfo cc = CultureInfo.CurrentCulture;
    for (int i = 0; i < LOOP; i++)
    {
        if (s1.StartsWith(s2, false, cc)) chk++;
    }
    watch.Stop();
    Console.WriteLine(watch.ElapsedMilliseconds + "ms; chk: " + chk);

it comes a lot closer.

If you use s1.StartsWith(s2, StringComparison.Ordinal) it's a lot faster than using CompareInfo.IsPrefix (depending on the CompareInfo of course). On my box the results are (not scientifically):

  • s1.StartsWith(s2): 6914ms
  • s1.StartsWith(s2, false, culture): 5568ms
  • compare.IsPrefix(s1, s2): 5200ms
  • s1.StartsWith(s2, StringComparison.Ordinal): 1393ms

Obviously that's because it's really just comparing 16 bit integers at each point, which is pretty cheap. If you don't want culture-sensitive checking, and performance is particularly important to you, that's the overload I'd use.

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