Boyer-Moore Practical in C#?

My bet is that setting that flag allows String.IndexOf to use Boyer-Moore itself. And its implementation is better than yours.

Without that flag it has to be careful using Boyer-Moore (and probably doesn't) because of potential issues around Unicode. In particular the possibility of Unicode causes the transition tables that Boyer-Moore uses to blow up.

Based on my own tests and the comments made here, I've concluded that the reason String.IndexOf() performs so well with StringComparision.Ordinal is because the method calls into unmanaged code that likely employs hand-optimized assembly language.

I have run a number of different tests and String.IndexOf() just seems to be faster than anything I can implement using managed C# code.

If anyone's interested, I've written everything I've discovered about this and posted several variations of the Boyer-Moore algorithm in C# at http://www.blackbeltcoder.com/Articles/algorithms/fast-text-search-with-boyer-moore.