What is the fastest, case insensitive, way to see if a string contains another string in C#?

Since ToUpper would actually result in a new string being created, StringComparison.OrdinalIgnoreCase would be faster, also, regex has a lot of overhead for a simple compare like this. That said, String.IndexOf(String, StringComparison.OrdinalIgnoreCase) should be the fastest, since it does not involve creating new strings.

I would guess (there I go again) that RegEx has the better worst case because of how it evaluates the string, IndexOf will always do a linear search, I'm guessing (and again) that RegEx is using something a little better. RegEx should also have a best case which would likely be close, though not as good, as IndexOf (due to additional complexity in it's language).

You peaked my curiousity (15,000 length string, 10,000 loop):

IndexOf-OrdinalIgnoreCase 00:00:00.0156251

RegEx-IgnoreCase 00:00:00.1093757

IndexOf-ToUpper 00:00:00.9531311

IndexOf-ToLower 00:00:00.9531311

Placement in the string also makes a huge difference:

At start:

Match00:00:00.6250040

IndexOf00:00:00.0156251

ToUpper00:00:00.9687562

ToLower00:00:01.0000064

At End:

Match00:00:00.5781287

IndexOf00:00:01.0468817

ToUpper00:00:01.4062590

ToLower00:00:01.4218841

Not Found:

Match00:00:00.5625036

IndexOf00:00:01.0000064

ToUpper00:00:01.3750088

ToLower00:00:01.3906339