Why is ToUpperInvariant() faster than ToLowerInvariant()?

Question

I read in CLR via C# by Jeffrey Richter that String.ToUpperInvariant() is faster than String.ToLowerInvariant(). He says that this is because the FCL uses ToUpperInvariant to normalise strings, so the method is ultra-optimised. Running a couple quick tests on my machine, I concur that ToUpperInvariant() is indeed slightly faster.

My question is if anybody knows how the function is actually optimised on a technical level, and/or why the same optimisations were not applied to ToLowerInvariant() as well.

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Concerning the "duplicate": The proposed "duplicate" question really doesn't provide an answer to my question. I understand the benefits of using ToUpperInvariant instead of ToLowerInvariant, but what I would like to know is how/why ToUpperInvariant performs better. This point is not addressed in the "duplicate".

Answer 1

Since it is now easier to read the CLR source which implements InternalChangeCaseString, we can see that it mostly calls down to the Win32 function LCMapStringEx. There appears to be no notes or any discussion on the performance of passing in LCMAP_UPPERCASE vs. LCMAP_LOWERCASE for the dwMapFlags parameter. Calling InternalChangeCaseString uses a flag isToUpper which, if true may result in better optimization by the compiler (or JITter), but since the call to LCMapStringEx has to setup a p/invoke call frame and the call itself has to do work, I'm not sure a lot of time is saved there.

Perhaps the recommendation is a hold over from some other implementation, but I can't see anything that would provide a significant speed advantage one way or the other.