Why is String.IsNullOrEmpty faster than String.Length?

Question

ILSpy shows that String.IsNullOrEmpty is implemented in terms of String.Length. But then why is String.IsNullOrEmpty(s) faster than

Answer 1

Your benchmark does not measure String.IsNullOrEmpty vs String.Length, but rather how different lambda expressions are generated to functions. I.e. it is not very surprising that delegate that just contains single function call (IsNullOrEmpty) is faster than one with function call and comparison (Length == 0).

To get comparison of actuall call - write code that calls them directly without delegates.

EDIT: My rough measurements show that delegate version with IsNullOrEmpty is slightly faster then the rest, while direct calls to the same comparision are in reverse order (and about twice faster due to significantly less number of extra code) on my machine. Results likely to wary between machines, x86/x64 mode, as well between versions of runtime. For practical purposes I would consider all 4 ways are about the same if you need to use them in LINQ queries.

Overall I doubt there will be measurable difference in real program cased by choice between these methods, so pick the one that is most readable to you and use it. I generally prefer IsNullOrEmpty since it gives less chance to get ==/!= wrong in a condition.

Removal of string manipulation altogether from time critical code will likley bring much higer benifit that picking between these choices, also dropping LINQ for critical code is an option. As always - make sure to measure overall program speed in real life scenario.