Alternatives to nullable types in C#

Question

I am writing algorithms that work on series of numeric data, where sometimes, a value in the series needs to be null. However, because this application is performance critical,

Answer 1

I am working on a large project that uses NaN as a null value. I am not entirely comfortable with it - for similar reasons as yours: not knowing what can go wrong. We haven't encountered any real problems so far, but be aware of the following:

NaN arithmetics - While, most of the time, "NaN promotion" is a good thing, it might not always be what you expect.

Comparison - Comparison of values gets rather expensive, if you want NaN's to compare equal. Now, testing floats for equality isn't simple anyway, but ordering (a < b) can get really ugly, because nan's sometimes need to be smaller, sometimes larger than normal values.

Code Infection - I see lots of arithmetic code that requires specific handling of NaN's to be correct. So you end up with "functions that accept NaN's" and "functions that don't" for performance reasons.

Other non-finites NaN is nto the only non-finite value. Should be kept in mind...

Floating Point Exceptions are not a problem when disabled. Until someone enables them. True story: Static intialization of a NaN in an ActiveX control. Doesn't sound scary, until you change installation to use InnoSetup, which uses a Pascal/Delphi(?) core, which has FPU exceptions enabled by default. Took me a while to figure out.

So, all in all, nothing serious, though I'd prefer not to have to consider NaNs that often.

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I'd use Nullable types as often as possible, unless they are (proven to be) performance / ressource constraints. One case could be large vectors / matrices with occasional NaNs, or large sets of named individual values where the default NaN behavior is correct.

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Alternatively, you can use an index vector for vectors and matrices, standard "sparse matrix" implementations, or a separate bool/bit vector.