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 somewhat disagree with Gravell on this specific edge case: a Null-ed variable is considered 'not defined', it doesn't have a value. So whatever is used to signal that is OK: even magic numbers, but with magic numbers you have to take into account that a magic number will always haunt you in the future when it becomes a 'valid' value all of a sudden. With Double.NaN you don't have to be afraid for that: it's never going to become a valid double. Though, you have to consider that NaN in the sense of the sequence of doubles can only be used as a marker for 'not defined', you can't use it as an error code in the sequences as well, obviously.

So whatever is used to mark 'undefined': it has to be clear in the context of the set of values that that specific value is considered the value for 'undefined' AND that won't change in the future.

If Nullable give you too much trouble, use NaN, or whatever else, as long as you consider the consequences: the value chosen represents 'undefined' and that will stay.