Why does IQueryable.All() return true on an empty collection?

Question

So I ran into a situation today where some production code was failing precisely because a method performed exactly as documented in MSDN. Shame on me for not reading the d

Answer 1

You will find this behaviour quite often in other areas of mathematics or computer science.

The SUM operator in Math will return 0 (the neutral element of +) in cases where the ranges are invalid (the SUM from 0 up to -1). The MULTIPYL operator will return 1 (neutral element for multiplication).

Now if you have Boolean expressions, it's quite similar: The neutral element for OR is false (a OR false = a) whereas the neutral element for AND is true.

Now on Linq's ANY and ALL: They are similar to this:

ANY = a OR b OR c OR d ...
ALL = a AND b AND c AND d ...

So this behavior is just what "you would expect" if you have a math/cs background.