Question

I just discovered that Except() will remove all elements in the second list from the first, but it also has the effect that it makes all elements in the returned result distinct.

Simple way around I am using is Where(v => !secondList.Contains(v))

Can anyone explain to me why this is the behavior, and if possible point me to the documentation that explains this?

Answer 1

The documentation for the Except function states:

Produces the set difference of two sequences by using the default equality comparer to compare values.

The set difference of two sets is defined as the members of the first set that do not appear in the second set.

The important word here is set, which is defined as:

…an abstract data structure that can store certain values, without any particular order, and no repeated values…

Because Except is documented as a set-based operation, it also has the effect of making the resulting values distinct.