c# - why does Enumerable.Except returns DISTINCT items? -

having spent on hour debugging bug in our code in end turned out enumerable.except method didn't know about:

var ilist = new[] { 1, 1, 1, 1 }; var ilist2 = enumerable.empty<int>(); ilist.except(ilist2); // returns { 1 } opposed { 1, 1, 1, 1 }

or more generally:

var ilist3 = new[] { 1 }; var ilist4 = new[] { 1, 1, 2, 2, 3 }; ilist4.except(ilist3); // returns { 2, 3 } opposed { 2, 2, 3 }

looking @ msdn page:

this method returns elements in first not appear in second. not return elements in second not appear in first.

i in cases this:

var ilist = new[] { 1, 1, 1, 1 }; var ilist2 = new[] { 1 }; ilist.except(ilist2); // returns empty array

you empty array because every element in first array 'appears' in second , therefore should removed.

but why distinct instances of other items not appear in second array? what's rationale behind behaviour?

i cannot sure why decided way. however, i'll give shot.

msdn describes except this:

produces set difference of 2 sequences using default equality comparer compare values.

a set described this:

a set collection of distinct objects, considered object in own right

TY