Assert that a collection does not contain any item in another collection

Question

Consider this code:

string[] myCollection = { "a", "b", "c", "d" }; 
string[] anotherCollection = { "c", "d" };

I want to test that myCollection does not contain any item of anotherCollection. How can I achieve that in NUnit?

I tried these:

CollectionAssert.DoesNotContain(anotherCollection, myCollection);
Assert.That(myCollection, Has.No.Member(anotherCollection));

...but the problem is that these functions operate on a single entity and I did not find any way to let them work with arrays.

Answer 1

To assert that a collection does not contain any of a set of items, we can check that the intersection of another collection is empty using LINQ:

string[] subject = { "a", "b", "c", "d" };
string[] forbidden = { "c", "d" };

Assert.IsEmpty(subject.Intersect(forbidden)); 
// Or:
Assert.That(subject.Intersect(forbidden), Is.Empty);

The test shown above will fail because both collections contain the values "c" and "d".

If we want the test to fail only when the collection contains all of the prohibited items:

Assert.That(subject.Intersect(forbidden), Is.Not.EquivalentTo(forbidden));
// Or:
Assert.That(subject, Is.Not.SupersetOf(forbidden));