Creating a NUnit constraint meaning "{collection} does not contain {item}"

Use the CollectionAssert method:

CollectionAssert.DoesNotContain(schedule.PendingItems, item);

If you are using NUnit 2.4 / 2.5 you may checkout the collection constraints.

Assert.That(schedule.PendingItems, Has.No.Member(item))

Only with NUnit 2.4 / 2.5