The DistinctBy operator ensures that only unique elements are included in the result sequence. The KeySelector property specifies the member, or set of members, to use to test the uniqueness of each element in the source sequence. The default EqualityComparer<T> for the type of the selected key is used to check whether each key is unique. If multiple non-unique keys are present in the source sequence, only the first element emitted with a given key will be included in the result sequence.