System.Collections — which of the following is NOT an interface defined in this namespace?

Introduction / Context:
Knowing the standard interfaces in System.Collections is essential for iteration and dictionary-like access patterns.

Concept / Approach:
Common interfaces include IEnumerable (sequence), IEnumerator (iterator), IComparer (ordering), IDictionaryEnumerator (dictionary enumeration). There is no standard IDictionaryComparer interface in System.Collections.

Why Other Options Are Wrong:
IComparer, IEnumerable, IEnumerator, and IDictionaryEnumerator are valid and widely used in the non-generic collections.

Final Answer:
IDictionaryComparer