System.Collections.Frozen Namespace with 4 public types Static Classes FrozenDictionary Provides a set of initialization methods for instances of the class. FrozenSet Provides a set of initialization methods for instances of the class. Abstract Classes FrozenDictionary`2 Provides an immutable, read-only dictionary optimized for fast lookup and enumeration. FrozenSet`1 Provides an immutable, read-only set optimized for fast lookup and enumeration.