DictionaryTKey

DictionaryTKey is a term used in generic programming to denote the key type parameter of a dictionary-like collection. It is not a standalone class or standard type; rather, it appears in documentation and code examples to emphasize that a data structure is keyed by some type. The conventional name for the key type parameter in many languages is TKey, with the corresponding value parameter commonly called TValue. In API descriptions you may encounter DictionaryTKey used as a stand-in for the actual key type when discussing generic types such as Dictionary<TKey, TValue> or custom dictionaries.

Usage and semantics

In generic dictionaries, the key type defines how entries are identified and looked up. Keys are typically required to implement a robust notion of equality and hashing, so that lookups, insertions, and removals are efficient. Languages like C# require or encourage constraints such as notnull for the key type to avoid null-key issues and to enable stable hashing and comparison.

Constraints and design considerations

- Keys must have a well-defined GetHashCode and Equals implementation to ensure correct dictionary behavior.

- Notnull or similar constraints help prevent null references for keys.

- Keys should be immutable or effectively immutable to avoid changing dictionary placement after insertion.

- An optional IEqualityComparer<TKey> can customize key equality behavior.

Relation to other concepts

DictionaryTKey is closely related to Dictionary<TKey, TValue>, KeyValuePair<TKey, TValue>, and IEqualityComparer<TKey>. It also contrasts with key-only or value-only data structures such as collections with different indexing strategies, like SortedDictionary or HashSet.

See also

Dictionary<TKey, TValue>, IEqualityComparer<TKey>, KeyValuePair<TKey, TValue>, HashSet<T>.