IEqualityComparerTKey

IEqualityComparerTKey is not the canonical name used in the standard API; in the widely used .NET libraries the generic interface is IEqualityComparer<TKey>. The term IEqualityComparerTKey may appear as a shorthand or misnomer for that interface, but the official API defines it as IEqualityComparer<TKey>. This interface is used to supply custom logic for determining equality and computing hash codes for keys of type TKey.

The interface IEqualityComparer<TKey> defines two members: bool Equals(TKey x, TKey y) and int GetHashCode(TKey obj). Implementations must provide a consistent equality relation and a hash function such that if Equals(x, y) is true, then GetHashCode(x) equals GetHashCode(y). Hash codes should be fast to compute and spread values well to minimize collisions. The behavior should be stable for objects used as keys, and the implementation should avoid throwing exceptions in common comparisons.

Usage of IEqualityComparer<TKey> occurs primarily with generic collections that rely on hashing, such as Dictionary<TKey, TValue> and HashSet<T>. By supplying a comparer to these collections, you can customize how keys are compared and how their hash codes are generated. This enables scenarios such as case-insensitive string matching, culture-aware comparisons, or domain-specific identity rules for complex key types.

Common patterns include using existing comparers provided by the framework (for example, StringComparer.OrdinalIgnoreCase for string keys) or implementing a custom, stateless comparer that encapsulates the specific equality criteria. When implementing your own comparer, ensure it remains immutable and thread-safe if shared across multiple collections. See also IEqualityComparer<T>, StringComparer, Dictionary<TKey, TValue>, and HashSet<T> for related concepts.