System.Collections.Immutable by Microsoft

<PackageReference Include="System.Collections.Immutable" Version="8.0.0-preview.6.23329.7" />

.NET API 253,056 bytes

 FrozenDictionary

public static class FrozenDictionary
public static FrozenDictionary<TKey, TValue> ToFrozenDictionary<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> source, IEqualityComparer<TKey> comparer = null)

public static FrozenDictionary<TKey, TValue> ToFrozenDictionary<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> source, bool optimizeForReading)

public static FrozenDictionary<TKey, TValue> ToFrozenDictionary<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> source, IEqualityComparer<TKey> comparer, bool optimizeForReading)

public static FrozenDictionary<TKey, TSource> ToFrozenDictionary<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer = null)

public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector, IEqualityComparer<TKey> comparer = null)