System.Collections.Concurrent.ConcurrentDictionaryPolyfills
Provides downlevel polyfills for instance methods on ConcurrentDictionary<T, U>.
public static TValue GetOrAdd<TKey, TValue, TArg>(this ConcurrentDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TArg, TValue> valueFactory, TArg factoryArgument)