ToDistinctDictionary.cs

public static Dictionary<TKey, TElement> ToDistinctDictionary<TSource, TKey, TElement>(
    this IEnumerable<TSource> source, 
    Func<TSource, TKey> keySelector,
    Func<TSource, TElement> elementSelector,
    IEqualityComparer<TKey> comparer)
{
    if (source is null) throw new ArgumentNullException(nameof(source));
    if (keySelector is null) throw new ArgumentNullException(nameof(keySelector));
    if (elementSelector is null) throw new ArgumentNullException(nameof(elementSelector));
    
    var dictionary = new Dictionary<TKey, TElement>(comparer ?? Comparer<TKey>.Default);
    foreach (var element in source)
    {
        dictionary[keySelector(element)] = elementSelector(element);
    }

    return dictionary;
}