/// <summary>
/// Look up in <paramref name="repository"/> for the <see cref="CategoryDTO"/> by the <paramref name="id"/> key.
/// If nothing found, null will be returned.
/// </summary>
/// <param name="id">Category unique ID.</param>
/// <param name="repository">Repository to lookup in.</param>
/// <returns>Found category instance or null otherwise.</returns>
public static CategoryDTO LookupIn(this int?id, ICategoriesRepository repository)
{
return(id.HasValue
? repository.ByKey(id.Value)
: null);
}