By default, search results are sorted by relevance. You can change the sort factor to OrderBy, ThenBy, OrderByDescending, or ThenByDescending.
client.Search<Article>() .OrderBy(x => x.PublicationDate) .ThenBy(x => x.Author.Name)
Sorting is supported for numerical types such as int, double, DateTime, and string.
Null values and SortMissing
By default, if a document has a null value in a field used to sort search results, it is sorted first when using the OrderBy and ThenBy methods, and last when using the OrderByDescending and ThenByDescending methods. For all types except string, you can change this behavior by supplying a second parameter of the SortMissing enum type.
client.Search<Article>() .OrderBy(x => x.PublicationDate, SortMissing.Last)