BooksConsolapp/Books.ConsoleApp/SearchByAuthor.cs

using System;
using System.Collections.Generic;
using System.Data.SqlTypes;
using System.Linq;
using System.Text;

namespace Books.ConsoleApp
{
    public static  class SearchByAuthor
    {
        public static IEnumerable<string> MatchBy(
            IEnumerable<Book> books, string searchCriteria)
        {
            return books
                .ExtractAuthors()
                .ListAuthorsMatching(searchCriteria);
        }

        public static IEnumerable<string> SuggestAuthors(
            IEnumerable<Book> books, int suggestionLength = 5)
        {
            return books
                .ExtractAuthors()
                .RandomizeOrder()
                .Take(suggestionLength);
        }

        public static IEnumerable<Book> ListBooks(IEnumerable<Book> books, string author)
        {
            return books
                .CatalogueByAuthor()
                .BooksBy(author);
        }

        public static AuthorCatalogue CatalogueByAuthor(this IEnumerable<Book> books)
        {
            return books
                    .ToLookup(b => b.author)
                // Mark Twain : {The Adventures of Huckleberry Finn},
                // Shakespeare: {Othello, Hamlet, ...}
                // ...
                ;
        }


        public static IEnumerable<Book> BooksBy(this AuthorCatalogue catalogue, string author)
        {
            return catalogue.Contains(author)
                ? catalogue[author]
                : Enumerable.Empty<Book>();
        }

        public static IEnumerable<string> ExtractAuthors(this IEnumerable<Book> books)
        {
            return books.Select(b =>
            {
                //do stuff here
                return b.author;

            }).Distinct();
        }

        public static IEnumerable<string> ListAuthorsMatching(
            this IEnumerable<string> authors, string searchCriteria)
        {
            var partOfNameLower = searchCriteria.ToLower();
            return authors.Where(a => a.ToLower().Contains(partOfNameLower));
        }

        private static IOrderedEnumerable<string> RandomizeOrder(this IEnumerable<string> authors)
        {
            return authors.OrderBy(_ => Guid.NewGuid());
        }

    }

    public class AuthorCatalogue
    {
    }

    public class Book
    {

    }
}