Operador Contains

Utilizando Query Expression

// retornar los elementos que pertenezcan a la categoria de Python
void Challenge_5(IEnumerable<Book> books)
{
    // extension method
    //var result = books.Where(b => b.Categories.Contains("Python"));

    // query expression
    var result = from b in books where b.Categories.Contains("Python") select b;

    PrintValues(result);
}

 public IEnumerable<Book> LibrosPorCategoria(string method, string categoria)
    {
        if (method is "method")
        {
            return _booksCollection.Where(x => x.Categories.Contains(categoria, StringComparer.CurrentCultureIgnoreCase));
        }
        else if (method is "query")
        {
            return from b in _booksCollection
                   where b.Categories.Contains(categoria, StringComparer.CurrentCultureIgnoreCase)
                   select b;
        }
        else
        {
            return Enumerable.Empty<Book>();
        }
    }

//Query expression (experiment)
        return from book in booksCollection where book.Categories.Contains("Python") select book; 

Operador Contains

Se usa para encontrar los registros que contienen un determinado string.