Question

Q:

I have a list of objects like this:

List<object> entities = GetallEntities();

The first object in the previous list is : List<Books>.

Now i wanna to get all the books which id

exist in the following list:

List<string> BookIds = new List<string>();

Answer 1

Assuming that your Books class has an Id property and one instance represents one book:

// Get the first element of your entity-list which is of type `List<Books>`
List<Books> bookList = entities.OfType<List<Books>>().First();
List<string> BookIds = new List<string>();
// fill your id list here ...

// filter your books by the list of ids
IEnumerable<Books> filteredBooks = 
               bookList.Where(b => BookIds.Any(id => id == b.Id);