[c#] Convert from List into IEnumerable format

IEnumerable<Book> _Book_IE
List<Book> _Book_List

How shall I do in order to convert _Book_List into IEnumerable format?

As far as I know List<T> implements IEnumerable<T>. It means that you do not have to convert or cast anything.

You can use the extension method AsEnumerable in Assembly System.Core and System.Linq namespace :

List<Book> list = new List<Book>();
return list.AsEnumerable();

This will, as said on this MSDN link change the type of the List in compile-time. This will give you the benefits also to only enumerate your collection we needed (see MSDN example for this).

IEnumerable<Book> _Book_IE;
List<Book> _Book_List;

If it's the generic variant:

_Book_IE = _Book_List;

If you want to convert to the non-generic one:

IEnumerable ie = (IEnumerable)_Book_List;

You need to

using System.Linq;

to use IEnumerable options at your List.

Why not use a Single liner ...

IEnumerable<Book> _Book_IE= _Book_List as IEnumerable<Book>;