ProjectLib/ProjectLibrary/Entites/Library.cs

using ProjectLibrary.Forms;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ProjectLibrary.Entites
{
    public class Library
    {
        public int Id { get; private set; }

        [DisplayName("Название")]
        public string Name { get; private set; } = string.Empty;
        
        [DisplayName("Адрес")]
        public string Address { get; private set; } = string.Empty;

        [Browsable(false)]
        public IEnumerable<Book_Library> BookLibrary
        {
            get;
            private set;
        }

        [DisplayName("Перечень книг в библиотеке")]
        public string ListBookInLibrary => BookLibrary != null ?
            string.Join("; ", BookLibrary.Select(x => $"{x.BookID},{x.Count}")): string.Empty;

        public static Library CreateEntity(int id, string name, string address, IEnumerable<Book_Library> bookLibrary)
        {
            return new Library
            {
                Id = id,
                Name = name ?? string.Empty,
                Address = address ?? string.Empty,
                BookLibrary = bookLibrary
            };
        }

        public void SetLibraryForBook(IEnumerable<Book_Library> book_Library)
        {
            if (book_Library != null && book_Library.Any())
            {
                BookLibrary = book_Library;
            }
        }
    }
}