Пример Inner Join на C# и LINQ

class Program
{
    static void Main(string[] args)
    {
        List<Book> bookList = new List<Book>
        {
            new Book{BookID=1, BookNm="DevCurry.com Developer Tips"},
            new Book{BookID=2, BookNm=".NET and COM for Newbies"},
            new Book{BookID=3, BookNm="51 jQuery ASP.NET Recipes"},
            new Book{BookID=4, BookNm="Motivational Gurus"},
            new Book{BookID=5, BookNm="Spiritual Gurus"}
        };

        List<Order> bookOrders = new List<Order>{
            new Order{OrderID=1, BookID=1, PaymentMode="Cheque"},
            new Order{OrderID=2, BookID=5, PaymentMode="Credit"},
            new Order{OrderID=3, BookID=1, PaymentMode="Cash"},
            new Order{OrderID=4, BookID=3, PaymentMode="Cheque"},
            new Order{OrderID=5, BookID=3, PaymentMode="Cheque"},
            new Order{OrderID=6, BookID=4, PaymentMode="Cash"}
        };
    }
}

public class Book
{
    public int BookID { get; set; }
    public string BookNm { get; set; }
}

public class Order
{
    public int OrderID { get; set; }
    public int BookID { get; set; }
    public string PaymentMode { get; set; }
}

var orderForBooks = from bk in bookList
            join ordr in bookOrders
            on bk.BookID equals ordr.BookID
            select new
            {
                bk.BookID,
                Name = bk.BookNm,
                ordr.PaymentMode
            };

foreach (var item in orderForBooks)
    Console.WriteLine(item);

Console.ReadLine();