using System;

using System.IO;

using System.Collections.Generic;

using System.Xml.Serialization;

​

public class Program

{

    public static void Main()

    {

        const string xml = @"<Invoices>

    <Invoice>

        <InvoiceId>12345</InvoiceId>

        <LineItems>

            <LineItem>

                <LineItemId>123</LineItemId>

            </LineItem>

            <LineItem>

                <LineItemId>456</LineItemId>

            </LineItem>

        </LineItems>

    </Invoice>

</Invoices>";

​

        var serializer = new XmlSerializer(typeof (InvoiceList));

        using (var reader = new StringReader(xml))

        {

            var list = (InvoiceList) serializer.Deserialize(reader);

            Console.WriteLine(list.Invoices.Count);

        }

    }

    [XmlRoot("Invoices")]

    public class InvoiceList

    {

        [XmlElement("Invoice")]

        public List<Invoice> Invoices { get; set; }

    }

​

    public class Invoice

    {

        public int InvoiceId { get; set; }

​

        public List<LineItem> LineItems { get; set; }

    }

​

    public class LineItem

    {

        public int LineItemId { get; set; }

    }

}