XmlSerializer serializer = new XmlSerializer(typeof(OrderedItem));

using (FileStream stream = File.Open("d:/123.txt", FileMode.Create))
{
    using (MemoryStream ms = new MemoryStream())
    {
        OrderedItem i = new OrderedItem();
        i.ItemName = "Widget1";
        i.Description = "Regular Widget1";
        i.Quantity = 10;
        i.UnitPrice = (decimal)2.30;

        serializer.Serialize(ms, i);

        int len = (int)ms.Length;
        stream.Write(BitConverter.GetBytes(len), 0, sizeof(int));
        stream.Write(ms.ToArray(), 0, (int)ms.Length);
    }
    using (MemoryStream ms = new MemoryStream())
    {
        OrderedItem i = new OrderedItem();
        i.ItemName = "Widget2";
        i.Description = "Regular Widget2";
        i.Quantity = 10;
        i.UnitPrice = (decimal)2.30;

        serializer.Serialize(ms, i);

        int len = (int)ms.Length;
        stream.Write(BitConverter.GetBytes(len), 0, sizeof(int));
        stream.Write(ms.ToArray(), 0, (int)ms.Length);
    }
}

var bytes = File.ReadAllBytes("d:/123.txt");
int index = 0;
{
    int len = BitConverter.ToInt32(bytes, index);
    index += sizeof(int);
    using (MemoryStream ms = new MemoryStream(bytes, index, len))
    {
        index += len;
        OrderedItem item = (OrderedItem)serializer.Deserialize(ms);
    }
}
{
    int len = BitConverter.ToInt32(bytes, index);
    index += sizeof(int);
    using (MemoryStream ms = new MemoryStream(bytes, index, len))
    {
        index += len;
        OrderedItem item = (OrderedItem)serializer.Deserialize(ms);
    }
}