实体框架在using语句中延迟加载

我很好奇，当我深入到实体框架，将很高兴帮助我理解这个例子中的延迟加载，我在这里展示:

public partial class Customer
{
    public int CustomerID { get; set; }
    public Nullable<int> PersonID { get; set; }
    public Nullable<int> StoreID { get; set; }
    public Nullable<int> TerritoryID { get; set; }
    public string AccountNumber { get; set; }
    public System.DateTime ModifiedDate { get; set; }
    public virtual Territory Territory { get; set; }
}
public partial class Territory
{
    public Territory()
    {
        this.Customers = new HashSet<Customer>();
    }
    public int TerritoryID { get; set; }
    public string Name { get; set; }
    public string CountryRegionCode { get; set; }
    public string C_Group_ { get; set; }
    public decimal SalesYTD { get; set; }
    public decimal SalesLastYear { get; set; }
    public virtual ICollection<Customer> Customers { get; set; }
}

public ActionResult LoadData()
{
    var draw = Request.Form.GetValues("draw").FirstOrDefault();
    var start = Request.Form.GetValues("start").FirstOrDefault();
    var length = Request.Form.GetValues("length").FirstOrDefault();
    var sortColumn = Request.Form.GetValues("columns[" + Request.Form.GetValues("order[0][column]").FirstOrDefault() + "][name]").FirstOrDefault();
    var sortColumnDir = Request.Form.GetValues("order[0][dir]").FirstOrDefault();
    var pageSize = length != null ? Convert.ToInt32(length) : 0;
    var skip = start != null ? Convert.ToInt32(start) : 0;
    int totalRecords = 0;
    using (MyDatabaseEntities dc = new MyDatabaseEntities())
    {           
        //1.
        var items = dc.Customers.Select(a => a);
        //2. 
        //var items = dc.Customers.Select(a => new
        //{
        //    a.CustomerID,
        //    a.PersonID,
        //    a.StoreID,
        //    TerritoryName = a.Territory.Name,
        //    a.AccountNumber,
        //    a.ModifiedDate
        //});
        if (!(string.IsNullOrEmpty(sortColumn) && string.IsNullOrEmpty(sortColumnDir)))
        {
            items = items.OrderBy(sortColumn + " " + sortColumnDir);
        }
        totalRecords = items.Count();
        var data = items.Skip(skip).Take(pageSize).ToList();
        return Json(new { recordsFiltered = totalRecords, recordsToral = totalRecords, data = data }, JsonRequestBehavior.AllowGet);
    }
}

我已经给一些零件编号了。在第一种情况下，如您所见，我选择了所有客户，延迟加载为true，这会抛出一个错误:

ObjectContext实例已被处置，不能再用于需要连接的操作

如果我取消第二部分的注释，它将正常工作而不会出现错误。它们之间唯一的区别是，第二种情况下，我选择我需要的列