Entity Framework using语句延迟加载

Question

我很好奇，当我深入研究 Entity Framework 时，我很乐意帮助我理解我在这里展示的这个例子中的延迟加载：

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);
    }
}

我已经对一些部分进行了编号。在第一种情况下，如您所见，我 select 所有客户，延迟加载是正确的，这会引发错误：

The ObjectContext instance has been disposed and can no longer be used for operations that require a connection

如果我取消注释第二部分，它将正常工作。它们之间的唯一区别是，在第二种情况下，我需要 select 列

Answer 1

这是因为JsonResult(来自return Json(...)语句)将在using块离开后由ASP执行，它会尝试序列化所有字段你的 Customer 对象，并会在你的 dbcontext 被处理后尝试访问 Territory 引用属性。

Entity Framework using语句延迟加载

Entity Framework lazy loading in using statement

c#

entity-framework

lazy-loading

eager-loading