如何解决错误的ObjectContext实例已设置，不能再被用于需要连接的操作 [英] How to solve the error The ObjectContext instance has been disposed and can no longer be used for operations that require a connection

问题描述

我试图填充 GridView控件使用实体Frameworkm但每次我收到以下错误时间：

I am trying to populate a GridView using Entity Frameworkm but every time I am getting the following error:

在对象属性访问器'LoanProduct'COSIS_DAL.MemberLoan
  发生以下异常：该ObjectContext的实例已
  布置和不再能够用于需要一个操作
  连接。

"Property accessor 'LoanProduct' on object 'COSIS_DAL.MemberLoan' threw the following exception: The ObjectContext instance has been disposed and can no longer be used for operations that require a connection."

我的code是：

     public List<MemberLoan> GetAllMembersForLoan(string keyword)
{
    using (CosisEntities db = new CosisEntities())
    {
        IQueryable<MemberLoan> query = db.MemberLoans.OrderByDescending(m => m.LoanDate);
        if (!string.IsNullOrEmpty(keyword))
        {
            keyword = keyword.ToLower();
            query = query.Where(m =>
                  m.LoanProviderCode.Contains(keyword)
                  || m.MemNo.Contains(keyword)
                  || (!string.IsNullOrEmpty(m.LoanProduct.LoanProductName) && m.LoanProduct.LoanProductName.ToLower().Contains(keyword))
                  || m.Membership.MemName.Contains(keyword)
                  || m.GeneralMasterInformation.Description.Contains(keyword)

                  );
        }
        return query.ToList();
    }
}


  protected void btnSearch_Click(object sender, ImageClickEventArgs e)
  {
    string keyword = txtKeyword.Text.ToLower();
    LoanController c = new LoanController();
    List<COSIS_DAL.MemberLoan> list = new List<COSIS_DAL.MemberLoan>();
    list = c.GetAllMembersForLoan(keyword);

    if (list.Count <= 0)
    {
        lblMsg.Text = "No Records Found";
        GridView1.DataSourceID = null;
        GridView1.DataSource = null;
        GridView1.DataBind();
    }

    else
    {
        lblMsg.Text = "";
        GridView1.DataSourceID = null;
        GridView1.DataSource = list;
        GridView1.DataBind();
    }


  }

该错误是提的GridView 的 LoanProductName 列。提到：我使用C＃，ASP.net，SQL-Server 2008作为后端数据库

The error is mentioning the LoanProductName column of the Gridview. Mentioned: I am using C#, ASP.net, SQL-Server 2008 as back end DB.

我很新的实体框架。我不明白为什么我收到此错误。谁能帮我吗？

I am quite new to Entity Framework. I can't understand why I am getting this error. Can anyone help me please?

推荐答案

默认情况下实体框架采用延迟加载导航性能。这就是为什么这些属性应标记为虚拟的 - EF为您创建的实体代理类，并覆盖导航属性允许延迟加载。例如。如果你有这样的实体：

By default Entity Framework uses lazy-loading for navigation properties. That's why these properties should be marked as virtual - EF creates proxy class for your entity and overrides navigation properties to allow lazy-loading. E.g. if you have this entity:

public class MemberLoan
{
   public string LoandProviderCode { get; set; }
   public virtual Membership Membership { get; set; }
}

实体框架将返回从这个实体中继承的代理和提供的DbContext实例添加到该代理，以便在以后允许会员延迟加载：

Entity Framework will return proxy inherited from this entity and provide DbContext instance to this proxy in order to allow lazy loading of membership later:

public class MemberLoanProxy : MemberLoan
{
    private CosisEntities db;
    private int membershipId;
    private Membership membership;

    public override Membership Membership 
    { 
       get 
       {
          if (membership == null)
              membership = db.Memberships.Find(membershipId);
          return membership;
       }
       set { membership = value; }
    }
}

所以，实体具有的DbContext的实例，它用于装载实体。那是你的问题。您有周围使用CosisEntities使用块。其中部署上下文中返回的实体之前。当一些code以后尝试使用延迟加载导航属性，它失败了，因为背景被设置在那一刻。

So, entity has instance of DbContext which was used for loading entity. That's your problem. You have using block around CosisEntities usage. Which disposes context before entities are returned. When some code later tries to use lazy-loaded navigation property, it fails, because context is disposed at that moment.

要解决此问题，您可以使用导航性能的渴望加载，稍后将需要：

To fix this behavior you can use eager loading of navigation properties which you will need later:

IQueryable<MemberLoan> query = db.MemberLoans.Include(m => m.Membership);

将pre-负载的所有成员和延迟加载将不会被使用。有关详情请参见加载相关实体 MSDN上的文章。

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