解决"ObjectContext实例已被放置，并且不能再用于需要连接的操作". InvalidOperationException [英] Solving "The ObjectContext instance has been disposed and can no longer be used for operations that require a connection" InvalidOperationException

问题描述

我试图使用Entity Frameworkm填充GridView，但是每次遇到以下错误时:

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

对象'COSIS_DAL.MemberLoan'上的属性访问器'LoanProduct' 引发以下异常:ObjectContext实例已 已废弃，不能再用于需要 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."

我的代码是:

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();
    }
}

该错误是提及Gridview的LoanProductName列.提及:我正在使用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.

我是实体框架的新手.我不明白为什么会收到此错误.有人可以帮我吗?

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，Entity Framework将延迟加载用于导航属性.这就是为什么应将这些属性标记为虚拟的原因-EF为您的实体创建代理类，并覆盖导航属性以允许延迟加载.例如.如果您有这个实体:

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; }
}

实体框架将返回从该实体继承的代理，并向该代理提供DbContext实例，以便稍后可以延迟加载成员资格:

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; }
    }
}

因此，实体具有用于加载实体的DbContext实例.那是你的问题.您在CosisEntities用法周围有using块.它在返回实体之前布置上下文.稍后当某些代码尝试使用延迟加载的导航属性时，它会失败，因为此时已处理上下文.

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，您可以使用后面将急需的导航属性加载:

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);

这将预加载所有成员资格，并且不会使用延迟加载.有关详细信息，请参见MSDN上的加载相关实体.

That will pre-load all memberships and lazy-loading will not be used. For details see Loading Related Entities article on MSDN.

这篇关于解决"ObjectContext实例已被放置，并且不能再用于需要连接的操作". InvalidOperationException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！