c# - 错误 :The ObjectContext instance has been disposed and can no longer be used for operations that require a connection

Question

我有一个列表框，当我从这个名为 ListofKBrands1 的列表框中选择一个项目时，我收到此错误消息:

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

在代码隐藏中，此错误的位置:

if (co.Company != null)

我的代码:

private void ListofKBrands1_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        RSPDbContext c = new RSPDbContext();

        if (ListofKBrands1.SelectedItem != null)
        {
            ListBoxItem item = ListofKBrands1.SelectedItem as ListBoxItem;
            KBrand co = item.Tag as KBrand;

            if (ListofKBrands1.SelectedItem != null)
                txtNewKBrand.Text = co.Name;
            else
                txtNewKBrand.Text = "";

            int count = 0;
            if (co.Company != null)
            {
                foreach (string a in cbCompany.Items)
                {
                    if (a == co.Company.Name)
                        cbCompany.SelectedIndex = count;
                    count++;
                }
            }
            else
                cbCompany.SelectedIndex = 0;
        }
    }

错误前:

我的 KBrand.cs:

public class KBrand {
    [Key]
    public int Id { get; set; }
    public String Name { get; set; }
    public virtual Company Company { get; set; }

    public override string ToString() {
        return Name;
    }
}

公司.cs:

public class Company
{
    [Key]
    public int Id { get; set; }
    public String Name { get; set; }

    public override string ToString() {
        return Name;
    }
}

如果选择的 KBrand 的公司为空，则不会出现此错误。但如果所选 KBrand 的公司不为空，我会收到此错误。我该如何解决此错误？提前致谢。

Answer 1

Company 在您的情况下应该是延迟加载的。但是您的实体现在处于分离状态(加载 KBrand 实体的上下文现在已被处置。因此，当您尝试访问 Company 属性时， Entity Framework 会尝试使用已处置的上下文来进行查询服务器。这给了你异常(exception)。

您需要将您的 KBrand 实体附加到当前上下文才能启用 lazy-loading

RSPDbContext c = new RSPDbContext();
KBrand co = item.Tag as KBrand;
c.KBrands.Attach(co);
// use co.Company

或者您需要使用 eager loading , 已加载 Company。当你得到元素时是这样的:

RSPDbContext db = new RSPDbContext();
var brands = db.KBrands.Include(b => b.Company).ToList();
// assign brands to listbox