LINQ 数据上下文处置问题

Question

在下面的 DAL 方法中，我收到 Cannot access object: DataContext after it's was dispose 。我想我可以在那里打电话处理。 result 是一个 IEnumurable，我认为是 IQueryable 导致了此类问题。

我做错了什么？我应该如何处置我的DataContext。有没有比 DataTable 返回更好的东西？这是一个指向 SQL 2005 的桌面应用程序。

导致此错误的示例方法 -->

---

    public static DataTable GetEnrolledMembers(Guid workerID)
    {
        var DB = CmoDataContext.Create();

        var AllEnrollees = from enrollment in DB.tblCMOEnrollments
                           where enrollment.CMOSocialWorkerID == workerID || enrollment.CMONurseID == workerID
                           join supportWorker in DB.tblSupportWorkers on enrollment.EconomicSupportWorkerID
                               equals supportWorker.SupportWorkerID into workerGroup
                           from worker in workerGroup.DefaultIfEmpty()
                           select
                               new
                               {
                                   enrollment.ClientID,
                                   enrollment.CMONurseID,
                                   enrollment.CMOSocialWorkerID,
                                   enrollment.EnrollmentDate,
                                   enrollment.DisenrollmentDate,
                                   ESFirstName = worker.FirstName,
                                   ESLastName = worker.LastName,
                                   ESPhone = worker.Phone
                               };

        var result = from enrollee in AllEnrollees.AsEnumerable()
                     where (enrollee.DisenrollmentDate == null || enrollee.DisenrollmentDate > DateTime.Now)
                     //let memberName = BLLConnect.MemberName(enrollee.ClientID)
                     let lastName = BLLConnect.MemberLastName(enrollee.ClientID)
                     let firstName = BLLConnect.MemberFirstName(enrollee.ClientID)
                     orderby enrollee.DisenrollmentDate ascending, lastName ascending
                     select
                         new
                         {
                             enrollee.ClientID,
                             //MemberName = memberName,
                             LastName = lastName,
                             FirstName = firstName,
                             NurseName = BLLAspnetdb.NurseName(enrollee.CMONurseID),
                             SocialWorkerName = BLLAspnetdb.SocialWorkerName(enrollee.CMOSocialWorkerID),
                             enrollee.EnrollmentDate,
                             enrollee.DisenrollmentDate,
                             ESWorkerName = enrollee.ESFirstName + " " + enrollee.ESLastName,
                             enrollee.ESPhone
                         };
        DB.Dispose();

        return result.CopyLinqToDataTable();
    }

我在其中创建 DataContext 的部分类 -->

---

partial class CmoDataContext
{
    public static bool IsDisconnectedUser
    {
        get
        {
            return Settings.Default.IsDisconnectedUser;
        }
    }

    public static CmoDataContext Create()
    {
        var cs = IsDisconnectedUser ? Settings.Default.CMOConnectionString : Settings.Default.Central_CMOConnectionString;
        return       new CmoDataContext(cs);
    }

Answer 1

您在处置 DataContext 后调用 CopyLinqToDataTable。由于 LINQ 使用延迟执行，因此它仅在 DataContext 被释放后枚举 result 时才执行查询。

您应该将 DataContext 放入包含 return 语句的 using 块中。
这将在调用CopyLinqToDataTable后处置DataContext，从而避免该问题。

例如：

using(var DB = CmoDataContext.Create()) {
    //Execute queries
    return result.CopyLinqToDataTable();
}

using 语句生成一个 finally 块，该块在最后处理 DataContext
using 块的。 （即使抛出异常）

Answer 2

在调用 Dispose 之前，您应该在查询中使用类似 ToList 的内容

Answer 3

你就不能这样做吗？

...
DataTable dt = result.CopyLinqToDataTable(); 
DB.Dispose(); 
return dt;