LINQ to SQL - >无法访问已处置的对象。对象名：'Dispose后访问的DataContext

Question

Let say I have 2 tables which is Student and School. Within my Student table I have a fkSchoolId linking to a School record. However, if I retrieve my record as below

public static List<Student> GetByType(string connString, int type)
{
    using (mydb_DataContext db = new mydb_dbDataContext(connString))
    {
        return (from t1 in db.Students
                where t1.type = type
                select t1).ToList();
    }
}

I will have the list of Student objects where I can access it in a foreach loop. But when I do as below, I will get an error when retrieving the school name.

foreach(Student student in DAL.StudentLogic.GetByType(5))
{
    string schoolName = student.School.Name;
}

System.ObjectDisposedException: 'Cannot access a disposed object. Object name: 'DataContext accessed after Dispose.'.'

May I know how can I get the foreign information stored in the return object so that I can access them? Or better way, if I can specify to load up just the school Name?

Updated: If I do as follow, it works, but not sure how much it will impact the performance. I will do a benchmark and update to this topic again next week.

public static List<Student> GetByType(string connString, int type)
{
    using (mydb_DataContext db = new mydb_dbDataContext(connString))
    {
        List<Student> students = (from t1 in db.Students where t1.type = type select t1).ToList();

        foreach(Student student in students)
        {
            student.School.Name = db.Schools.Where(q => q.SchoolId == student.fkSchoolId).FirstOrDefault().Name;
        }
    }
}

I'll be able to access to student.School.Name in my return object.

Answer 1

Set the DeferredLoadingEnabled property to false :

Gets or sets a value that indicates whether to delay-load one-to-many or one-to-one relationships.

Therefor the related data will be retrieved when materializing the query and not retrieved in a later stage (after context was disposed)

public static List<Student> GetByType(string connString, int type)
{
    using (mydb_DataContext db = new mydb_dbDataContext(connString))
    {
        db.DeferredLoadingEnabled = false;
        return (from t1 in db.Students
                where t1.type = type
                select t1).ToList();
    }
}

However consider thinking (depends on overall design/ program requirements and load) of having the context kept open for the lifetime of the class this functions sits in (in seems like a DAL class). Then implement the IDisposable interface and dispose the context in it. (Remember that the Dispose must be called explicitly).

---

If all you want is the school name and you are using C# 7.0 you can use the named tuples this way:

public static List<(Student Student, string SchoolName)> GetByType(string connString, int type)
{
    using (mydb_DataContext db = new mydb_dbDataContext(connString))
    {
        return (from t1 in db.Students
                join school in db.Schoold on t1.SchoolId equals school.Id
                where t1.type = type
                select (t1, school.Name)).ToList();
    }
}

If you get compilation error CS8137 then you need to install the Nuget package of System.ValueTuple

Answer 2

With Linq2Sql you can use LoadWith , eg

using (mydb_DataContext db = new mydb_dbDataContext(connString))
{
    DataLoadOptions op = new DataLoadOptions();
    op.LoadWith<Student>(o => o.School);

    db.LoadOptions = op;
    return (from t1 in db.Students
        where t1.type = type
        select t1).ToList();
}

The one downside is that this is retrieving all columns in the school table.