Asked  7 Months ago    Answers:  5   Viewed   173 times

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

"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."

My code is:

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();
    }
}

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

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

 Answers

70

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; }
}

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; }
    }
}

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.

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);

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

Tuesday, June 1, 2021
 
Sethunath
answered 7 Months ago
86

I am guessing that the problem is that the execution of your LINQ query has been deferred until starting to access them on your view. At this point db has already been disposed.

Try this:

return View(Users.ToList());

Added ToList()

That will force the fetch from the DB before disposing db.

Saturday, July 3, 2021
 
Noob_Programmer
answered 6 Months ago
78

View -> Other windows -> Package Manager Console then run install-package entityframework -version 5.0.0.0.

Add -project <project.name> if you want to install it in a specific project.

Sunday, August 1, 2021
 
nasty
answered 5 Months ago
28

Be eager in the controller and call .ToList() before disposing in order to schedule the execution of the query immediately before you have left the using block (as after that it is too late, the context is gone):

using (MyEntities db = new MyEntities())
{
    var model = 
        from c in db.Categories
        select new Category 
        { 
            CategoryID = c.CategoryID, 
            Name = c.Name 
        };
    return View(model.ToList()); // <-- .ToList() here
}

Now, while this will solve your particular problem, what developers or dependency injection frameworks usually do is to instantiate the DbContext inside the BeginRequest event, store it inside the HttpContext.Items so that it is available throughout the execution of the entire request and inside the EndRequest method, retrieve it from HttpContext and dispose it.


UPDATE:

Also it is a good practice to use a view model containing only the properties you would like to use in this particular view:

public class CategoryViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }
}

and then inside your action:

public ActionResult Index()
{
    using (MyEntities db = new MyEntities())
    {
        var model = 
            from c in db.Categories
            select new CategoryViewModel
            { 
                Id = c.CategoryID, 
                Name = c.Name 
            };
        return View(model.ToList());
    }
}

and in the view:

@model IEnumerable<MyProject.Models.CategoryViewModel>

@foreach (var category in Model)
{
    <p>
        Id: @category.Id 
        Name: @category.Name
    </p>
}
Saturday, August 14, 2021
 
muncherelli
answered 4 Months ago
14

Open your EDMX file, click on an empty space in your designer view and in the properties window set Pluralize New Objects to False.

Tuesday, August 31, 2021
 
ysquared
answered 4 Months ago
Only authorized users can answer the question. Please sign in first, or register a free account.
Not the answer you're looking for? Browse other questions tagged :
 
Share