Entity Framework context issue

Question

Alright...Entity Framework 6 question for all y'alls.

I'm writing a small intranet portal using EF. It's a pretty simple CRUD using a gridview. Everthings to be going swimmingly but I'm having an issue while attempting to save changes.

For the sake of paging, sorting and filtering on the gridview, I create a session variable that holds a list of entity objects then I bind that session variable to the gridview.

//Class level variables
private nsdc_supplyEntities _context = new nsdc_supplyEntities();
private VlanClass _vc;
private EnvironmentTypesClass _ec;

protected void Page_Load(object sender, EventArgs e)
{
    _vc = new VlanClass(_context);
    _ec = new EnvironmentTypesClass(_context);

    if (!IsPostBack)
    {
        Session["vlans"] = _vc.GetAllVlans();
        BindData();
    }
}

protected void BindData()
{
    GridView1.DataSource = Session["vlans"];
    GridView1.DataBind();
}

protected void GridView1_EditVlan(object sender, GridViewEditEventArgs e)
{
    GridView1.EditIndex = e.NewEditIndex;
    BindData();
}

This way, if the user filters the list down to a smaller result set or sorts and pages then does some other event that triggers a postback, the gridview's set remains constant.

But from what I can tell, the list of Vlans in Session["vlans"] no longer has a context (due to a postback event?). So when I try to edit the selected vlan and save it (particularily when I try to add an environment_type child entity to the vlan entity) I get an error.

I've tried reattaching the vlan entry back into _context but that brought up a whole 'nother ball of wax. What is the best way to do this?

Here's my update as it stands:

protected void UpdateVlan(object sender, GridViewUpdateEventArgs e)
{
    CheckBoxList environmenttypesCBL = (GridView1.Rows[e.RowIndex].FindControl("EnvironmenttypesCBL") as CheckBoxList);
    int vlanid = Convert.ToInt32(((Label)GridView1.Rows[e.RowIndex].FindControl("lblid")).Text);

    //At first I was creating a new instance of vlan in question and saving that, 
    //but then database and the Session["vlan"] variable (and gridview) is out of sync
    //vlan editedVlan = new vlan();
    //editedVlan = _vc.GetVlanByID(vlanid);

    //Get the vlan I am editing from Session["vlan"]
    List<vlan> list = Session["vlans"] as List<vlan>;
    vlan editedVlan = list.Find(i => i.id == vlanid);

    //Tried reattaching but "The object cannot be detached because it is not attached to the ObjectStateManager."
    //_context.Attach(editedVlan);

    #region EnvironmentTypes
    editedVlan.environment_type.Clear();
    if (environmenttypesCBL.Items.Count > 0)
    {
        foreach (ListItem l in environmenttypesCBL.Items.Cast<ListItem>())
        {
            //tried adding an environment_type to the editedVlan but "The object cannot be detached because it is not attached to the ObjectStateManager."
            if (l.Selected)
                editedVlan.environment_type.Add(_ec.GetEnvironmentTypeByID(Convert.ToInt32(l.Value)));
        }
    }
    #endregion

    _vc.UpdateSelectedVlan();

    GridView1.EditIndex = -1;
    BindData();
}

T McKeown · Accepted Answer · 2014-01-10 20:54:13Z

1

Your _context is created EVERY time you POST or GET, and the list of vlans are bound to a different context as a result.

Session doesn't care about POST or GET, I think you should change this:

_vc = new VlanClass(_context);
_ec = new EnvironmentTypesClass(_context);

if (!IsPostBack)
{
    Session["vlans"] = _vc.GetAllVlans();
    BindData();
}

To this:

if (Session["vlans"] == null )
{
   _context = new nsdc_supplyEntities();  // only create the context here not in the instance var declaration.
   _vc = new VlanClass(_context);
   _ec = new EnvironmentTypesClass(_context);
    Session["vc"] = _vc;
    Session["vlans"] = _vc.GetAllVlans();
    Session["context"] = _context;
}
else
{
    _context = (DbContext)Session["context"];  // cast to your Context
   _vc = (VlanClass) Session["vc"]  ;
}

if (!IsPostBack)
{
    BindData();
}

edited Jan 10, 2014 at 20:54

answered Jan 10, 2014 at 20:17

T McKeown

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3 Comments

Jeremy Over a year ago

That is interesting. This is more elegant that what I was doing. I'm a little confused about the second line of the else. The session variable is a list of vlans. My guess is you're trying to attach it to _vc or something but _vc is just an instance of a class with a bunch of query methods. I would think List<vlan> list = (List<"vlans">)Session["vlans"]; would make more sense but I'm not sure that's what you're getting at.

T McKeown Over a year ago

ah, yes... I screwed that up (will edit). But essentially you are losing your context because you are creating a new one.

Jeremy Over a year ago

Ok, so I posted a couple times on here with issues then realized it those issues were caused by my own stupidity. (so I posted and deleted a few times) However, I'm excited to say, this solution works as advertised and I am very greatful. Thank you TMK.

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