Question

So I am trying to Delete some items from the database before I Insert some data. For some reason, C# Entity Framework and ASP.NET do not enjoy me doing this simple operation and gives out another one of those vague errors.

I am trying to update a set of rows of information in a table (using foreach loops and Insert/Delete), as well as some properties in another table (using update)

How do I fix it? The error occurs on the Delete function, on the line:
this.entities.tableModule.Attach(module);

I get: “System.InvalidOperationException: An entity object cannot be referenced by multiple instances of IEntityChangeTracker.”

public void Insert(tableModule module)
    {
        this.entities.tableModule.AddObject(module);
        this.entities.SaveChanges();
    }    
public void Delete(tableModule module)
    {
        this.entities.tableModule.Attach(module); // error HERE
        this.entities.DeleteObject(module);
        this.entities.SaveChanges();
    }

Further down the code I call Delete and Insert and Update for another table.

try
        {
            DifferentClass.Update(tu);
            foreach (tableModule mitem in list_to_del)
            {
                moduleclass.Delete(mitem); // remove all super roles by the user.
            }
            foreach (tableModule m_to_add in add_list)
            {
                moduleclass.Insert(m_to_add);
            }
        }

From what I gather, the attach function is maybe being run more than once and that for some reason is something that the designers of C# Entity Framework are completely caught by surprise for, and so they produce an error. Though it’s most likely my complete lack of understanding of entity framework.

EDIT:
Here’s how I get the list_to_del array.

List<tableModule> list_to_del = (from m in entities.tableModule where m.userid == (long)userID select m).ToList(); 

EDIT2:

New test code:

public void InsertQueue(tableModule module)
{
    this.entities.tableModule.AddObject(module);
}
public void DeleteQueue(tableModule module)
{
    this.entities.tableModule.Attach(module);
    this.entities.DeleteObject(module);
}
public void FinallySave(){
    this.entities.SaveChanges();
}

EDIT 3 —- SOLUTION —–

The test code worked sort of (though inserts didn’t for some odd reason). The problem was, I was calling list_to_del query to get that array of tableModules from a tableUser class which has its own entityset/context. So the results in list_to_del were of a different context, than the call to the class that does DeleteQueue / Insert etc.

I changed:

List<tableModule> list_to_del = (from m in entities.tableModule where m.userid == (long)userID select m).ToList(); 

to the appropriate class that has the SAME query:

List<tableModule> list_to_del = ModuleClass.GetListofModulesByUserID(userID);

Of course ModuleClass.InsertQueue and ModuleClass.DeleteQueue is called.

So everything uses the same entities object (context/entityset).

So the error happens when you call a query on a context, that returns a set of results that are ALREADY attached to that context. Then when you call Attach again, a problem occurs because they already are in a context on some other class/instance.

Answer 1

You don’t need to call Attach if you loaded module from the same context instance – you can call DeleteObject directly. When you load entity from the context it is by default tracked by the context (= it is attached) and you cannot attach it again. You also cannot attach it to different context so make sure that list_to_del is loaded by the same context instance as you use for deletion (= it must be loaded in the same request processing because you should have a new context instance per request).