Question

I am getting an error when calling entities.savechanges() on my EF 4.3.1. My database is a sql ce v4 store and I am coding in the mvvm pattern. I have a local version of my context that I send to an observable collection and modify etc. This works fine, and when I call savechanges() when no rows exist in the database the objects persist fine. When I reload the application, the objects are populated in my listbox as they should, however if I add another object and call savechanges() I get an error saying that a duplicate value cannot be inserted into a unique index.

From my understanding it means that the context is trying to save my entities to the datastore, but it seems to be adding my untouched original objects as well as the new one. I thought it would leave them alone, since their state is unchanged.

private void Load()
    {
        entities.Properties.Include("Images").Load();
        PropertyList = new ObservableCollection<Property>();
        PropertyList = entities.Properties.Local;        

        //Sort the list (based on previous session stored in database)
        var sortList = PropertyList.OrderBy(x => x.Sort).ToList();
        PropertyList.Clear();
        sortList.ForEach(PropertyList.Add);

        propertyView = CollectionViewSource.GetDefaultView(PropertyList);
        if (propertyView != null) propertyView.CurrentChanged += new System.EventHandler(propertyView_CurrentChanged);     


        private void NewProperty()
    {
        try
        {
            if (PropertyList != null)
            {                                             
                Property p = new Property()
                    {
                        ID = Guid.NewGuid(),
                        AgentName = "Firstname Lastname",
                        Address = "00 Blank Street",
                        AuctioneerName = "Firstname Lastname",
                        SaleTitle = "Insert a sales title",
                        Price = 0,
                        NextBid = 0,
                        CurrentImage = null,
                        Status = "Auction Pending",
                        QuadVis = false,
                        StatVis = false, //Pause button visibility
                        Sort = PropertyList.Count + 1,                            
                    };

                PropertyList.Add(p);
                SaveProperties();
            }

        private void SaveProperties()
    {
        try
        {               
            foreach (var image in entities.Images.Local.ToList())
            {
                if (image.Property == null)
                    entities.Images.Remove(image);
            }                
        }

        catch (Exception ex)
        {
            System.Windows.MessageBox.Show(ex.Message);
        }

        entities.SaveChanges();
    }

Answer 1

Without commenting on all the code here this is the bit that’s causing the specific problem you bring up:

//Sort the list (based on previous session stored in database) 
var sortList = PropertyList.OrderBy(x => x.Sort).ToList(); 
PropertyList.Clear(); 
sortList.ForEach(PropertyList.Add); 

This code:

• Starts with entities that have been queried and are being tracked by the context as Unchanged entities. That is, entities that are known to already exist in the database.
• Creates a new sorted list of these entities.
• Calls Clear on the local collection causing each tracked entity to be marked as deleted and removed from the collection.
• Adds each entity back to the context putting it now in an Added state meaning that it is new and will be saved to the database when SaveChanges is called,

So effectively you have told EF that all the entities that exist in the database actually don’t exist and need to be saved. So it tries to do this and it results in the exception you see.

To fix this don’t clear the DbContext local collection and add entities back. Instead you should sort in the view using the local collection to back the view.