Multiplicity constraint violated (In XAF When createdObject.ObjectSpace.ModifiedObjects.Count has not been accessed.)

Question

Question Summary:

Why would a "Multiplicity constraint violated" be able to be worked around by the introduction of the following line of code?

var numModifiedObjects =createdObject.ObjectSpace.ModifiedObjects.Count;

Is there a setting that would make the work around unnecessary?

Background of the problem followed by the work around:

In my winforms xaf, EF6.2 Code First project I have the following business object which is self referencing.

[NavigationItem("Main")]
public class H2Category : IHCategory 
{
    public H2Category()
    {
        Children = new BindingList<H2Category>();
    }
    [Browsable(false)]
    public Int32 ID { get; protected set; }
    public String Name { get; set; }

    public H2Category Parent { get; set; }

    public virtual IList<H2Category> Children { get; set; }

    [NotMapped, Browsable(false), RuleFromBoolProperty("H2CategoryCircularReferences", DefaultContexts.Save, "Circular refrerence detected. To correct this error, set the Parent property to another value.", UsedProperties = "Parent")]
    public Boolean IsValid
    {
        get
        {
            H2Category currentObj = Parent;
            while (currentObj != null)
            {
                if (currentObj == this)
                {
                    return false;
                }
                currentObj = currentObj.Parent;
            }
            return true;
        }
    }
    IBindingList ITreeNode.Children => Children as IBindingList;

    ITreeNode IHCategory.Parent
    {
        get => Parent as IHCategory;
        set => Parent = value as H2Category;
    }
    ITreeNode ITreeNode.Parent => Parent as ITreeNode;
}

and in the dbContext OnModelCreating I use

   modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();

   modelBuilder.Entity<H2Category>().HasMany(x => x.Children).WithOptional(x => x.Parent);

Parent_Id);

The database looks correct in the SQL Server Object Explorer

and the data looks correct.

In the XAF Treeview, when the user adds a child record, I want the new child's parent to be set to the current record.

To do this I am using a NewObjectViewController

public partial class H2CategoryController : ViewController
{
    private NewObjectViewController controller;
    public H2CategoryController()
    {
        InitializeComponent();
        TargetObjectType = typeof(H2Category);
    }
    protected override void OnActivated()
    {
        controller = Frame.GetController<NewObjectViewController>();
        controller.ObjectCreated += controller_ObjectCreated;
        base.OnActivated();
    }
    private void controller_ObjectCreated(object sender, ObjectCreatedEventArgs e)
    {
        var createdObject = e.CreatedObject as H2Category;
        var currentObject = View.CurrentObject as H2Category;
        // inspection here shows that currentObject and createdObject are different but they have the same proxy
        var s = $"created object Id is {createdObject.ID}  current object id is {currentObject.ID}";
        Console.WriteLine(s);

        var propertyCollectionSource = (View as ListView)?.CollectionSource as PropertyCollectionSource;
        if (!(propertyCollectionSource?.MasterObject is H2Category master)) return;
        createdObject.Parent = master;
        createdObject.Name = "t";
        master.Children.Add(createdObject);

        Console.WriteLine(master.ID);

    }

    protected override void OnDeactivated()
    {
        controller.ObjectCreated += controller_ObjectCreated;
        base.OnDeactivated();
    }
}

As soon as the user starts typing into the new record the following error appears

    Multiplicity constraint violated. The role 'H2Category_Children_Source' of the relationship 'SBD.GL.Module.BusinessObjects.H2Category_Children' has multiplicity 1 or 0..1.

    at System.Data.Entity.Core.Objects.EntityEntry.AddDetectedRelationship[T](Dictionary`2 relationships, T relatedObject, RelatedEnd relatedEnd)
    at System.Data.Entity.Core.Objects.EntityEntry.AddRelationshipDetectedByGraph(Dictionary`2 relationships, Object relatedObject, RelatedEnd relatedEndFrom, Boolean verifyForAdd)
    at System.Data.Entity.Core.Objects.EntityEntry.DetectChangesInRelationshipsOfSingleEntity()
   at System.Data.Entity.Core.Objects.ObjectStateManager.DetectChangesInNavigationProperties(IList`1 entries)
   at System.Data.Entity.Core.Objects.ObjectStateManager.DetectChanges()
   at System.Data.Entity.Core.Objects.ObjectContext.DetectChanges()
   at DevExpress.ExpressApp.EF.EFObjectSpace.DetectChanges()
   at DevExpress.ExpressApp.EF.EFObjectSpace.GetModifiedObjects()
   at DevExpress.ExpressApp.EF.EFObjectSpace.get_ModifiedObjects()
   at DevExpress.ExpressApp.Win.SystemModule.CustomCollectModifiedObjectsEventArgs.CollectModifiedObjects(Boolean checkObjectSpaceIsModified)
   at DevExpress.ExpressApp.Win.SystemModule.LockController.GetModifiedObjects()
   at DevExpress.ExpressApp.Win.SystemModule.LockController.CheckLocking(LockController controller)
   at DevExpress.ExpressApp.Win.SystemModule.LockController.CheckLocking(Object obj)
   at DevExpress.ExpressApp.Win.SystemModule.LockController.ViewObjectSpace_ObjectChanged(Object sender, ObjectChangedEventArgs e)
   at System.EventHandler`1.Invoke(Object sender, TEventArgs e)
   at DevExpress.ExpressApp.BaseObjectSpace.OnObjectChanged(ObjectChangedEventArgs args)
   at DevExpress.ExpressApp.EF.EFObjectSpace.SetModified(Object obj, ObjectChangedEventArgs args)
   at DevExpress.ExpressApp.BaseObjectSpace.SetModified(Object obj, IMemberInfo memberInfo)
   at DevExpress.ExpressApp.DetailView.Editor_ControlValueChanged(Object sender, EventArgs e)
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at DevExpress.ExpressApp.Editors.PropertyEditor.OnControlValueChanged()
   at DevExpress.ExpressApp.Win.Editors.DXPropertyEditor.Editor_EditValueChanged(Object sender, EventArgs e)
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at DevExpress.XtraEditors.Repository.RepositoryItem.RaiseEditValueChangedCore(EventArgs e)
   at DevExpress.XtraEditors.Repository.RepositoryItem.RaiseEditValueChanged(EventArgs e)
   at DevExpress.XtraEditors.BaseEdit.RaiseEditValueChanged()
   at DevExpress.XtraEditors.BaseEdit.OnEditValueChanged()
   at DevExpress.XtraEditors.TextEdit.OnEditValueChanged()
   at DevExpress.XtraEditors.BaseEdit.OnEditValueChanging(ChangingEventArgs e)
   at DevExpress.XtraEditors.TextEdit.OnEditValueChanging(ChangingEventArgs e)
   at DevExpress.XtraEditors.BaseEdit.set_EditValue(Object value)
   at DevExpress.XtraEditors.TextEdit.OnMaskBox_ValueChanged(Object sender, EventArgs e)
   at DevExpress.XtraEditors.Mask.MaskBox.RaiseEditTextChanged()
   at DevExpress.XtraEditors.Mask.MaskBox.MaskStrategy.SimpleStrategy.DoAfterTextChanged(EventArgs e)
   at DevExpress.XtraEditors.Mask.MaskBox.OnTextChanged(EventArgs e)
   at System.Windows.Forms.TextBoxBase.WmReflectCommand(Message& m)
   at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
   at System.Windows.Forms.TextBox.WndProc(Message& m)
   at DevExpress.XtraEditors.Mask.MaskBox.BaseWndProc(Message& m)
   at DevExpress.XtraEditors.Mask.MaskBox.MaskStrategy.SimpleStrategy.DoWndProc(Message& m)
   at DevExpress.XtraEditors.Mask.MaskBox.WndProc(Message& m)
   at DevExpress.XtraEditors.TextBoxMaskBox.WndProc(Message& msg)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

I note that the debug values look correct when I set the parent

But am confused that all the H2Category objects have the same proxy value.

I have asked about the issue at Dev Express but ask here as well as it is Entity Framework related.

[Update -Work Around]

I added the IObjectSpaceLink interface to the business object. This exposes the ObjectSpace as a property of the business object.

When I put a break in the controller_ObjectCreated method I was able to see that the ModifiedObjects property was showing an error

I found that if I assign createdObject.ObjectSpace.ModifiedObjects.Count to a variable then the problem does not occur.

       private void controller_ObjectCreated(object sender, ObjectCreatedEventArgs e)
    {
        var createdObject = e.CreatedObject as H2Category;
        var numModifiedObjects =createdObject.ObjectSpace.ModifiedObjects.Count;

        var currentObject = View.CurrentObject as H2Category;
        var s = $"created object Id is {createdObject.ID}  current object id is {currentObject.ID}";

        var propertyCollectionSource = (View as ListView)?.CollectionSource as PropertyCollectionSource;
        if (!(propertyCollectionSource?.MasterObject is H2Category master)) return;

        var m = e.ObjectSpace.GetObject(master);

        createdObject.Parent = m;
        createdObject.Name = "t";
        m.Children.Add(createdObject);
        numModifiedObjects = createdObject.ObjectSpace.ModifiedObjects.Count;
        Console.WriteLine(master.ID);

    }

I tried disabling Proxy creation but it did not help.

Answer 1

From the Dev Express support answer.

The relationship works incorrectly in my project because of a mistake in my H2Category class. The Parent property must be declared virtual.

Also, my ICategoryController conflicts with the built-in TreeNodeController. This controller also subscribes to the NewObjectViewController.ObjectCreated event and links a newly created ITreeNode object with a parent node selected in the source list view. By default, objects are linked when changes are saved. To link the new object immediately upon creation, set the NewObjectViewController.LinkNewObjectToParentImmediately property to true.