WP7 LINQ Delete not working - Getting NullReferenceException - linq

Question

I'm working on a WP7 app where i need to delete a row using the DeleteOnSubmit() method, but i keep getting a NullReferenceException error. I can't find where the issue is.
public void HardDeleteOrder(int deleteOrderId)
{
var oResult = from o in App.orderDataContext.orders
where o.OrderId == deleteOrderId
select o;
foreach (var oRow in oResult)
{
App.orderDataContext.Orders.DeleteOnSubmit(oRow);
}
App.orderDataContext.SubmitChanges();
}
When i run this, the code crashes on the ending brace of the method with an exception message "NullReferenceException was unhandled".
Here's the Stack trace:
System.NullReferenceException was unhandled
Message=NullReferenceException
StackTrace:
at System.Data.Linq.Mapping.MetaAccessor`2.SetBoxedValue(Object& instance, Object value)
at System.Data.Linq.ChangeProcessor.ClearForeignKeysHelper(MetaAssociation assoc, Object trackedInstance)
at System.Data.Linq.ChangeProcessor.ClearForeignKeyReferences(TrackedObject to)
at System.Data.Linq.ChangeProcessor.PostProcessUpdates(List`1 insertedItems, List`1 deletedItems)
at System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode)
at System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode)
at System.Data.Linq.DataContext.SubmitChanges()
at orders.viewmodels.OrderViewModel.HardDeleteOrder(Int32 deleteOrderId)
at orders.OrderView.RemoveOrderFromDatabase()
at orders.OrderView.RemoveOrder()
at orders.OrderView.detailsBarCancel_Click(Object sender, EventArgs e)
at orders.App.detailsBarCancel_Click(Object sender, EventArgs e)
at Microsoft.Phone.Shell.ApplicationBarItemContainer.FireEventHandler(EventHandler handler, Object sender, EventArgs args)
at Microsoft.Phone.Shell.ApplicationBarIconButton.ClickEvent()
at Microsoft.Phone.Shell.ApplicationBarIconButtonContainer.ClickEvent()
at Microsoft.Phone.Shell.ApplicationBar.OnCommand(UInt32 idCommand)
at Microsoft.Phone.Shell.Interop.NativeCallbackInteropWrapper.OnCommand(UInt32 idCommand)
What am I missing here ?

Answer 1

This is mostly due to foreign key constraint.
According to
http://msdn.microsoft.com/en-us/library/bb386925.aspx
LINQ to SQL does not support or recognize cascade-delete operations. If you want to delete a row in a table that has constraints against it, you must complete either of the following tasks:
Set the ON DELETE CASCADE rule in the foreign-key constraint in the database.
Use your own code to first delete the child objects that prevent the parent object from being deleted.