Question

I have a webpage with a form that is used to edit some object. This object contains a Collection of other objects defined like this:

Public Overridable Property Employees As List(Of Employee)

On a form I can delete an employee, add a new one or modify existing one. When I click save new values are sent to the server. On a server I check if the user exists. If exists then I modify its values, if it does not exist then I add it. All employees that exist on the server and were not sent are marked as deleted (State changed to EntityState.Deleted). I try to use the following code (dbCollection = database entities, newCollection = collection sent from the form):

    For Each item In dbCollection
        Dim dbItem = item
        Dim newTask = newCollection.FirstOrDefault(Function(i) i.Id = dbItem.Id)


        If newTask Is Nothing Then
            Me.Entry(item).State = EntityState.Deleted
        Else
            Me.Entry(item).CurrentValues.SetValues(newTask)
            newCollection.Remove(newTask)
        End If
    Next

    For Each item In newCollection
        dbCollection.Add(item)
    Next

    Me.SaveChanges() 

This code does not work, because changing to EntityState.Deleted removes the object from collection, and for each loop breaks, since the collection is modified…

I know that I can overcome this problem by using a for loop or adding objects to delete to some other list first, but I hope maybe there is a pattern that would make my code nicer.

Thanks in advance for all suggestions.

Answer 1

Replace…

For Each item In dbCollection

…by:

For Each item In dbCollection.ToList()

ToList() will create a copy of the collection (only the references, not the objects themselves). dbCollection.ToList() is another collection than dbCollection so that you safely can modify the dbCollection without getting the “collection has been modified” exception in the For Each loop.