Correct way to refresh datagrid bound to a CollectionViewSource from database using EF 6.0

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WPF

DataGrid

entity-framework

I have a datagrid bound to a CollectionViewSource:

    <Window.Resources>
        <CollectionViewSource x:Key="vs"></CollectionViewSource>
    </Window.Resources>


<DataGrid x:Name="dg" ItemsSource="{Binding Source={StaticResource vs}}" AutoGenerateColumns="true"/>

I populate the CollectionViewSource.Source using entity framework:

Private vsAs CollectionViewSource
vs = CType(FindResource("vs"), CollectionViewSource)

dim db as new dbcontext
vs.Source = db.SomeEntities.ToList

I want this data to refresh itself from the database every so often, at the moment I have it on a timer like so:

    'invoker is used so objects of a different thread can be accessed
    Me.Dispatcher.Invoke(Sub()
                           dim db as new dbcontext
                           vs.Source = db.SomeEntities.ToList
                         End Sub)


End Sub

However this causes any sorting from the datagrid, along with any selected items to be lost, it also causes a noticable delay on screen.

What is the correct way to update a collectionviewsource from a database using entity framework?

Thanks