Auto-increment a primary key in the LINQ to SQL DataContext “Insert” partial method

Partial Public Class MyDataContext

    Private Sub InsertProduct(ByVal instance As Product)

        Dim id As Integer = Me.Products.Max(Function(p As Product) p.ProductID) + 1
        instance.ProductID = id

        Me.ExecuteDynamicInsert(instance)

    End Sub

End Class

Using context As New MyDataContext

    Dim product1 As New Product
    context.Products.InsertOnSubmit(product1)
    context.SubmitChanges() 'This works

    Dim product2 As New Product
    context.Products.InsertOnSubmit(product2)
    context.SubmitChanges() 'DuplicateKeyException

End Using

Private Sub InsertProduct(ByVal instance As Product)

    Dim id As Guid = Guid.NewGuid
    instance.ProductID = id

    Me.ExecuteDynamicInsert(instance)

End Sub