How do I update ComboBox SelectedItem when bound to a viewmodel?

See more:

VB

XAML

combobox

I've got a Combobox bound in the following fashion:

<combobox itemssource="{Binding BoomSections}" selecteditem="{Binding XPath=SelectedRodConnectionBoomSection, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" displaymemberpath="BoomSectionInstance.Description" issynchronizedwithcurrentitem="False" />

The problem I'm having is when I initially populate the value of SelectedRodConnectionBoomSection, the Combobox does not show the value of BoomSectionInstance.Description. If I put a breakpoint in the property get, the value is correct, but the value shown in the combobox is blank. If I select a value, I see all of the items populated in BoomSections (with BoomSelectionInstance.Description). I can actually select a value and the value populates on the Combobox as expected. So... The problem appears to be in the initialization of the value. Any thoughts?

BoomSections is defined as:

Public Property BoomSections As ObservableCollection(Of BoomSectionViewModel)
   Get
      Return _boomSections
   End Get
   Set(value As ObservableCollection(Of BoomSectionViewModel))
      _boomSections = value
      NotifyPropertyChanged(Function() BoomSections)
   End Set
End Property

SelectedRodConnectionBoomSection is defined as:

Public Property SelectedRodConnectionBoomSection As BoomSectionViewModel
   Get
      Return _selectedRodConnectionBoomSection
   End Get
   Set(value As BoomSectionViewModel)
      _selectedRodConnectionBoomSection = value
      NotifyPropertyChanged(Function() SelectedRodConnectionBoomSection)
   End Set
End Property

The BoomSectionViewModel contains a property called BoomSectionInstance is defined as:

Public Property BoomSectionInstance As BoomSection
   Get
      Return _boomSectionInstance
   End Get
   Set(value As BoomSection)
      _boomSectionInstance = value
      NotifyPropertyChanged(Function() BoomSectionInstance)
   End Set
End Property

The BoomSection represents an entity defined by:

Namespace Entities

   ''' <summary>
   ''' Class containing Entity Definition for a Boom
   ''' </summary>
   ''' <remarks></remarks>
   <Table("BoomSections")>
   Public Class BoomSection
      Inherits ViewModelBase
      Implements IBaseProperties

      Public Sub New()
         Id = Guid.NewGuid()
      End Sub
      <Key>
      Public Property Id As Guid Implements IBaseProperties.Id
      Public Property Description As String Implements IBaseProperties.Description
   End Class