Combobox и IDataErrorInfo

Question

Это моя проблема. Когда я выбираю элемент из поля со списком, (поле со списком заполнено), у меня всегда красный контур.

Почему?

<ComboBox x:Name="Cmb_USER" 
      TabIndex="5" 
      ItemsSource="{Binding User_USER, 
      ValidatesOnExceptions=True, 
      ValidatesOnDataErrors=True,NotifyOnValidationError=True}" 
      IsEditable="True"  
      DisplayMemberPath ="DescUser" 
      Text="{Binding Path=USER}" Margin="114,105,98,179"/> 

Public Property User_USER As ObservableCollection(Of Model_User) 
Private p_USER As String 
Public Property USER() As String 
    Get 
     Return p_USER 
    End Get 
    Set(ByVal value As String) 
     p_USER = value 
     OnPropertyChanged("USER") 
    End Set 
End Property 

#Region "IDataErrorInfo Members" 
Public ReadOnly Property [Error]() As String Implements System.ComponentModel.IDataErrorInfo.Error 
    Get 
     Return Nothing 
    End Get 
End Property 
Default Public ReadOnly Property Item(ByVal Name As String) As String Implements System.ComponentModel.IDataErrorInfo.Item 
    Get 
     Dim result As String = Nothing 

     Select Case Name 
      Case "User_USER" 
       If USER = "" Then 
        result = "ERROR" 
       End If 
     End Select 
     Return result 
    End Get 
End Property 
#End Region 

Answer 1

решенный .. Это решение

Public Property USER() As String 
Get 
    Return p_USER 
End Get 
Set(ByVal value As String) 
    p_USER = value 
    OnPropertyChanged("USER") 
    OnPropertyChanged("User_USER") 
End Set 

End Property